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The role of inherited crustal structures and magmatism in the development of rift segments: Insights from the Kivu basin, western branch of the East African Rift
Abstract
The study of rift basin’s morphology can provide good insights into geological features influencing the development of rift valleys and the distribution of volcanism. The Kivu rift segment represents the central section of the western branch of the East African Rift and displays morphological characteristics contrasting with other rift segments. Differences and contradictions between several structural maps of the Kivu rift make it difficult to interpret the local geodynamic setting. In the present work, we use topographic and bathymetric data to map active fault networks and study the geomorphology of the Kivu basin. This relief-based fault lineament mapping appears as a good complement for field mapping or mapping using seismic reflection profiles. Results suggest that rifting reactivated NE-SW oriented structures probably related to the Precambrian basement, creating transfer zones and influencing the location and distribution of volcanism. Both volcanic provinces, north and south of the Kivu basin, extend into Lake Kivu and are connected to each other with a series of eruptive vents along the western rift escarpment. The complex morphology of this rift basin, characterized by a double synthetic half-graben structure, might result from the combined action of normal faulting, magmatic underplating, volcanism and erosion processes.
... Geodetic and seismic studies could infer the depth, direction and size of the dyke which was also shown to reach the bottom of Lake Kivu. The characteristics determined for these two intrusions are very similar; the propagation was initially radial to the south-east before moving approximately north-south in line with the eruptive fissures and fault lineaments in the Kivu basin (Smets et al., 2016); the intrusion plunged from the volcanic edifice to a maximum of 10 km depth under Lake Kivu. The main differences are the identification of two dykes during the 2002 eruption compared to only one in 2021, and openings of a maximum of 1 m in 2002 compared to 2.5 m in 2021 (Wauthier et al., 2012;Smittarello et al., 2022). ...
... In the case of Nyiragongo volcano, there are evidences suggesting that preexisting faults play a role. In particular, Smittarello et al. (2022) proposed that the Nyabihu fault mapped by Smets et al. (2016) stopped the propagation of the dyke during the 2021 eruption. However, this cessation of propagation only occurred after the dyke had propagated laterally for 20 km to the south. ...
... However, the crust in the Kivu basin is far from being a homogeneous medium as numerous fractures have been mapped. Most of them are strongly aligned with the direction of the rift, but some have different directions (Wadge and Burt, 2011;Smets et al., 2016). In addition, magma remaining trapped at depth, either from failed eruptions or from the cooled part of ancient dykes that fed eruptive vents, also produces crustal heterogeneities. ...
... During the period between July to December 2023, there were a number of earthquake events within a radius of 20-25 km from Mt. Hanang with magnitude ranging from 3.8 to 4.5 Richter Scale, i.e. four of these events occurred in the month of August, 2023(USGS 2023ISC 2024). These events to a large existent tend to reactivate both pre-existing, recent faults and shear fractures leading to slope instability hence increasing chances of landslide (Smets et al. 2016;Roche et al. 2020). ...
... 3). Using DEM, multiple directions of topographic lineaments representing fault systems, fracture zones, dyke, or shear zones were identified (e.g., Kervyn et al. 2006;Smets et al. 2016;Mulaya et al. 2022). These lineaments were then traced in ArcGIS software and calibrated by using existing structural maps of the Hanang area (e.g., Kervyn et al. 2006;Fig. ...
... The slope was computed from DEM layer using a slope tool under spatial analyst tools in the ArcMap version 10.8 to identify the steepness at each cell of a raster surface (Fig. 3). The slope was then used to define morphological characteristics of relief and infer slope stability following methods described by Smith and Clark (2005), Smets et al. (2016) and in Fig. 3. ...
The December 3rd 2023 catastrophic landslide which claimed 89 lives in the Hanang Town in Northern Tanzania is undeniable fact of how deadly the landslide can strike. Geological fieldwork, structural measurement, and SRTM-DEM data reveal that the Hanang area is located within a tectonically active zone of the East African Rift System where the interplay of tectonics, groundwater, and structural geometry affects the hydromechanical behavior of the rocks leading to landslide. The slope stability and failure behavior are influenced primarily by geometry of brittle inherited basement fabrics and conjugate structures involving faults, fractures, shear zones, and dykes. The Hanang area exhibit several patterns of fractures, faults, and shear zones with varied trends, i.e., azimuthal directions in the E-W, NW–SE, and NE-SW which crosscut each other at an angle forming conjugate structure systems. These conjugate structure patterns provide a primary source of rock weakness and have an implication on kinematics triggering landslide events in the area. This study deduces that the landslide on the 3rd December, 2023, involved slope failure of basement rocks and associated boulders (rock fall) which primarily paved a way for the catastrophic debris flow in the Katesh Town, Gendabi, Ganana, and Jordom villages downhill. The aftermath of this landslide event had initiated new abrupt water channels (creeks) which imply breaching of a long term accumulated groundwater reservoirs hence leading to abrupt leakage similar to collapse of a “water dam.” Understanding of these complex interactions of geological, seismic, hydrological, and topographical factors which trigger landslide catastrophic events is crucial for formulating effective monitoring and mitigation strategies.
... Some of the larger, historical, landslides (i.e. landslides that do not appear active in our oldest source 145 of information) clearly occurred more than 10,000 years ago (Dewitte et al., 2021), i.e. over a period of time that underwent changes in environmental conditions (Felton et al., 2007;Wassmer et al., 2013;Ross et al., 2014;Smets et al., 2016). ...
... For example, no earthquake-induced recent deepseated landslides were observed (Dewitte et al., 2021), whereas seismicity is an important component of the old deep-seated landslide model. In addition, the climatic and seismic conditions have evolved over the past tens of thousands of years (Felton et al., 2007;Wassmer et al., 2013;Ross et al., 2014;Smets et al., 2016). For example, the region experienced an abrupt shift from drier conditions to more humid conditions around 13,000 BP (Felton 570 et al., 2007;Wassmer et al., 2013). ...
... This change in the lake level was not only due to a shift in the climatic conditions but also to the formation of the Virunga Volcano Province that created a dam on the upstream part of the Rift basin that used to drain northwards (Figure 1b; Haberyan and Hecky, 1987). During that period of volcano formation, the regional geodynamics and 575 the seismicity pattern were different (Smets et al., 2016). Hence a large part of the old deep-seated landslides may have been triggered under different conditions (Dewitte et al., 2021). ...
Tropical mountainous regions are often identified as landslide hotspots with particularly vulnerable populations. Anthropogenic factors are assumed to play a role in the occurrence of landslides in these populated regions, yet the relative importance of these human-induced factors remains poorly documented. In this work, we aim to explore the impact of forest cover dynamics, roads and mining activities on the occurrence of landslides in the Rift flank west of Lake Kivu in the DR Congo. To do so, we compile an inventory of 2730 landslides using © Google Earth imagery, high resolution topographic data, historical aerial photographs from the 1950’s and extensive field surveys. We identify old and recent (post 1950’s) landslides, making a distinction between deep-seated and shallow landslides, road landslides and mining landslides. We find that susceptibility patterns and area distributions are different between old and recent deep-seated landslides, which shows that natural factors contributing to their occurrence were either different or changed over time. Observed shallow landslides are recent processes that all occurred in the past two decades. The analysis of their susceptibility indicates that forest dynamics and the presence of roads play a key role in their regional distribution pattern. Under similar topographic conditions, shallow landslides are more frequent, but of smaller size, in areas where deforestation has occurred since the 1950’s as compared to shallow landslides in forest areas, i.e. in natural environments. We attribute this size reduction to the decrease of regolith cohesion due to forest loss, which allows for a smaller minimum critical area for landsliding. In areas that were already deforested in 1950’s, shallow landslides are less frequent, larger, and occur on less steep slopes. This suggests a combined role between regolith availability and soil management practices that influence erosion and water infiltration. Mining activities increase the odds of landsliding. Mining and road landslides are larger than shallow landslides but smaller than the recent deep-seated instabilities. The susceptibility models calibrated for shallow and deep-seated landslides do not predict them well, highlighting that they are controlled by environmental factors that are not present under natural conditions. Our analysis demonstrates the role of human activities on the occurrence of landslides in the Lake Kivu region. Overall, it highlights the need to consider this context when studying hillslope instability characteristics and distribution patterns in regions under anthropogenic pressure. Our work also highlights the importance of considering the timing of landslides over a multi-decadal period of observation.
... Overall, geological structures have enormous contributions during decision making that aim to advance geological investigations of a certain area on interest. However, structure mapping exercise may end up being difficult if proxies of these structures are obscured on the earth's surface by young supracrustal rocks (e.g., Smets et al. 2016). This has been the case in the Rungwe Volcanic Province (RVP), SW Tanzania (Figure 1), where piles of volcanic materials have covered most of the RVP area and resulted into difficulties in understanding the structure setting of the area. ...
... In the last three decades, remote sensing has proved to be a robust supplementary tool for geological mapping (van der Meer et al. 2012, van der Meer et al. 2014). Geoscientists are intensively using different remote sensing data sets for geological mapping and in exploration of earth resources (e.g., Hewson et al. 2005, van Ruitenbeek et al. 2005, Macheyeki 2008, Macheyeki et al. 2008, Masoud and Koike 2011, Mshiu 2011, Grohmann and Miliaresis 2013, Mshiu et al. 2015, Smets et al. 2016, Cudahy et al. 2017, Faulds et al. 2017. For example, Mshiu et al. (2015) used the 90 m resolution Shuttle Radar Topographical Mission (SRTM) data and Landsat Enhanced Thematic Mapper (ETM+) data to map crustal pathways of paleo-hydrothermal fluids in the Sukumaland Greenstone Belt in Tanzania. ...
... For example, Mshiu et al. (2015) used the 90 m resolution Shuttle Radar Topographical Mission (SRTM) data and Landsat Enhanced Thematic Mapper (ETM+) data to map crustal pathways of paleo-hydrothermal fluids in the Sukumaland Greenstone Belt in Tanzania. A recent study in the western branch of the EARS has revealed topographic and bathymetric data can offer reliable results to interpret fault systems (Smets et al. 2016). This shows that applying geomorphology characteristics for identification of geological structures, for example, faults and folds is not new. ...
This study aimed to assess the potentiality of Digital Elevation Model (DEM)-derived flow direction in mapping geologic structures, specifically in areas with thick overburden of soil or volcanic materials such as in the Rungwe Volcanic Province, SW Tanzania. Clustering of flow directions based on trends of structure sets found in the study area was applied and it successfully revealed a large number of geological structures through demarcated tectonic blocks from patterns created by flow directions. Northwest-southwest (NW-SE) and NE-SW-trending lineaments are the dominant structural sets in the area whereas N-S and E-W-trending lineaments constitute the minor sets. Moreover, the approach has also unveiled the NW-SE-trending faults that control most of the volcanic centers. The extracted lineaments by the method presented in this research are consistent with the known geological structures and structure surface manifestations such as volcanic centers, hot springs and earthquake epicenters. Findings of this study has revealed that the method can be applied as a robust technique to map crustal structures mostly in early phases of exploration, and has shown good results in delineating faults in areas with thick overburden such as soils and volcanic materials.
... East African Rift volcanism spans the entire range of magmatic compositions, from felsic to ultramafic [Andersen et al. 2012; Barette et al. 2017] The rift system has been suggested to divide into two branches around a microplate centered on the Tanzanian Craton [Calais et al. 2006;Saria et al. 2014]. Within the western branch of this division lies the VVP [Smets et al. 2016], located at the border between the DRC, Uganda, and Rwanda. This province contains eight volcanoes, only two of which are active [Demant et al. 1994]: Nyamuragira (commonly: Nyamulagira) and Nyiragongo. ...
... These are the most active volcanoes in Africa [Coppola and Cigolini 2013;Smets et al. 2015;Wright et al. 2015], with Nyiragongo erupting some of the lowest viscosity lavas on Earth [Barette et al. 2017;Chakrabarti et al. 2009a;Chakrabarti et al. 2009b;Giordano et al. 2007]. While these two volcanoes are separated by only 13 km, they produce very different lavas suggesting the volcanoes sample different sources and/or are fed by magma chambers with different degrees of differentiation [Andersen et al. 2012;Barette et al. 2017;Chakrabarti et al. 2009a;Smets et al. 2016]. ...
... The main crater is roughly 1.3 km in diameter with a lava lake approximately 250 m across making it the largest persistent lava lake in the world [Sawyer et al. 2008;Smets et al. 2017]. Nyiragongo lavas are Smets et al. [2016], based on previous field observations and mapping from Sahama [1954], Thonnard and Denaeyer [1965], Denaeyer and Schellinck [1965], Smets et al. [2010], and Smets et al. [2015]. Bold black lines are the main rift segments of the western branch of the East African Rift, based on the mapping of Smets et al. [2016]. ...
Nyiragongo (Democratic Republic of Congo) erupts fluid, fast-moving, foidite lavas. Nearby Nyamuragira frequently erupts less mobile tephrite lavas. Nyamuragira flows rarely threaten urbanized areas, but Nyiragongo flows threaten Goma (pop. ~900,000) and surrounding villages, resulting in fatalities in 1977 and 2002. We report new laboratory measurements of viscosity evolution during cooling and crystallization from both volcanoes by concentric cylinder viscometry. Melt viscosity is ~33 Pa s at the liquidus (~1220°C) for Nyiragongo lavas, similar to Hawaiian basalts. Lavas remain fluid over ~75°C of undercooling (ϕc
... Section of the western branch of the East African Rift including the Kivu Rift and the northern tip of Lake Tanganyika. The main fault lineaments of this section and its two volcanic provinces are indicated (Smets et al., 2016). The Ikoma landslide is located within the South Kivu Volcanic Province (SKVP),~15 km west of the city of Bukavu. ...
... The weathering of their morphology however suggests that these landslides are at least a few thousands year old. As evidenced by hanging valleys and knickpoints, this swarm of landslides located along an active fault (Smets et al., 2016) is of a probable seismo-tectonic origin. Depicker et al. (under review) have inventoried N1000 of such old deep-seated landslides in the region. ...
... First, this landslide has faced large deformations episodes during recent years, as indicated by highly apparent deformation features at its surface, allowing following its near-complete development through remote sensing and fieldbased approaches. Second, the landslide developed in the South Kivu Volcanic Province (Smets et al., 2016;Delvaux et al., 2017, Fig. 1), that is characterised by deeply weathered (up to tens of metres) sequences of late Miocene to Pleistocene basaltic lava layers (Pasteels et al., 1989;Moeyersons et al., 2004). Third, the landslide is located in a rural area where subsistence agriculture can be assumed to be of negligible influence on the activity of a deep-seated process. ...
Studying the causes and triggers of landslides is essential to understand the key process of hillslope evolution and the hazards they generate. Such understanding is crucial in tropical areas where landslide impacts are high and on the rise, and the dearth of accurate processes characterisation is large. Here we investigate the timing and the mechanisms of relatively slow-moving deep-seated landslides in weathered tropical environments through the analysis of a landslide located in the Kivu Rift (DR Congo). This landslide, developed in weathered basalt, shows obvious deformation features at its surface indicating large deformations during recent years, making it a unique natural laboratory in an underexplored area. High-resolution topographic data, historical aerial photographs, satellite imagery and careful field investigations are used to detail the landslide mechanisms and investigate failure development over a 60-year record. By confronting rainfall time series and earthquake sequences with the different deformation episodes, we show that the relation between instability triggers and slope failure is not straightforward; e.g., the largest instability occurred at the end of a dry season during a period of relatively low seismicity. Instead of direct influence of external triggers, we show that some phases of instability may be caused by the intrinsic evolution of the hillslope associated with weathered-related weakening of the slope strength through time. Our results question the relative weight of the commonly recognized causes and triggers of slope instability in this area. Analysis of landslide causes and triggers provided here should help improve the understanding of how surface processes influence the pace of hillslope evolution. It also contributes to a more accurate evaluation of the landslide hazard in the area and across other regions where similar environmental conditions are met.
... The western branch of the EARS is more seismically active than the eastern branch (Chorowicz, 2005;Mulibo and Nyblade, 2016). It has a length of approximately 2100 km from the Albertine-Rhino grabens (Chorowicz, 2005;Katumwehe et al., 2015) in the north through the Kivu graben and Tanganyika Rift (Ebinger, 1989a;Smets et al., 2016;Wood et al., 2017), which are represented by lakes Albert, Edward, Kivu and Tanganyika on Fig. 2. To the southeast, it continues into the Rukwa Rift Basin and then enters the Malawi Rift in the south (lakes Rukwa and Malawi/Nyasa on Fig. 2). ...
... Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) (Fig. 4) with a spatial resolution of 30 m (Farr et al., 2007) is used to delineate rift-related faults and lineaments in the study area. Following the methodology of Smets et al. (2016), hillshade (shaded relief) and slope images were derived from these data for analysis. Hillshade provides information on slope aspect and helps in the interpretation of structures and morphological characteristics of relief but the choice of illumination may mislead interpretation (Smets et al., 2016). ...
... Following the methodology of Smets et al. (2016), hillshade (shaded relief) and slope images were derived from these data for analysis. Hillshade provides information on slope aspect and helps in the interpretation of structures and morphological characteristics of relief but the choice of illumination may mislead interpretation (Smets et al., 2016). Slope images generally highlight rapid changes at the base and the top of a slope: concave at the base and convex at the summit allowing a clear indication of breaks of slope and ridges (Smith and Clark, 2005). ...
... The NE-trending Mesoproterozoic Kibaran Belt lies to the west of the Tanzanian craton and east of Lake Kivu (Figure 3). The northeastern part of the Kivu rift comprises N-S trending structures, whereas crustal structures in the southern part of the rift are NW-trending (Smets et al., 2016;Wood et al., 2017). The Kivu rift comprises two major basins bounded on one or both sides by large offset border fault systems ( Figure 3). ...
... Magmatism has affected the Kivu rift zone since at least 12 Ma when lavas at the base of the Virunga province were erupted (Kampunzu et al., 1998). Large parts of the North Kivu basin are covered by extensive lava flows of the VVP, including Nyiragongo and Nyamuragira, two of the most active volcanoes in Africa (Smets et al., 2016;Wood et al., 2017). ...
... All of the stations located on volcanic strata in the VVP have null measurements and high standard deviations of ϕ, similar to those in the RVP sector. The NW and N-S ϕ patterns parallel the two magmatic trends: active dike intrusion from reservoirs beneath Nyiragongo strike approximately N-S (Wauthier et al., 2012), whereas the aligned chain of volcanoes, faults, and fissures west of the Kivu rift strike NW (Smets et al., 2016;Wood et al., 2017). Data from southwestern Uganda are highly complementary to our results, and they extend coverage northward along the Western rift. ...
Although the East African rift system formed in cratonic lithosphere above a large‐scale mantle upwelling, some sectors have voluminous magmatism, while others have isolated, small‐volume eruptive centers. We conduct teleseismic shear wave splitting analyses on data from 5 lake‐bottom seismometers and 67 land stations in the Tanganyika‐Rukwa‐Malawi rift zone, including the Rungwe Volcanic Province (RVP), and from 5 seismometers in the Kivu rift and Virunga Volcanic Province, to evaluate rift‐perpendicular strain, rift‐parallel melt intrusion, and regional flow models for seismic anisotropy patterns beneath the largely amagmatic Western rift. Observations from 684 SKS and 305 SKKS phases reveal consistent patterns. Within the Malawi rift south of the RVP, fast splitting directions are oriented northeast with average delays of ~1 s. Directions rotate to N‐S and NNW north of the volcanic province within the reactivated Mesozoic Rukwa and southern Tanganyika rifts. Delay times are largest (~1.25 s) within the Virunga Volcanic Province. Our work combined with earlier studies shows that SKS‐splitting is rift parallel within Western rift magmatic provinces, with a larger percentage of null measurements than in amagmatic areas. The spatial variations in direction and amount of splitting from our results and those of earlier Western rift studies suggest that mantle flow is deflected by the deeply rooted cratons. The resulting flow complexity, and likely stagnation beneath the Rungwe province, may explain the ca. 17 Myr of localized magmatism in the weakly stretched RVP, and it argues against interpretations of a uniform anisotropic layer caused by large‐scale asthenospheric flow or passive rifting.
... The VVP is a transition zone between the Lake Kivu and Lake Edward rift basins, in the central part of the western branch of the East African Rift System (EARS) (Fig. 1a) (Ebinger 1989;Smets et al. 2016). In this part of the EARS, the extension rate reaches 2.8 mm/year (Saria et al. 2013;Ji et al. 2017). ...
... The main edifice is also characterized by two satellite cones: Baruta on the northern flank and Shaheru on the southern flank. Its eruptive activity is mostly characterized by persistent lava lake activity in its summit crater, sometimes becoming the largest lava lake surface on Earth (Smets et al. 2016). Only three flank eruptions have been documented since the arrival of the first European explorers in the nineteenth century. ...
Volcano monitoring requires simple techniques to rapidly identify the cause of volcanic unrest. The so-called RSAM (real-time seismic amplitude measurements) technique, used in many observatories, is a good example of extracting information from seismograms with minimal processing. Built on a similar principle, the more recent seismic amplitude ratio analysis (SARA) technique allows locating migrating seismicity at high frequency (> 2 Hz, e.g., due to dike intrusions) under certain assumptions. However, such analysis generally requires a dense distribution of stations close to the seismic sources (depending on the magnitude) and/or station sites undisturbed by human activity. In a more straightforward and qualitative approach, computing amplitude ratios between station pairs can also allow for the detection of temporal and (2D) spatial changes of volcanic activity. In this work, we adopt such a simplified approach of SARA in order to characterize seismic tremors originating from two open-vent neighboring volcanoes, Nyiragongo and Nyamulagira, in the Virunga Volcanic Province (VVP) in the Democratic Republic of the Congo (DRC). In contrast with previous studies, we focus here on the low-frequency band (0.3–1 Hz), free from anthropogenic noise and sensitive to shallow volcanic tremors linked to intermittent or permanent intra-crater eruptive activity recorded through the large-aperture local network. We apply for the first time the SARA methodology for volcanic sources predominantly generating surface waves and propagating over long distances. The analysis is performed on more than two years of continuous seismic data. Seismic amplitude analysis in this frequency band is strongly influenced by the short-period microseisms originating from nearby Lake Kivu. Despite this diurnal to seasonal amplitude variability, SARA successfully detects continuous volcanic tremor activity and its arrest at both volcanoes. In light of these findings, we discuss the applicability of the method to the continuous, real-time detection, and characterization of long-period shallow volcanic tremor sources in this region.
... For example, no earthquake-induced recent deep-seated landslides were observed (Dewitte et al., 2021), whereas seismicity is an important component of the old deep-seated landslide model. In addition, the climatic and seismic conditions have evolved over the past tens of thousands of years (Felton et al., 2007;Wassmer et al., 2013;Ross et al., 2014;Smets et al., 2016). For example, the region experienced an abrupt shift from drier conditions to more humid conditions around 13 000 BP (Felton et al., 2007;Wassmer et al., 2013). ...
... This change in the lake level was due to not only a shift in the climatic conditions but also the formation of the Virunga Volcano Province that created a dam on the upstream part of the rift basin that used to drain northwards (Fig. 1b;Haberyan and Hecky, 1987). During that period of volcano formation, the regional geodynamics and the seismicity pattern were different (Smets et al., 2016). Hence a large part of the old deep-seated landslides may have been triggered under different conditions. ...
Tropical mountainous regions are often identified as landslide hotspots with growing population pressure. Anthropogenic factors are assumed to play a role in the occurrence of landslides in these densely populated regions, yet the relative importance of these human-induced factors remains poorly documented. In this work, we aim to explore the impact of forest cover dynamics, roads and mining activities on the characteristics and causes of landslides in the rift flank west of Lake Kivu in the Democratic Republic of the Congo (DR Congo). To do so, we compile a comprehensive multi-temporal inventory of 2730 landslides. The landslides are of different types and are grouped into five categories that are adapted to study the impact of human activities on slope stability: old (pre-1950s) and recent (post-1950s) deep-seated landslides, shallow landslides, landslides associated with mining and landslides associated with road construction. We analyse the landslides according to this classification protocol via frequency–area statistics, frequency ratio distribution and logistic regression susceptibility assessment. We find that natural factors contributing to the cause of recent and old deep-seated landslides were either different or changed over time. Under similar topographic conditions, shallow landslides are more frequent, but of a smaller size, in areas where deforestation has occurred since the 1950s. We attribute this size reduction to the decrease in regolith cohesion due to forest loss, which allows for a smaller minimum critical area for landsliding. In areas that were already deforested in the 1950s, shallow landslides are less frequent, larger and occur on less steep slopes. This suggests a combined role between regolith availability and soil management practices that influence erosion and water infiltration. Mining activities increase the odds of landsliding. Landslides associated with mining and roads are larger than shallow landslides but smaller than the recent deep-seated instabilities, and they are controlled by environmental factors that are not present under natural conditions. Our analysis demonstrates the role of human activities on the occurrence of landslides in the Lake Kivu region. Overall, it highlights the need to consider this context when studying hillslope instability characteristics and distribution patterns in regions under anthropogenic pressure. Our work also highlights the importance of using landslide classification criteria adapted to the context of the Anthropocene.
... The NTK Rift, in which Bujumbura is located, is situated in the central section of the western branch of the East African Rift (Figure 1a). In this region, tectonic uplift, accompanied with seismic activity and faulting, has initiated landscape rejuvenation through knickpoint retreat, enforcing topographic steepening [16,[36][37][38]. This tectonic setting, combined with a tropical climate that favors the occurrence of intense rainfall events and deep weathering, makes the region a landslide hotspot in Africa [6,16]. ...
... Rift sediments are thick, while gneiss and granites are usually highly weathered. The region is crossed by active faults related to the rifting dynamics [36,37]. Typically of the NTK Rift, the climate of Bujumbura is tropical, with an average annual rainfall of 1400 mm (the southern catchment of Bujumbura [45], with most of the precipitation occurring during the October-May rainy season [46]. ...
Accurate and detailed multitemporal inventories of landslides and their process characterization are crucial for the evaluation of landslide hazards and the implementation of disaster risk reduction strategies in densely-populated mountainous regions. Such investigations are, however, rare in many regions of the tropical African highlands, where landslide research is often in its infancy and not adapted to the local needs. Here, we have produced a comprehensive multitemporal investigation of the landslide processes in the hillslopes of Bujumbura, situated in the landslide-prone East African Rift. We inventoried more than 1200 landslides by combining careful field investigation and visual analysis of satellite images, very-high-resolution topographic data, and historical aerial photographs. More than 20% of the hillslopes of the city are affected by landslides. Recent landslides (post-1950s) are mostly shallow, triggered by rainfall, and located on the steepest slopes. The presence of roads and river quarrying can also control their occurrence. Deep-seated landslides typically concentrate in landscapes that have been rejuvenated through knickpoint retreat. The difference in size distributions between old and recent deep-seated landslides suggests the long-term influence of potentially changing slope-failure drivers. Of the deep-seated landslides, 66% are currently active, those being mostly earthflows connected to the river system. Gully systems causing landslides are commonly associated with the urbanization of the hillslopes. Our results provide a much more accurate record of landslide processes and their impacts in the region than was previously available. These insights will be useful for land management and disaster risk reduction strategies.
... More specifically, we focus on the northern part of Lake Tanganyika and 100 the Kivu Rift ( Fig. 1), where rifting was initiated about 11 Ma due to the divergence of the Nubia and Victoria Plate (Delvaux & Barth, 2010;Saria et al., 2014;Pouclet et al., 2016). The Kivu rift currently opens up at a rate of 2 mm/yr (Saria et al., 2014) and is associated with active volcanism and seismic activity (Smets et al., 2016;Delvaux et al., 2017). 105 The study area has a diverse and complex geology, whereby most rock outcrop dates from the Mesoproterozoic, consisting mainly of granites, quartzites, and pelites. ...
... Delvaux et al. (2017) conducted a seismic hazard assessment for the study area, resulting in a 2.2 km resolution PGA model that we used in this study. The distance to the fault 280 systems was derived from a layer composing of active and inactive fault systems (Smets et al., 2016;Delvaux et al., 2017). ...
Predicting landslide occurrence is of key importance for understanding the geomorphological development of mountain environments as well as to assess the potential risk posed by landsliding to human societies in such environments. Global landslide susceptibility models use a generic model formulation to predict landslide susceptibility anywhere on the planet from openly available data. Regional models, on the contrary, use local information on landslide occurrence to constrain model parameters and may also benefit from better spatial information with respect to controlling factors. This study aims to investigate the added value of the construction of regional landslide susceptibility models (versus global and continental models) in the western branch of the East African Rift, a data-scarce landslide-prone tropical environment. First, a comprehensive landslide database containing 6446 instances was compiled for the study area using Google Earth imagery. Second, three regional data-driven landslide susceptibility models were developed. Third, the efforts to construct these regional models were quantified by analysing how their quality is impacted by (1) the use of more accurate, regional peak ground acceleration and geology data, and (2) an increasing inventory size. Fourth, regional and global/continental models were compared in terms of predictive power and geomorphological plausibility. We observe that global/continental landslide susceptibility models are capable of identifying landslide-prone areas, but lack prediction power and geomorphological plausibility when compared to our regional models. Importantly, this difference in quality is not driven by the use of more accurate and detailed geology and peak ground acceleration data, but rather by the use of a detailed regional landslide inventory to calibrate the models. We also show that the model quality only increases marginally beyond a certain inventory size. We conclude that the regional landslide susceptibility assessment does provide an added value compared to existing global models in terms of geomorphological plausibility and model performance, whereby the largest gain is to be found in the construction of a regional landslide inventory, rather than the investment in more detailed covariates or the application of more complex modelling techniques. The latter suggests that the role of controlling variables depends, to some extent, on the regional context: making adequate susceptibility predictions proves difficult when local conditions are not accounted for by means of a regional inventory.
... It is commonly assumed that the Cenozoic rifts have avoided the cratons and follow the mobile belts (McConnell, 1972;Mohr, 1982), which serve as the weakest pathways for rift propagation. Structural control exerted by the pre-existing heterogeneities within the Proterozoic belts at the scale of individual faults or rifts has also been demonstrated (Corti, van Wijk, Cloetingh, & Morley, 2007;Katumwehe, Abdelsalam, & Atekwana, 2015;Morley, 2010;Ring, 1994;Smets et al., 2016;Theunissen, Klerkx, Melnikov, & Mruma, 1996;Versfelt & Rosendahl, 1989). However, as shown by Koptev, Calais, Burov, Leroy, and Gerya (2015), Koptev et al. (2016), the formation of two rift zones on opposite sides of a thick lithosphere segment can be explained without appealing to pre-imposed heterogeneities at the crustal level. ...
... Very small variations in initial plume position with respect to the cratonic bodies (up to several tens of km only) appear to be able to change the relation between these three segments of separated plume head, which, in turn, alters the degree of development of the corresponding rift branches. We argue, thus, that the resulting rifting pattern is largely controlled by the relative position of the initial mantle plume anomaly with respect to first-order lithospheric thickness differences rather than by secondorder crustal and/or lithospheric compositional heterogeneities as commonly assumed (Corti et al., 2007;Katumwehe et al., 2015;Smets et al., 2016;Theunissen et al., 1996). ...
Using numerical thermo-mechanical experiments we analyze the role of an active mantle plume and pre-existing lithospheric-thickness differences in the structural development of the central and southern East African Rift system. The plume–lithosphere-interaction model setup captures the essential features of the studied area: two cratonic bodies embedded into surrounding lithosphere of normal thickness. The results of the numerical experiments suggest that localization of rift branches in the crust is mainly defined by the initial position of the mantle plume relative to the cratons. We demonstrate that development of the Eastern branch, the Western branch and the Malawi rift can be the result of non-uniform splitting of the Kenyan plume, which has been rising underneath the southern part of the Tanzanian craton. Major features associated with Cenozoic rifting can thus be reproduced in a relatively simple model of the interaction between a single mantle plume and pre-stressed continental lithosphere with double cratonic roots. This article is protected by copyright. All rights reserved.
... Different maps of rift faults have been proposed based on reconnaissance field surveys (Boutakoff, 1939), interpretation of aerial photographs and satellite images coupled with field work (Degens et al., 1973;Pouclet, 1977Pouclet, , 1978Villeneuve, 1978Villeneuve, , 1980Villeneuve, , 1983Ebinger, 1989b;Laerdal and Talbot, 2002), and on slopeshaded topography and bathymetry (d'Oreye et al., 2011;Wood et al., 2015;Smets et al., 2016). A deterministic seismic hazard assessment requires a map of active faults for which the past activity of each faults (time of last seismic activation, recurrence rate, maximal magnitude, dimensions, segmentation) is well known. ...
... For the faults in Lake Kivu and their continuation on-shore (see Fig. 3 for the geographic names), we followed the interpretation of Wood et al. (2015) but in more details on-shore and thus have generally shorter border faults. North of the lake (Virunga volcanic region and Rutshuru basin), we followed more or less closely the interpretation of Smets et al. (2016). Along the western shore of the southern part of the lake, in Katana area, we used the interpretation of Villeneuve (1983), supplemented by our field observation. ...
In the frame of the Belgian GeoRisCA multi-risk assessment project focusing on the Kivu and northern Tanganyika rift region in Central Africa, a new probabilistic seismic hazard assessment has been performed for the Kivu rift segment in the central part of the western branch of the East African rift system. As the geological and tectonic setting of this region is incompletely known, especially the part lying in the Democratic Republic of the Congo, we compiled homogeneous cross-border tectonic and neotectonic maps.
... Nyamulagira's caldera rim is outlined with a white dashed line. Faults are from [13], and rose diagrams of VVP's faults (yellow) and eruptive fissures (red) are from [9]. The white rectangle gives the extent of Figure 2. ...
... We also suggest that the 2010 sources unclamped the deep part of the potential dike surface oriented ~N45E, south of the caldera, in which magma could have intruded from a deep reservoir. Magma could have then migrated laterally following this ~N45E SC orientation, which may represent a plane of weakness due to pre-existing faults buried under the lava field [13] toward the 2011-2012 eruption site location. Unusual seismic tremor data [10] also suggest magma movement at a depth along a similar ~N45E orientation. ...
A summit and upper flank eruption occurred at Nyamulagira volcano, Democratic Republic of Congo, from 2–27 January 2010. Eruptions at Nyamulagira during 1996–2010 occurred from eruptive fissures on the upper flanks or within the summit caldera and were distributed along the ~N155E rift zone, whereas the 2011–2012 eruption occurred ~12 km ENE of the summit. 3D numerical modeling of Interferometric Synthetic Aperture Radar (InSAR) geodetic measurements of the co-eruptive deformation in 2010 reveals that magma stored in a shallow (~3.5 km below the summit) reservoir intruded as two subvertical dikes beneath the summit and southeastern flank of the volcano. The northern dike is connected to an ~N45E-trending intra-caldera eruptive fissure, extending to an ~2.5 km maximum depth. The southern dike is connected to an ~N175E-trending flank fissure extending to the depth of the inferred reservoir at ~3.5 km. The inferred reservoir location is coincident with the reservoir that was active during previous eruptions in 1938–1940 and 2006. The volumetric ratio of total emitted magma (intruded in dikes + erupted) to the contraction of the reservoir (rv) is 9.3, consistent with pressure recovery by gas exsolution in the small, shallow modeled magma reservoir. We derive a modified analytical expression for rv, accounting for changes in reservoir volume induced by gas exsolution, as well as eruptive volume. By using the precise magma composition, we estimate a magma compressibility of 1.9–3.2 × 109 Pa−1 and rv of 6.5–10.1. From a normal-stress change analysis, we infer that intrusions in 2010 could have encouraged the ascent of magma from a deeper reservoir along an ~N45E orientation, corresponding to the strike of the rift transfer zone structures and possibly resulting in the 2011–2012 intrusion. The intrusion of magma to greater distances from the summit may be enhanced along the N45E orientation, as it is more favorable to the regional rift extension (compared to the local volcanic rift zone, trending N155E). Repeated dike intrusions beneath Nyamulagira’s SSE flank may encourage intrusions beneath the nearby Nyiragongo volcano.
... This metamorphic basement is intruded by large Late Proterozoic S-type granitic batholites [40][41][42]. Since the Miocene, the tectonic activity linked to the opening of the East African Rift has led to the formation of the Kivu graben and the correlated uplift of its shoulders [43,44]. Surprisingly, though the area is seismically active, landslide events in the North Tanganyika-Kivu rift are not correlated with earthquakes [34,37]. ...
Rwanda, in eastern tropical Africa, is a small, densely populated country where climatic disasters are often the cause of considerable damage and deaths. Landslides are among the most frequent hazards, linked to the country’s peculiar configuration including high relief with steep slopes, humid tropical climate with heavy rainfall, intense deforestation over the past 60 years, and extensive use of the soil for agriculture. The Karongi region, in the west-central part of the country, was affected by an exceptional cluster of more than 700 landslides during a single night (6–7 May 2018) over an area of 100 km2. We analyse the causes of this spectacular event based on field geological and geomorphology investigation and CHIRPS and ERA5-Land climate data. We demonstrate that (1) the notably steep slopes favoured soil instability; (2) the layered soil and especially the gravelly, porous C horizon allowed water storage and served as a detachment level for the landslides; (3) relatively low intensity, almost continuous rainfall over the previous two months lead to soil water-logging; and (4) acoustic waves from thunder or mechanical shaking by strong wind destabilized the water-logged soil through thixotropy triggering the landslides. This analysis should serve as a guide for forecasting landslide-triggering conditions in Rwanda.
... Ma) volcanic rocks in the area of Bukavu are deeply weathered (Pouclet et al., 2016;Dille et al., 2019). Contrarily, the lava-dominated landscapes in the Virunga Volcanic Province are much younger (2 Ma -present) and much less weathered (Smets et al., 2016;Pouclet et al., 2016). The formation of the Virunga Volcanic Province blocked the original drainage of Lake Kivu towards Lake Edward. ...
... A superimposição de eventos e a inversão de esforços tectônicos podem ser aferidas quando são interpretados mais de um campo de tensões a partir de dados estruturais de falha/estria (Riccomini, 1989;Bezerra et al., 2020;Shi et al., 2020), ou se há evidências geofísicas de reativação e inversão de falhas (Williams et al., 1989;Maia e Bezerra, 2014;Stephenson et al., 2020). Quando a análise in situ é dificultada, seja por questões de acesso, falta de afloramentos ou pela escala de trabalho, a análise estrutural por sensoriamento remoto é uma técnica bastante utilizada (Rahiman e Pettinga, 2008;Bezerra et al., 2014;Cianfarra e Salvini, 2014;Smets et al., 2016;Brêda et al., 2018;Pinheiro et al., 2019). Esse método, associado com ferramentas de modelagem numérica, permite interpretar e associar os principais trends de lineamentos de uma área com a tectônica regional ou de áreas adjacentes. ...
A correlação entre tensão e deformação é um desafio em estudos geológicos, principalmente em regiões onde ocorre superposição de eventos com diferentes arranjos de esforços. Uma região propícia para esse tipo de estudo é a Bacia Potiguar, no Nordeste do Brasil, que registra abundantes evidências de deformações tectônicas cenozoicas, e tem dois eventos admitidos na literatura para este período: do Albiano ao Mioceno médio, com tensão horizontal máxima (SH) N-S; e do Mioceno médio ao Recente, com SH variando de E-W a NW-SE. O presente estudo modelou numericamente esses dois eventos tectônicos em uma região no noroeste da Bacia Potiguar, correlacionando as tensões modeladas com a trama estrutural registrada em análise de fotolineamentos. Os cenários numéricos foram produzidos com base no método dos elementos finitos. Foram aplicadas tensões compressivas relacionadas aos dois eventos tectônicos, de modo a simular como se comportam os padrões de ruptura e as tensões horizontais máximas em uma trama estrutural pretérita. Como resultados, foi observado que as feições estruturais predominantes na região apresentam orientação E-W, WNW-ESE e NE-SW a NNE-SSW, sendo interpretadas principalmente como reativações da trama preexistente, já que são subparalelas à estruturação da Bacia Potiguar ou da Margem Equatorial. Os modelos numéricos sugerem que estruturas formadas com orientação próxima a N-S, como os lineamentos NE-SW/NNE-SSW, estariam relacionados ao campo de esforços atuante do Albiano ao Mioceno médio. Já as estruturas próximas a E-W e WNW-ESE seriam formadas principalmente pelo campo de esforços atuante do Mioceno médio ao Recente. Os resultados numéricos mostram ainda que ocorre reorientação das tensões nas adjacências de descontinuidades estruturais, favorecendo, nos dois eventos, a formação de novas estruturas paralelas à
... The neotectonic setting, related to the evolution of the late Cenozoic rifting, has been highlighted by mapping the fault pattern from combined interpretation of the topography from the NASA Shuttle Radar Topography Mission (SRTM) at 30 m resolution, existing geological maps and additional field work (Smets et al. 2016). We also added the known thermal springs, quaternary volcanic centers, and seismic epicenters . ...
The Kivu rift, in the middle of the western branch of the East African Rift system, has a particular setting within the African continent. It represents the most recent (late Cenozoic) evolution of the Mesoproterozoic Karagwe-Ankole Belt of the Great Lakes Region in Central Africa. Its architecture and evolution have been profoundly influenced by the tectonic framework inherited from the Kibaran and Pan-African orogenic events. In order to build a new detailed seismic hazard map, we have compiled regional geological and neotectonic maps, re-examined the tectonic evolution, investigated the brittle structures, and determined the paleo-stress field evolution. The Kivu rift appears heterogeneous and complex. It probably started as an isolated segment that progressively linked with the adjacent segments of the Western Rift Branch. Its architecture and structural inheritance are reflected in the seismic activity and the current stress field. This results in a marked lateral variability of the Gutenberg–Richter parameters and seismic hazard estimates.
... (2) Faults: This kind of geological structure has a prominent effect on the stability of rock mass [51,52]. In the study area there is a spectacular thrust nappe structure characterized by strong faulting activity. ...
Landslides are one of the major geohazards threatening human society. The objective of this study was to conduct a landslide hazard susceptibility assessment for Ruijin, Jiangxi, China, and to provide technical support to the local government for implementing disaster reduction and prevention measures. Machine learning approaches, e.g., random forests (RFs) and support vector machines (SVMs) were employed and multiple geo-environmental factors such as land cover, NDVI, landform, rainfall, lithology, and proximity to faults, roads, and rivers, etc., were utilized to achieve our purposes. For categorical factors, three processing approaches were proposed: simple numerical labeling (SNL), weight assignment (WA)-based and frequency ratio (FR)-based. Then 19 geo-environmental factors were respectively converted into raster to constitute three 19-band datasets, i.e., DS1, DS2, and DS3 from three different processes. Then, 155 observed landslides that occurred in the past decades were vectorized, among which 70% were randomly selected to compose a training set (TS1) and the remaining 30% to form a validation set (VS1). A number of non-landslide (no-risk) samples distributed in the whole study area were identified in low slope (
... The long rainy season during nine months per year explains a high density of the hydrographic network with active rivers which crossing the steep topography and then incising vertically and generating river landslides and bank erosion (Moeyersons and Trefois 2012). For geomorphological setting, low slope angles are founded in the West followed by very steep topography associated to recent tectonic uplift with North-Southern oriented active faults system (Smets et al. 2016;Delvaux et al. 2017) and the Eastern with moderate slopes. The lithology is dominated by complex rocks of Precambrian age. ...
The aim of this research is the modelling of landslide susceptibility in the hillslopes of Bujumbura using the Weights-of-Evidence model, a probabilistic data modelling approach relevant for predicting future landslides at a regional scale. Initially, characteristics and spatial mapping of different landslides type were identified (fall, flow, slide, complex) by thorough interpretation of high-resolution remote sensing data (mountainous areas with difficult access) and intensive fieldwork. Subsequently, the main landslides controlling factors were selected (lithology, fault density, land use, drainage density, slope aspect, curvature, slope angle, and elevation) using in-depth field knowledge and relevant literature. A landslide inventory map with a total of 569 landslide sites was constructed using the data from various sources. Out of those 569 landslide sites, 285 (50.1%) of the data taken before the 2000s was used for training and the remaining 284 (49.9%) sites (post-2000 events) were used for the accuracy assessment purpose. Thereafter, a prediction map of future landslides was generated with an accuracy of 73.7%. The main geo-environmental landslides factors retained are the high density of drainage networks, the lithology often made with weathered gneiss, the high fault density, the steep topography and the convex slope curvature. The landslide susceptibility map validated was reclassified into very high, high, moderate, low and very low zones. The established susceptibility map will allow with the interaction of the real terrain to locate roads, dwellings, urban extension areas, dams located in high landslides risk zones. These infrastructures will require intervention to address their vulnerability with new facilities, slope stabilization, creation of bypass roads, etc. The susceptibility map produced will be a powerful decision-making tool for drawing up appropriate development plans. Such an approach will make it possible to mitigate the socio-economic impacts due to slope instabilities.
... (2) Faults: This kind of geological structure has a prominent effect on the stability of rock mass [51,52]. In the study area there is a spectacular thrust nappe structure characterized by strong faulting activity. ...
Landslides are one of the major geohazards threatening human society. This study was aimed at conducting such a hazard risk prediction and zoning based on an efficient machine learning approach, Random Forest (RF), for Ruijin, Jiangxi, China. Multiple geospatial and geo-environmental data such as land cover, NDVI, landform, rainfall, stratigraphic lithology, proximity to faults, to roads and to rivers, depth of the weathered crust, etc., were utilized in this research. After pre-processing, including digitization, linear feature buffering and value assignment, 19 hazard-causative factors were eventually produced and converted into raster to constitute a 19-band geo-environmental dataset. 155 observed landslides that had truly taken places in the past 10 years were utilized to establish a vector layer. 70 % of the disaster sites (points) were randomly selected to compose a training set (TS) and the remained 30 % to form a validation set (VS). A number of non-risk samples were identified in low slope (
... The NTK Rift, which initiated about 11 Ma, opens up at a current rate of 2 mm/year (Kampunzu et al. 1998;Saria et al. 2014) and is associated with active volcanism, seismic activity and fault structures Delvaux et al. 2017;Oth et al. 2017). Its lithology is diverse and of various ages, leading to a wide range of weathering processes (Kampunzu et al. 1998;Pouclet et al. 2016;Smets et al. 2016;Delvaux et al. 2017;Laghmouch et al. 2018). Numerous knickpoints attest the occurrence of recent relief erosion waves. ...
Understanding when landslides occur and how they evolve is fundamental to grasp the dynamics of the landscapes and anticipate the dangers they can offer up. However, knowledge on the timing of the landslides remains overlooked in large parts of the world. This is particularly the case in low-capacity regions, where infrastructures are weak or absent and data scarcity is the norm. The tropics stand out as such regions, despite being affected by high and increasing landslide impacts. There, persistent cloud cover, rapid natural vegetation regeneration, cultivation practices and high weathering rates further challenge the harvest of timing information. Based on a synthesis of our recent work, we present new findings on the characterisation of the timing of the landslides in the North Tanganyika-Kivu Rift region, a tropical environment with very low capacity and high population density. Our aim is also to highlight the methodological approaches and research strategies that we adopt to investigate such slope processes in a large region lacking baseline studies. From an inventory of more than 9000 landslides with various timing accuracy (from daily to thousands of years), we identify causes and triggers of the slope instabilities in a context of important human-induced landscape changes. This is achieved through a holistic approach that combines field work, satellite remote sensing, historical photograph processing and geomorphic marker understanding. The role of the needs of the local stakeholders in the setting up of the research strategy is also highlighted, and research perspectives are discussed.
... Rift basins commonly evolve on structurally heterogeneous crust that has undergone numerous deformation events prior to the final phase of extension (e.g., Fazlikhani et al., 2017;Morley, 2014;Mortimer et al., 2016). Numerous studies utilizing a wide variety of techniques have proposed that basement structures play a key role in shaping the geometry of rift basins by controlling basin location and the orientation of major riftrelated faults, for example, using gravity and magnetic data (Katumwehe et al., 2015), seismic and well data (Bird et al., 2015;Fazlikhani et al., 2017;Fossen et al., 2017;Morley, 2014;Mortimer et al., 2016;Paton & Underhill, 2004;Phillips et al., 2016;Qi et al., 2015;Ye et al., 2018), analogue and numerical modeling (Aanyu & Koehn, 2011;Autin et al., 2013;Chattopadhyay & Chakra, 2013;Deng et al., 2017Deng et al., , 2018Tong et al., 2014), and geochronological and topographic data (Rotevatn et al., 2018;Smets et al., 2016). However, the exact manner in which basement structures reactivate and the degree that basement structures exert their influence on the overlying rift basins remains poorly understood, although some studies have paid attention to the basement structures and their relationship with individual rift-related faults and subbasins (Bird et al., 2015;Fazlikhani et al., 2017;Morley, 2014;Phillips et al., 2016). ...
Crustal heterogeneity has a fundamental influence on rift development; however, the detailed architecture of structures within crystalline basement is commonly poorly understood, resulting in the exact manner in which basement structures control rift development being rather enigmatic. The Jianghan Basin evolved on a crystalline basement covered by thick prerift strata, providing an excellent opportunity to unravel the enigma. Using extensive 2‐D and 3‐D seismic, borehole and field data from the Jianghan Basin in combination, we investigate the deformation characteristics of basement structures and unravel their relationship with the overlying rift basin. A most striking feature of the basement structures in the Jianghan Basin is that prerift strata near the major rift‐related fault planes in the hanging walls are older than these in the footwalls and for most faults prerift strata in the hanging walls become increasingly older while approaching the fault planes, suggesting that these rift‐related faults have reactivated preexisting thrusts or thrusts associated with unroofed folds through negative structural inversion. All the major rift‐related faults have very high Dmax/L ratios (0.11–0.66) compared to typical normal faults, likely resulting from either constant length fault model or low frictional strength. By unraveling the prerift deformation intensity and structural division of basement structures in the Jianghan Basin, we propose that basement structures not only have crucial influence on rift geometry but also control rift localization. The reactivation of multidirectional basement structures and multidirectional extension with rift localization in the Jianghan Basin provides yet unrecognized insights into the development of tripartite rift systems.
... The second S model is the regional-scale model of [49] which was calibrated for a representative part of the WEAR and extrapolated within this study for the entire WEAR. This model includes all landslide types and is trained at a 0.0003° resolution using logistic regression with a 1:1 L/NL ratio based on a local inventory and 11 global/continental predictor variables [49]: slope (~30 m SRTM [57]), peak ground acceleration [58], distance to active faults and inactive faults [52,60], lithology [59], land cover [61], distance to drainage network (~30 m SRTM [57]), planar curvature (~30 m SRTM [57]), profile curvature (~30 m SRTM [57]), aspect (~30 m SRTM [57]), and two-day 15 mm rainfall accumulation threshold exceedance [62]. Note that the rainfall predictor was of minor importance in the model and had no significant impact on the susceptibility pattern in the study area [49]. ...
Determining rainfall thresholds for landsliding is crucial in landslide hazard evaluation and early warning system development, yet challenging in data-scarce regions. Using freely available satellite rainfall data in a reproducible automated procedure, the bootstrap-based frequentist threshold approach, coupling antecedent rainfall (AR) and landslide susceptibility data as proposed by Monsieurs et al., has proved to provide a physically meaningful regional AR threshold equation in the western branch of the East African Rift. However, previous studies could only rely on global-and continental-scale rainfall and susceptibility data. Here, we use newly available regional-scale susceptibility data to test the robustness of the method to different data configurations. This leads us to improve the threshold method through using stratified data selection to better exploit the data distribution over the whole range of susceptibility. In addition, we discuss the effect of outliers in small data sets on the estimation of parameter uncertainties and the interest of not using the bootstrap technique in such cases. Thus improved, the method effectiveness shows strongly reduced sensitivity to the used susceptibility data and is satisfyingly validated by new landslide occurrences in the East African Rift, therefore successfully passing first transferability tests.
... Its progression during the next hours follows a direction SW-NE. This orientation corresponds to the direction of the principal rift faults of the Precambrian basement in the VVP indicating some preferential structure weaknesses for magma intrusion at depth ( Smets et al., 2016). More detailed investigations about this swarm and other potential intrusions associated with lava lake drops as well as a dedicated analysis of the source mechanism of the repetitive (master) events will be the subject of future studies. ...
(free access to the article before November 09, 2019 : https://authors.elsevier.com/a/1ZmFe,Ig4H0Uo)
Active lava lakes at volcanoes can be regarded as open windows to their magmatic systems. The dynamics of such lakes may therefore provide decisive insights into deeper magmatic processes, potentially leading to fundamental theoretical implications and volcano monitoring improvements. Among the rare volcanoes on Earth hosting a persistent lava lake, Nyiragongo in D.R. Congo directly threatens a massive population of roughly 1 million inhabitants. Here we analyze close-range (i.e., summit) and distant (around 17 km) seismic measurements acquired at this African volcano between 2011 and 2018 in order to better understand the seismic signature of the lava lake activity and how it relates to the deeper volcanic processes. Both summit and distant seismic records contain a similar continuous tremor pattern attributable to the lava lake activity. Combining this information with time-lapse camera images and lava lake level measurements confirms the mechanism of gas pistoning at Nyiragongo, which is characterized by short-duration (a few minutes long) and meter-scale level variations during the period of observation. We also characterize the dominant periodicity of this shallow tremor signature of about a few tens of minutes. Because this marked periodic pattern varies during a significant one-month fluctuation of SO2 emissions (estimated from space), we suggest that this particular seismic periodicity corresponds to the convective lake movement driven by the persistent degassing typical of active open-vent volcanoes. Finally, new seismic evidence reveals the effect of deep magmatic intrusion and consequent major pressure changes in the plumbing system, resulting in sudden and large drops of the lava lake level associated with strong degassing. Such transient episodes have similar characteristics to total lava lake drainage associated with flank eruptions already observed at this volcano in 1977 and 2002, or at Kīlauea volcano in 2018.
... However, subsidence due to this process is a transient phenomenon that would not persist for more than seven years, as we observe here, and potential post-seismic slip would follow an exponential decay. There is no significant fault-related seismic activity (Hamaguchi et al., 1983;d'Oreye et al., 2011;Wauthier et al., 2015a), nor traces of normal faulting at ground surface ( Fig. 1, (Smets et al., 2016)), in the Bulengo area. However, two gravity surveys performed in 1959 (Evrard and Jones, 1963) and 1982 (Hamaguchi et al., 1983) suggest that a normal fault is possible in this area, west of the subsiding area in Bulengo (Fig. 4). ...
We processed InSAR time series of ENVISAT ASAR descending and ascending datasets and identified two new deforming areas on the northern shore of Lake Kivu, characterized by steady ground subsidence of up to ~1 cm/year over a time period of about seven years (December 2002/January 2003–March 2010). Two distinct areas can be identified: one centered on the Rumoka volcanic cone, which built up during the 1912 eruption of Nyamulagira volcano, and a broader one centered on the Bulengo area on the northern shoreline of Lake Kivu. Both areas include high density of diffuse magmatic degassing areas (“mazuku”), which are topographic depressions in which substantial amount of carbon dioxide accumulates. The steady deflation of at least two fluid reservoirs is consistent with geodetic data but the presence of weak, porous layers, possibly connected with hydrothermal and hydrogeological processes in aquifers and Lake Kivu, could also account for the observed subsidence. The subsidence could be due to pore pressure decrease in porous layers, which may also serve as preferential pathways for escaping gases percolating from the aquifers and Lake Kivu.
... Another natural potential factor that could affect the displacement rates of active landslides is seismicity (e.g., [66][67][68][69][70]). The city of Bukavu is commonly affected by earthquakes [41,42,71,72] and the Mw 5.8 August 2015 earthquake with the epicentre located~40 km north of the centre of Bukavu ([52]- Figure 1) could have potentially affected the slope stability over the city [70,[73][74][75]. The displacement time series in Figure 7, however, show no change (anticipated it could be acceleration) in the deformation rates after this event. ...
Landslides can lead to high impacts in less developed countries, particularly in tropical environments where a combination of intense rainfall, active tectonics, steep topography, and high population density can be found. However, the processes controlling landslide initiation and their evolution through time remains poorly understood. Here we show the relevance of the use of the multi-temporal differential radar interferometric (DInSAR) technique to characterise ground deformations associated with landslides in the rapidly-expanding city of Bukavu (DR Congo). We use 70 COSMO-SkyMed synthetic aperture radar images acquired between March 2015 and April 2016 with a mean revisiting time of eight days to produce ground deformation rate maps and displacement time series using the small baseline subset approach. We find that various landslide processes of different ages, mechanisms, and states of activity can be identified. Ground deformations revealed by DInSAR are found consistent with field observations and differential GPS measurements. Our analysis highlights the ability of DInSAR to grasp landslide deformation patterns affecting the complex tropical-urban environment of the city of Bukavu. However, longer time series will be needed to infer landside responses to climate, seismic, and anthropogenic drivers.
... Les failles décrites à Muranga et Bweremana sur la route Sake-Minova donnent des résultats compatibles avec la tectonique en extension qui caractérise le rift du Kivu. Il s'agirait de segments de faille secondaires associés à une grande zone de faille bordière mal définie de direction NNE-SSW (PASTEELS, 1961;WOOD et al., 2015) qui passerait dans le grand escarpement, mais dont le tracé le long des rivages de la baie de Kabuno est peu clair car mal exprimé dans le paysage (BUSCHTEIN et al., 1967, SMETS et al, 2016. Comme ces failles semblent réactiver la stratification d'une alternance de quartzites, carbonates et schistes, il est possible que le mouvement total soit distribué en un nombre de failles de moindre importantes, distribuées selon une large zone de faille qui définirait l'escarpement morphologique. ...
The sismotectonic investigation of the western side of the Kivu rift allowed to identify and characterise some sismogenic faults on the basis of tectonic parameters such as the tensor of the orthogonal principal stress (?1, ?2 and ?3), the tectonic regime index (R and R') and the azimuths of the maximal (SHmax) and minimal (Shmin) horizontal principal stresses. Those parameters were obtained with the help of the Win-Tensor software using orientation data of striated fault planes and various fractures measured during field work and also of earthquake focal mechanisms. Using this approach, the morphotectonic markers observed in the field have been associated with two distinct large sismotectonic zones of the western branch of the East-African rift system. The first one comprises the northern part of the basin of Lake Kivu in which we have observed faults with general NNE-SSW orientation. The second zone comprises the southern part of Lake Kivu, including the depression of the Rusizi River until the northern tip of the Lake Tanganyika basin. The faults in this zone are preferably N-S oriented.
... Elle a une superficie de 44 km² et un relief assez marqué (plus de 600 m de dénivelé). La ville est localisée dans la branche ouest du Rift est-africain, une région sismiquement active, traversée par de nombreuses failles (d'OREYE et al., 2011;SMETS et al., 2016;DELVAUX et al., 2017) et reconnue pour être favorable aux glissements de terrain (par. ex. ...
Ground deformations in urban areas can lead to environmental constraints that must be considered in risk management and urban planning. These deformations can damage the infrastructures exposed to them. Here, the objective is to assess impacts of ground deformations on water and electricity distribution networks in the city of Bukavu (DR Congo); this city being known to be affected by deformations linked in particular to the occurrence of landslides. First, an inventory of active deformation zones was carried out. Then, the water distribution (REGIDESO) and electricity (SNEL) infrastructures were identified in the field in order to establish updated network maps. Finally, on the basis of these maps, analyses using field knowledge were made to deduce relationships between ground deformation zones, distribution networks and their physical vulnerability. The study shows that the three communes of Bukavu are affected by damage caused by these deformations. The most important vulnerability problems are logically concentrated in the deformation zones which are more inhabited and are linked to the temporal evolution of the city.
... Proximal basins are mostly oriented perpendicular to the main extension (e.g., Jeanne d'Arc and Lusitanian Basins). However, crustal inheritance can influence the orientation and location of these basins (e.g., Kivu Basin in the East African Rift (Smets et al., 2016)). ...
We focus on the southern North Atlantic rifted margins to investigate the partitioning and propagation of deformation in hyper-extended rift systems using plate kinematic modelling. The kinematic evolution of this area is well determined by oceanic magnetic anomalies after the Cretaceous normal polarity superchron. However, the rift and early seafloor spreading evolution (200-83 Ma) remain highly disputed due to contentious interpretations of the J magnetic anomaly on the Iberia-Newfoundland conjugate margins. Recent studies highlight that the J-anomaly is probably polygenic, related to polyphased magmatic events and therefore does not correspond to an isochron. We present a new palinspastic restoration without using the J magnetic anomaly as the chron M0. We combine 3D gravity inversion results with local structural, stratigraphic and geochronological constraints on the rift deformation history. The restoration of the southern North Atlantic itself is not the primary aim of the study but rather is used as a method to investigate the spatiotemporal evolution of hyper-extended rift systems. We include continental micro-blocks that enable the partitioning of the deformation between different rift segments, which is of particular importance for the evolution of the Iberia-Eurasia plate boundary. Our modelling highlights: 1) the segmentation of the Iberia-Newfoundland rift system during continental crust thinning, 2) the northward V-shape propagation of mantle exhumation and seafloor spreading, 3) the complex partitioning of deformation along the Iberia-Eurasia plate boundary, and 4) a three plates propagation model which implies transtension.
... However crustal inheritances can influence the orientation and location of these basins (e.g. Kivu basin in the East African rift (Smets et al., 2016)). With the resolution of our model, it is too ambitious to discuss the influence of inheritance for the stretching phase. ...
Deformation modes of magma-poor hyper-extended rift systems evolve through time and space. Hence the observed structures and architectures vary along a depth section as well as on a map. This study aims to characterize the deformation modes of hyper-extended systems and their propagation using the examples of the southern North Atlantic. The architecture of the continental crust termination has been compared to the critical Coulomb wedge theory because it has a wedge shape, the final deformation is brittle/frictional and this wedge is gliding over a basal detachment. This theory highlights the distinct behavior of the two conjugate margins. Moreover it constrains crustal architecture of the continental crust termination, integrates continentward dipping faults and explains the formation of extensional allochthons in a sequential faulting model. The integration of deformation modes in an evolving 3D model necessitates a reliable kinematic context, which is not the case for the opening of the southern North Atlantic Ocean. This is linked to the interpretation of the J-magnetic anomaly as an oceanic isochron. Re-investigations of this anomaly revealed its polygenic and polyphased formation, which is inconsistent for an oceanic isochrons or a domain boundary making it unusable for plate reconstruction. The evolution of rift deformation has been analyzed with a new plate reconstruction of the southern North Atlantic. It appears that the continental crust deformation is segmented whereas oceanic crust propagates in a V-shape. The approach developed in this thesis also asks new geodynamical questions on the influence of inheritance and the effect of triple junction.
... Interestingly, most of these products erupted from NE e SW oriented cone and fissure systems, which are inferred to be associated with deep Precambrian basement structures reactivated by rifting processes (Smets et al., 2016). Hence, these structures potentially represent direct pathways for deep primitive magma to reach the surface. ...
We present an integrated, spatially-explicit database of existing geochemical major-element analyses available from (post-) colonial scientific reports, PhD Theses and international publications for the Virunga Volcanic Province, located in the western branch of the East African Rift System. This volcanic province is characterised by alkaline volcanism, including silica-undersaturated, alkaline and potassic lavas. The database contains a total of 908 geochemical analyses of eruptive rocks for the entire volcanic province with a localisation for most samples. A preliminary analysis of the overall consistency of the database, using statistical techniques on sets of geochemical analyses with contrasted analytical methods or dates, demonstrates that the database is consistent. We applied a principal component analysis and cluster analysis on whole-rock major element compositions included in the database to study the spatial variation of the chemical composition of eruptive products in the Virunga Volcanic Province. These statistical analyses identify spatially distributed clusters of eruptive products. The known geochemical contrasts are highlighted by the spatial analysis, such as the unique geochemical signature of Nyiragongo lavas compared to other Virunga lavas, the geochemical heterogeneity of the Bulengo area, and the trachyte flows of Karisimbi volcano. Most importantly, we identified separate clusters of eruptive products which originate from primitive magmatic sources. These lavas of primitive composition are preferentially located along NE-SW inherited rift structures, often at distance from the central Virunga volcanoes. Our results illustrate the relevance of a spatial analysis on integrated geochemical data for a volcanic province, as a complement to classical petrological investigations. This approach indeed helps to characterise geochemical variations within a complex of magmatic systems and to identify specific petrologic and geochemical investigations that should be tackled within a study area.
... Although the Virunga Volcanic Province (VVP) is mostly known for its effusive volcanism, the presence of phreatomagmatic centers with a young morphology along Lake Kivu's densely populated northern shoreline signals the need to consider the future hazard of explosive magmawater interactions. The VVP lies at a transfer zone that separates two rift segments (i.e., Kivu and Albert rifts ;Ebinger 1989;Smets et al. 2016) within the western branch of the East African Rift (Fig. 1a, b). The VVP hosts eight large volcanic edifices, of which Nyiragongo and Nyamulagira, in the western, intra-rift part of the VVP, have frequently erupted over the past 150 years. ...
The Virunga Volcanic Province (VVP) represents the most active zone of volcanism in the western branch of the East African Rift System. While the VVP’s two historically active volcanoes, Nyamulagira and Nyiragongo, have built scoria cones and lava flows in the adjacent lava fields, several small phreatomagmatic eruptive centers lie along Lake Kivu’s northern shoreline, highlighting the potential for explosive magma-water interaction. Their presence in the densely urbanized Sake-Goma-Gisenyi area necessitates an assessment of their eruptive mechanisms and chronology. Some of these eruptive centers possess multiple vents, and depositional contacts suggest distinct eruptive phases within a single structure. Depositional facies range from polymict tuff breccia to tuff and loose lapilli, often impacted by blocks and volcanic bombs. Along with the presence of dilute pyroclastic density current (PDC) deposits, indicators of magma-water interaction include the presence of fine palagonitized ash, ash aggregates, cross-bedding, and ballistic impact sags. We estimate that at least 15 phreatomagmatic eruptions occurred in the Holocene, during which Lake Kivu rose to its current water level. Radiocarbon dates of five paleosols in the top of volcanic tuff deposits range between ∼2500 and ∼150 cal. year BP and suggest centennial- to millennial-scale recurrence of phreatomagmatic activity. A vast part of the currently urbanized zone on the northern shoreline of Lake Kivu was most likely impacted by products from phreatomagmatic activity, including PDC events, during the Late Holocene, highlighting the need to consider explosive magma-water interaction as a potential scenario in future risk assessments.
In rifted margins or rifted basins, the structure of the basin is often complex due to the multistage development of fault systems. As we can only see the result of the basin evolution, it is difficult to judge the influence of early fault activity on the later stages. Here, we present a case study from the southern part of Qiongdongnan Basin (QDNB) where the influence of the early stage can be clearly recognised. Using newly acquired high‐resolution 3D seismic datasets, we analyse the prototype and temporal evolution of the study area since the Cenozoic. Three fault systems (FS1, FS2 and FS3) were identified according to fault activities and fault strike. Observation results show a rotation of the stress field at the end of the Eocene, dividing the whole rifting into two stages. FS1 initiated in NW‐SE extension at rift stage I (42.5–33.9 Ma), showing a typical basement‐involved structure while FS2 and FS3 developed in N‐S extension at rift stage II (33.9–23.03 Ma). The faults in FS1 were either crosscut to form a zigzag plane geometry by FS2 and FS3 faults, or became long‐lived active faults throughout the rifting period, resulting in a localisation of strain and extremely thinning of the crust. It can also be compared with adjacent basins which have undergone the same regional tectonic evolution history at the northern margin of the South China Sea. The long‐lived NE–SW trending faults cause significant variation in the subsidence history, basin structure, and crust thickness along strike in the study area, indicating that faults developed in rift stage I play a significant role in basin evolution at rift stage II.
The objective of this work is to evaluate the geotechnical characteristics of pozzolanic materials from Kanyaruchinya in the Democratic Republic of Congo, in order to define the practical modalities of their use in road construction. After a detailed macroscopic description of the materials in the field, the taken samples were subject to a geotechnical identification. The geotechnical work focused on the realization of identification and compaction tests by reference to French standards. The obtained results show that the studied materials are porous and loose. The Kanyaruchinya pozzolanas are of vacuolar and scoriaceous structure, and present a variability of color going from brick red to dark brown, yellow and black due to the iron oxidation states. These materials consist of high gravel content (47–76%), moderate sand content (21–44%) and low to moderate pebble content (2–30%). With a maximum grain diameter (25–40 mm) smaller than 60 mm, these materials can be used in road construction without risk of segregation. The studied pozzolanas are less fine (3 % < C80µm < 4 %) and non-plastic. The CBR (California Bearing Ratio) of the studied pozzolana has an average value of 31%. These materials can be used as sub-base layer for low volume traffic T1 despite their maximum dry density (MDD) values lower than 1.80. This use is confirmed by their maximum grain diameter of less than 60 mm and grading modulus values between 1.65 and 2.70. The optimization of their use in sub-base layer for all volume traffics and in base course requires an improvement by adding a fine-grained, high-density material such as clay sand, sandy clay, clay lateritic. This operation could raise the MDD values. However, a compaction energy higher than that obtained for traditional materials is necessary for the implementation of these materials in their natural state.
The longevity of lava lakes in open-vent volcanoes reflects a hydraulic connection between the lake and the deep
part of the magma plumbing system. Constraining the size of the shallow magmatic system and resolving the
rheology of magma filling in the system is essential to evaluate potential hazards like lava flow and other activities.
As the lava lake is often perturbed by degassing bursts, rockfall, and even convection, seismic waves
radiated from the oscillation of fluid and its mechanical coupling with the surrounding solid walls provide
invaluable information on probing system geometry and magma rheology. In this report, I show the first
observation of very long-period signals in Nyiragongo volcano, to uncover the sloshing of the world’s largest
known lava lake and its dynamic interaction with a deep reservoir during the relatively quiet period. The signal is
manifested as the ground oscillations with two isolated spectral peaks at ~15 s and ~16 s sustaining up to half an
hour and a spectral peak at a longer period of ~76 s. The radiated seismic energy can be well recognized by the
stations with distances of <50 km to the lava lake. The traveling time, particle-motion polarization, and
deformation inversion suggest that the 15 s’ and 16 s’ modes are related to two orthogonal horizontal forces at a
very shallow depth, likely pointing to the sloshing dynamics of the lava lake. The 76 s’ mode is considered as the
dynamic coupling between the lake bottom to a deep reservoir at a depth of 8–16 km through a conduit driven by
the sloshing. The dynamic modeling of the 76 s’ mode points to a deep reservoir storativity of ~8 m3/Pa and a
spherical reservoir with a radius of ~7.5 km. High-frequency seismic waves before the onset of the 15 s’ and 16 s’
modes suggest that the signals may be excited by rigorous degassing or rockfall. Variations in the period and
quality factor of the modes reflect the changes in the lake/reservoir geometry and magma rheology. This finding
may improve our ability to understand the magmatic plumbing system, track magma evolution in Nyiragongo,
and further probe the formation of lava lakes in active volcanoes.
Le bassin versant topographiue du lac Kivu se tend sur deux pays, la RDC et le Rwanda. Il a à son centr la plus grande ile lacustre du Continent (Idjwi).
dans cette Présentation, nous avons porté une intervation sur la pétrographie du Flanc Ouest sur deux territoires, Masisi (Nord Kivu) et Kalehe (Sid Kivu).
Les intrusions basiues logent les bords du lac, celles acides au sommet de Mutumb. le terrain est fragmenté ci et là en micro grabens par la tectonogénèse
L’Afrique évolue tectoniquement avec finalité la fragmentation en microplaques. La trace est l’activité du rift en graben servant du futur plancher océanique ; les rides océaniques étant les points chauds, cas de Virunga. Le SREA a deux branches (Rift valley, albertin) toutes actives à volcans. La branche Ouest, dont fait le Rift Kivucien est une dépression NE-SW à NW-SE. Le lac Kivu à l’aplomb de l’axe sert de point de virgation. Dans cette partie rift, une série des failles E-W jouant périodiquement comme failles transformantes avec agitation des compartiments, glissement du profil pédogènique.
La faille E-W dans la vallée de Renga, au Nord-Kivu, serait borgne sous les alluvions. L’écartement cisaillant est périodiquement la cause de rupture de l’axe routier. Selon l’histoire et dire des riverains, au Sud de Minova, à la confluence Lulunga et Mubimbi la haute tension connaissait une casse des files, ce qui traduit un cisaillement des lèvres jointives. Notre lever y dénichant une brèche tectonique longue sur des Kilomètre.
Une faille au Sud-Kivu, est un exemple illustratif. Bordée de la cité stannifère de Nyabibwe, la rupture routiere perpétuelle et basculement des ponts au NE de la cité, la fissuration des murs de l’école primaire Nyabibwe centre cité, l’affaissement de la zone Sud de la cité est une attestation de la faille. Les arbres formant un arc dont la concavité tourne vers le sommet est aussi une matérialisation du jet des masses. Il s’agit d’une faille courbe dont l’activité évolue vers le Nord avec trop de glissements et éboulements.
In the context of rift basin formation, structural inheritance describes the influence of pre-existing basement structures on new, rift-related structures, including faults. Examples of basin features influenced by inheritance include rift localisation and segmentation at the plate scale, as well as variations in the geometries, orientations, and kinematics of individual rift-related faults. Given that continental rifts commonly form in pre-deformed crust, structural inheritance is likely to be the norm, not the exception. As such, structural inheritance has implications for reconstructing the paleotectonic history of rifts, investigating seismic hazards, and understanding the fluid transport and storage capabilities of natural fracture systems in the context of geo-energy and ore deposits.Our review of the literature shows that inheritance is driven by several mechanisms, which include frictional reactivation and local re-orientation of the far-field strain and/or stress. Here we highlight how insights from field observations, geophysics, and analogue and numerical models can be used to classify these mechanisms in terms of hard-linked and soft-linked inheritance. We demonstrate how different inheritance mechanisms can produce different geometric and kinematic relationships between pre-existing basement structures and rift-related faults, and that these mechanisms can be active at different depths within the same rift. Our aim is to provide a framework for recognising various expressions of structural inheritance and their underlying mechanism(s) in natural rifts, so that we can better interpret basement structures under cover and are equipped with additional constraints for understanding the multi-stage evolution of basement-influenced rift basins worldwide.
We investigate the spatiotemporal patterns of strain accommodation during multiphase rift evolution in the Shire Rift Zone (SRZ), East Africa. The NW-trending SRZ records a transition from magma-rich rifting phases (Permian-Early Jurassic: Rift-Phase 1 (RP1), and Late Jurassic-Cretaceous: Rift-Phase 2 (RP2)) to a magma-poor phase in the Cenozoic (ongoing: Rift-Phase 3 (RP3)). Our observations show that although the rift border faults largely mimic the pre-rift basement metamorphic fabrics, the rift termination zones occur near crustal-scale rift-orthogonal basement shear zones (Sanangoe (SSZ) and the Lurio shear zones) during RP1-RP2. In RP3, the RP1-RP2 sub-basins were largely abandoned, and the rift axes migrated northeastward (rift-orthogonally) into the RP1-RP2 basin margin, and northwestward (strike-parallel) ahead of the RP2 rift-tip. The northwestern RP3 rift-axis side-steps across the SSZ with a rotation of border faults across the shear zone, and terminates farther northwest at another regional-scale shear zone. We suggest that over the multiple pulses of tectonic extension and strain migration in the SRZ, pre-rift basement fabrics acted as: 1) favorably-oriented zones of mechanical strength contrast that localized the large rift faults, and 2) mechanical ‘barriers’ that refracted and possibly, temporarily halted the lateral propagation of the rift zone. Further, the cooled RP1-RP2 mafic dikes localized later-phase deformation in the form of border fault hard-linking transverse faults that exploited strength contrasts within the dike clusters and served as mechanically-strong zones that arrested some of the RP3 fault-tips. Overall, we argue that during pulsed rift propagation, inherited crustal strength anisotropies may serve as both strain-localizing, refracting, and ‘strain barrier’ tectonic structures.
Tectonic and paleo‐environmental reconstructions of rift evolution typically rely on the interpretation of sedimentary sequences, but this is rarely possible in early‐stage rifts where sediment volumes are low. To overcome this challenge, we use geomorphology to investigate landscape evolution and the role of different forcing mechanisms during basin development. Here, we focus on the humid Middle Shire River basin, located within the zone of progressive interaction and linkage between the southern Malawi Rift and Shire Rift Zone, East Africa. We used a digital elevation model to map knickpoints and knickpoint morphologies in the Middle Shire River basin and examined the relationships with pre‐rift and syn‐rift structures within the rift interaction zone. The main axial stream, Shire River, descends steeply, 372 m over a 50 km distance, across exposed metamorphic basement along the rift floor, exhibiting a strongly disequilibrated longitudinal elevation profile with both ‘mobile’ and ‘fixed’ knickpoints. In particular, we identify two clusters of mobile knickpoints, which we interpret as associated with baselevel fall events at the downstream end of the exposed basement that triggered knickpoint migration through the fluvial network since at least the Mid. Pleistocene. We infer that after the integration of the axial stream across the Middle Shire Basin, the knickpoints migrate upstream in response to fault‐related subsidence in the Shire Rift Zone. Conversely, the fixed knickpoints are interpreted to reflect local differential bedrock erodibility at lithologic contacts or basement‐hosted fault scarps along the basin floor. The results suggest that Middle Shire basin opening, associated with rift linkage, is likely a recent event (at least Mid. Pleistocene) relative to the Late Oligocene activation of Cenozoic rifting in the East African Rift’s Western Branch. These findings support the hypothesis that the Western Branch developed from the gradual propagation, linkage, and coalescence of initially nucleated distinct rift basins.
We investigate the spatiotemporal patterns of strain accommodation during multiphase rift evolution in the Shire Rift Zone (SRZ), East Africa. The NW-trending SRZ records a transition from magma-rich rifting phases (Permian-Early Jurassic: Rift-Phase 1 (RP1), and Late Jurassic-Cretaceous: Rift-Phase 2 (RP2)) to a magma-poor phase in the Cenozoic (ongoing: Rift-Phase 3 (RP3)). Our observations show that although the rift border faults largely mimic the pre-rift basement metamorphic fabrics, the rift termination zones occur near crustal-scale rift-orthogonal basement shear zones (Sanangoe (SSZ) and the Lurio shear zones) during RP1-RP2. In RP3, the RP1-RP2 sub-basins were largely abandoned, and the rift axes migrated northeastward (rift-orthogonally) into the RP1-RP2 basin margin, and northwestward (strike-parallel) ahead of the RP2 rift-tip. The northwestern RP3 rift-axis side-steps across the SSZ, with a rotation of border faults across the shear zone and terminates further northwest at another regional-scale shear zone. We suggest that over the multiple pulses of tectonic extension and strain migration in the SRZ, pre-rift basement fabrics acted as: 1) zones of mechanical strength contrast that localized the large rift faults, and 2) mechanical ‘barriers’ that refracted and possibly, temporarily halted the propagation of the rift zone. Further, the cooled RP1-RP2 mafic dikes facilitated later-phase deformation in the form of border fault hard-linking transverse faults that exploited mechanical anisotropies within the dike clusters and served as mechanically-strong zones that arrested some of the RP3 fault-tips. Overall, we argue that during pulsed rift propagation, inherited strength anisotropies can serve as both strain-localizing, refracting, and transient strain-inhibiting tectonic structures.
The Rukwa Rift Basin, Tanzania is regarded as a modern example of cratonic rift zone despite complex polyphase extensional and episodic inversion structures. We interpret 2D seismic reflection data tied to wells to identify and describe structures controlling stratigraphic sequences (Late Carboniferous to Pleistocene) in two main segmented Rukwa Rift domains, A and B, which are controlled by the Chisi and Saza shear zones. Fault geometry and stratal patterns are illustrated in relation to their kinematic interaction with folds. Fold structures reflect both extensional and compressional deformation and were mapped with a particular interest for their helium potential. We illustrate fault bend folds, fault propagation folds and fault propagation monoclines that are related to extension events. Folds related to compression exhibit various structural styles reflecting at least two phases of episodic and widespread inversion. Firstly, Early Jurassic inversion phase which involved multi-faulted anticlines in the Karoo strata. Secondly, a mild and widespread inversion structures during the Pleistocene which are characterized by both symmetrical and asymmetrical anticlines styles. Taken together, the extensional and compressional fold structures, stratal juxtapositions and unconformities define stratigraphic packages that are widely distributed in the Rukwa Rift Basin, and form potential subsurface traps for helium-nitrogen rich gases, from which some seep to the surface, evidently documented in thermal springs across the region.
Оn the basis of the analysis of the results of modern domestic, foreign and author's research, the problems of tectonic inheritance in the course of supercontinental cyclicity are considered, examples of it in the form of repeated manifestations of Wilson cycles within a specific zones, paleomagnetic data on the similarity of reconstructions of supercontinents of different ages are given. The deep, predetermining causes of this inheritance, consisting in the existence of weakened zones of the lithospheric mantle, controlling the processes of formation and decay of supercontinents, are shown.
Historical aerial photographs provide salient information on the historical state of the landscape. The exploitation of these archives is often limited by accessibility and the time-consuming process of digitizing the analogue copies at a high resolution and processing them with a proper photogrammetric workflow. Furthermore, these data are characterised by limited spectral information since it occurs very often in a single band. Our work presents a first application of deep learning for the extraction of land cover from historical aerial panchromatic photographs of the African cities of Goma, Bukavu and Bujumbura. We evaluate the suitability of deep learning for land cover generation from a challenging dataset of photographs from the 1940s and 1950s that covers large geographical extents and is characterised by radiometric variations between dates and locations. A fully convolutional approach is investigated by considering two network architectures with different strategies of exploiting contextual information: one used atrous convolutional layers without downsampling, whereas the second network has both downsampling and learned upsampling convolutional layers (U-NET). The networks are trained to detect three main classes namely, buildings, high vegetation and a mixed class of bare land and low vegetation class. High overall accuracies of >90% in Goma-Gisenyi and Bukavu, and >85% in Bujumbura are obtained. This work provides a novel methodology that outperforms a baseline standard machine learning classifier for the exploitation of the vast archives of historical aerial photographs that can aid long-term environmental baseline studies. Future work will entail developing domain adaptation strategies in order to make the trained network robust for different image mosaics.
Warm and cold subaquatic groundwater discharge into Lake Kivu forms the large-scale density gradients presently observed in the lake. This structure is pertinent to maintaining the stratification that locks the high volume of gases in the deepwater. Our research presents the first characterisation of these inflows. Temperature and conductivity profiling was conducted from January 2010 to March 2013 to map the locations of groundwater discharge. Water samples were obtained within the lake at the locations of the greatest temperature anomalies observed from the background lake-profile. The isotopic and chemical signatures of the groundwater were applied to assess how these inflows contribute to the overall stratification. It is inferred that Lake Kivu's deepwater has not been completely recharged by the groundwater inflows since its turnover that is speculated to have occurred within the last ~1000 yrs. Given a recent salinity increase in the lake constrained to within months of seismic activity measured beneath the basin, it is plausible that increased hydrothermal-groundwater inflows into the deep basin are correlated with episodic geologic events. These results invalidate the simple two-component end-member mixing regime that has been postulated up to now, and indicate the importance of monitoring this potentially explosive lake.
The Lake Edward basin lies within the Albertine Rift Valley of Uganda and the Democratic Republic of Congo which forms the northern end of the western arm of the East African Rift System. It is a frontier petroleum prospective area, which, at the outset of this study, had no exploration wells drilled within it or any deep reflection seismic surveys. There have been some previous studies in the basin, but none produced a geological map subdividing the onshore rift-fill sediments or established a workable stratigraphic framework for them. Between 2007 and 2010, Dominion Uganda Ltd., in collaboration with Trinity College Dublin and the Petroleum Exploration and Production Department of the Ministry of Energy, Uganda, undertook a geological mapping survey of the south-eastern onshore part of the basin, known as petroleum ‘Exploration Area 4B’ (EA4B). Five rift sediment formations were identified and mapped across the area to produce a new geological map of EA4B. Palynological analyses suggest that all exposed rift sediments are (Late to Mid) Pleistocene-Holocene. EA4B is dominated by a north-east to south-west trending fault zone which underwent significant extension within the last 130,000 years to produce a trough, or sub-basin, to the south-east against the rift margin. This trough subsequently filled, initially with ponded swamp clays, followed by coarse fluvial and alluvial clastics. There is field evidence for minor inversion and ‘pop-up’ structures along some footwall crests, suggesting that the neotectonic phase is compressional or transpressional, and this has caused stream rejuvenation and incision.
The East African Rift (EAR) is a type-locale for investigating the processes that drive continental rifting and breakup. The current kinematics of this ~5000-km long divergent plate boundary between the Nubia and Somalia plates is starting to be unraveled thanks to a recent augmentation of space geodetic data in Africa. Here, we use a new data set combining episodic GPS measurements with continuous measurements on the Nubian, Somalian, and Antarctic plates, together with earthquake slip vector directions and geologic indicators along the Southwest Indian Ridge to update the present-day kinematics of the EAR. We use geological and seismological data to determine the main rift faults and solve for rigid block rotations while accounting for elastic strain accumulation on locked active faults. We find that the data is best fit with a model that includes three microplates embedded within the EAR, between Nubia and Somalia (Victoria, Rovuma, and Lwandle), consistent with previous findings but with slower extension rates. We find that earthquake slip vectors provide information that is consistent with the GPS velocities and helps to significantly reduce uncertainties of plate angular velocity estimates. We also find that 3.16 My MORVEL average spreading rates along the Southwest Indian Ridge are systematically faster than prediction from GPS data alone. This likely indicates that outward displacement along the SWIR is larger than the default value used in the MORVEL plate motion model.
In this study, we explore the origin of lower crustal seismicity and the factors controlling rift propagation using seismological data recorded within the youngest part of the East African Rift System, the North Tanzanian Divergence (NTD). Most earthquakes below Lake Manyara occur at depth ranging between 20 and 40 km and have a swarm-like distribution. Focal mechanisms of 26 events indicate a combination of strike-slip and normal faulting involving Archaean basement structures and forming a relay zone. The derived local stress regime is transtensive and the minimum principal stress is oriented N110°E. Crustal seismic tomography reveals low velocity anomalies below the rifted basins in the NTD, interpreted as localized thermo-mechanical perturbations promoting fluid release and subsequent seismicity in the lower crust. SKS splitting analysis in the NTD indicates seismic anisotropy beneath 17 stations most likely due to aligned magma lenses and/or dikes beneath the rift and to the lithospheric fabrics. Our results favor a strain pattern intermediate between purely mechanical and purely magmatic. We suggest that melt products arising from a large asthenospheric thermal anomaly enhance lithospheric weakening and facilitate faulting and creeping on critically oriented inherited structures of the Precambrian lower crust. Although the crust is unlikely weakened at a point comparable to other parts of the East African Rift System, this deep-seated thermo-mechanical process is efficient enough to allow slow rift propagation within the eastern Tanzanian cratonic edge.
Geophysical, geochemical and biological data are integrated to unravel the origin and evolution of an unusual rift lake. The northern basin of Lake Kivu contains about 0.5 km of sediments which overlie a basement believed to be crystalline rocks of Precambrian age. Volcanic rocks at the northern end of the lake have created large magnetic anomalies of up to 300γ. Heat flow varies from 0.4 to 4 hfu. The extreme variability may be due in part to sedimentation or recent changes in the temperature of the bottom water. Sharp boundaries in the vertical temperature and salinity structure of the water across the lake can best be explained as separate convecting layers. Such convecting cells are the result of the increase in both temperature and salinity with depth.
Although the East African Rift (EAR) System is often cited as the archetype for models of continental rifting and break-up, its present-day kinematics remains poorly constrained. We show that the currently available GPS and earthquake slip vector data are consistent with (1) a present-day Nubia-Somalia Euler pole located between the southern tip of Africa and the Southwest Indian ridge and (2) the existence of a distinct microplate (Victoria) between the Eastern and Western rifts, rotating counter-clockwise with respect to Nubia. Geodetic and geological data also suggest the existence of a (Rovuma) microplate between the Malawi rift and the Davie ridge, possibly rotating clockwise with respect to Nubia. The data indicate that the EAR comprises at least two rigid lithospheric blocks bounded by narrow belts of seismicity (< 50 km wide) marking localized deformation rather than a wide zone of quasi-continuous, pervasive deformation. On the basis of this new kinematic model and mantle flow directions interpreted from seismic anisotropy measurements, we propose that regional asthenospherie upwelling and locally focused mantle flow may influence continental deformation in East Africa.
The Western rift, the western branch of the East African rift system, is bordered by high-angle normal fault systems bounding one side of spoon-shaped basins. Depth-to-detachment estimates of 20-30 km, the rollover geometry of asymmetric basins, and seismicity throughout the depth range 0-30 km suggest that planar border faults along one side of rift basins penetrate the crust. Along the length of the rift, ~100-km long en echelon border-fault segments are linked by oblique-slip transfer faults, ramps, and monoclines within comparatively high-strain accommodation zones. Kinematic constraints from several basins indicate a regional east-west extension direction that is consistent with existing seismicity data. Volcanism began at ~12 Ma in the north and at ~7 Ma in the south prior to or concurrent with the initial development of Western rift sedimentary basins. During Pleistocene time, asymmetric basins have narrowed through progressive hanging-wall collapse and with uplift of the rift flanks. -from Author
Earthquake source parameter determination is of great importance for hazard assessment, as well as for a variety of scientific studies concerning regional stress and strain release and volcano-tectonic interaction. This is especially true for poorly instrumented, densely populated regions such as encountered in Africa, where even the distribution of seismicity remains poorly documented. In this paper, we combine data from satellite radar interferometry (InSAR) and teleseismic waveforms to determine the source parameters of the Mw 5.9 earthquake that occurred on 2008 February 3 near the cities of Bukavu (DR Congo) and Cyangugu (Rwanda). This was the second largest earthquake ever to be recorded in the Kivu basin, a section of the western branch of the East African Rift (EAR). This earthquake is of particular interest due to its shallow depth and proximity to active volcanoes and Lake Kivu, which contains high concentrations of dissolved carbon dioxide and methane. The shallow depth and possible similarity with dyking events recognized in other parts of EAR suggested the potential association of the earthquake with a magmatic intrusion, emphasizing the necessity of accurate source parameter determination. In general, we find that estimates of fault plane geometry, depth and scalar moment are highly consistent between teleseismic and InSAR studies. Centroid-moment-tensor (CMT) solutions locate the earthquake near the southern part of Lake Kivu, while InSAR studies place it under the lake itself. CMT solutions characterize the event as a nearly pure double-couple, normal faulting earthquake occurring on a fault plane striking 350° and dipping 52° east, with a rake of -101°. This is consistent with locally mapped faults, as well as InSAR data, which place the earthquake on a fault striking 355° and dipping 55° east, with a rake of -98°. The depth of the earthquake was constrained by a joint analysis of teleseismic P and SH waves and the CMT data set, showing that the earthquake occurred in the shallow crust, at approximately 8 km depth. Inversions of ENVISAT (Environment Satellite) and ALOS (Advanced Land Observation Satellite) data place the earthquake at 9 km. A comparison of the scalar moment (9.43 ± 0.06 × 1017 Nm from seismology and 8.99 ± 0.010 × 1017 Nm from the joint InSAR solution) shows good agreement between the two data sets. Such an agreement is in contrast to the large discrepancies observed (up to an order of magnitude) in other places along the EAR where similar earthquake sequences are associated with magmatic intrusion. From this, we infer that the rupture was brittle and occurred with little aseismic deformation as might be expected from magma injection. Our results provide insights into the style of rifting occurring in the South Kivu Volcanic Province and hence will aid future studies on seismic risk in the context of Lake Kivu and underline the importance of systematic monitoring of the EAR area.
On 17 January 2002, Nyiragongo volcano erupted along a 20 km-long
fracture network extending from the volcano to the city of Goma. The
event was captured by InSAR data from the ERS-2 and RADARSAT-1
satellites. A combination of 3D numerical modeling and inversions is
used to analyze these displacements. Using Akaike Information Criteria,
we determine that a model with two subvertical dikes is the most likely
explanation for the 2002 InSAR deformation signal. A first, shallow
dike, 2 km high, is associated with the eruptive fissure, and a second,
deeper dike, 6 km high and 40 km long, lies about 3 km below the city of
Goma. As the deep dike extends laterally for 20 km beneath the gas-rich
Lake Kivu, the interaction of magma and dissolved gas should be
considered as a significant hazard for future eruptions. A likely
scenario for the eruption is that the magma supply to a deep reservoir
started ten months before the eruption, as indicated by LP events and
tremor. Stress analysis indicates that the deep dike could have
triggered the injection of magma from the lake and shallow reservoir
into the eruptive dike. The deep dike induced the opening of the
southern part of this shallow dike, to which it transmitted magma though
a narrow dike. This model is consistent with the geochemical analysis,
the lava rheology and the pre- and post-eruptive seismicity. We infer
low overpressures (1-10 MPa) for the dikes. These values are consistent
with lithostatic crustal stresses close to the dikes and low magma
pressure. As a consequence, the dike direction is probably not
controlled by stresses but rather by a reduced tensile strength,
inherited from previous rift intrusions. The lithostatic stresses
indicate that magmatic activity is intense enough to relax tensional
stresses associated with the rift extension.
Nyamulagira (3058 m a.s.l.), a volcano of the Virunga volcanic province in the western branch of the East African Rift, is Africa’s most active volcano with one eruption every 2–4 years. It represents a hazard for the Virunga National Park and its vicinity. Despite such a frequent activity, Nyamulagira remains poorly studied. The only existing volcanological map was produced in the sixties by Thonnard et al. (1965). The occurrence of 19 eruptions since its publication makes it obsolete. In the present study we mapped the Nyamulagira lava flows from 1938 up to the last eruption to date in 2010 using optical (Landsat, ASTER) and radar (ENVISAT-ASAR, ERS, JERS) imagery. The results are integrated into a Geographical Information System (GIS) and coupled with additional data sources. GIS use makes the new database a flexible – and easy-to-update – tool for scientific purposes as well as for risk, environmental and humanitarian management. Here a new lava flow map was produced. Volumes of the successive lava flows and affected areas of the Virunga National Park were estimated.
The word “mazuku” in Swahili means “evil wind”. It corresponds to lowland (depressions) where carbon dioxide is released and, being heavier than air, accumulates at high – often lethal – concentrations (10 vol.% of CO2 in atmosphere can be considered as the deadly threshold, even for a short time exposure). Mazuku are abundant in Goma and surrounding areas and particularly in the area south of the large volcanic edifices of Nyiragongo and Nyamulagira volcanoes located in the most eastern part of DR Congo, W branch of the East African Rift System (EARS). Our extensive field surveys have indicated that mazuku are concentrated within to and around the densely populated city of Goma close to the N shores of Lake Kivu, mainly near fault or fissure networks. At a more local scale, depressions allowing CO2-rich gas accumulation are created by lava flow superposition, lava tunnels or cavity collapses, or directly associated with open fractures. People are killed by mazuku every year. Given political and social unrest coupled with the current important demographic and urban growths around Goma, the risks associated to mazuku are increasing accordingly. Mazuku are currently the most important natural risk in terms of human loss for the area and there is an urgent need for further research, more systematic mapping and monitoring of mazuku and for appropriate risk management to be implemented. This paper summarizes the current scientific knowledge on mazuku as well as new advances and a preliminary risk assessment performed recently in the frame of the GORISK project.
1] Continental rifting is accompanied by lithospheric thinning and decompressional melting. After extrac-tion, melt is intruded at shallower depth thereby heating and weakening the lithosphere. In a feedback mechanism this weakening may assist rifting and melt production. A one-dimensional kinematic litho-spheric thinning model is developed including decompressional melting and intrusional magma deposition. The intrusional heating effect is determined as a function of thinning rate and amount, melting parameters, potential temperature, and the depth range of emplacement. The temperature increases approximately pro-portionally to the square root of the thinning rate and to the square of the supersolidus potential temperature. Simple scaling laws are derived allowing predicting these effects and the surface heat flux for arbitrary sce-narios. Two-dimensional thermomechanical extension models are carried out for a multicomponent (crust-mantle) two-phase (melt-matrix) system with a rheology based on laboratory data including magmatic weakening. In good agreement with the 1-D kinematic models it is found that the lithosphere may heat up by several 100 K. This heating enhances viscous weakening by one order of magnitude or more. In a feedback mechanism rifting is dynamically enforced, leading to a significant increase of rift induced melt generation. Including the effect of lateral focusing of magma toward the rift axis the laws are applied to dif-ferent segments of the East African Rift System. The amount of intrusional heating increases with maturity of the rift from O(10 K) to up to 200 K or 400 K at the Afar Rift depending on the depth range of the mag-matic emplacement. (2012), Magmatic lithospheric heating and weakening during continental rifting: A simple scaling law, a 2-D thermomechanical rifting model and the East African Rift System, Geochem. Geophys. Geosyst., 13, Q08001, doi:
a b s t r a c t In recent decades, civil wars in the eastern provinces of the Democratic Republic of Congo have caused mas-sive social disruptions, which have been exacerbated by volcanic and earthquake disasters. Seismic data were gathered and analysed as part of an effort to monitor the volcanoes and quantitatively assess the earth-quake hazard. This information can be used to regulate the settlement of displaced people and to ''build back better". In order to investigate volcanic processes in the Virunga area, a local seismic velocity model was derived and used to relocate earthquake hypocenters. It was found that swarm-type seismicity, composed mainly of long-period earthquakes, preceded both the 2004 and 2006 eruptions of Nyamuragira. A steady increase in seismicity was observed to commence ten or eleven months prior to the eruption, which is attributed to the movement of magma in a deep conduit. In the last stage (1 or 2 months) before the erup-tion, the hypocenters of long-period earthquakes became shallower. Seismic hazard maps were prepared for the DRC using a 90-year catalogue compiled for homogeneous M w magnitudes, various published atten-uation relations, and the EZ-Frisk software package. The highest levels of seismic hazard were found in the Lake Tanganyika Rift seismic zone, where peak ground accelerations (PGA) in excess of 0.32 g, 0.22 g and 0.16 g are expected to occur with 2%, 5% and 10% chance of exceedance in 50 years, respectively.
1] The kinematics of the East African Rift (EAR) is the least well-known of all major plate boundaries. Here, we show that present-day data (a GPS+DORIS geodetic solution and earthquake slip vectors) are consistent with 3.2 Myr-average spreading rates and transform-fault azimuths along the Southwest Indian Ridge and support a kinematic model that includes three subplates (Victoria, Rovuma, and Lwandle) between Nubia and Somalia. Continental rifting in the EAR appears to involve localized strain along narrow rift structures that isolate large lithospheric blocks.
In the Kivu lake region, the faults of the rift are concentrated in three netting following three main directions NE-SW, N-S and NW-SE. Some small tectonics trenchs. aligned along these three directions are intersecting in a zone lengthening in the N-S direction and 100–200 km wide.
It does not look as if there was a chronological and regular succession in the coming out of faults belonging to a netting defined by a certain direction with regard to the faults of another netting; for, on the occasion of short tectonics events (eruptions, earthquakes), it is noticeable than any of these three nettings, are reactivated.
This overview paper considers the East African rift system (EARS) as an intra-continental ridge system, comprising an axial rift. It describes the structural organization in three branches, the overall morphology, lithospheric cross-sections, the morphotectonics, the main tectonic features—with emphasis on the tension fractures—and volcanism in its relationships with the tectonics. The most characteristic features in the EARS are narrow elongate zones of thinned continental lithosphere related to asthenospheric intrusions in the upper mantle. This hidden part of the rift structure is expressed on the surface by thermal uplift of the rift shoulders. The graben valleys and basins are organized over a major failure in the lithospheric mantle, and in the crust comprise a major border fault, linked in depth to a low angle detachment fault, inducing asymmetric roll-over pattern, eventually accompanied by smaller normal faulting and tilted blocks. Considering the kinematics, divergent movements caused the continent to split along lines of preexisting lithospheric weaknesses marked by ancient tectonic patterns that focus the extensional strain. The hypothesis favored here is SE-ward relative divergent drifting of a not yet well individualized Somalian plate, a model in agreement with the existence of NW-striking transform and transfer zones. The East African rift system comprises a unique succession of graben basins linked and segmented by intracontinental transform, transfer and accommodation zones. In an attempt to make a point on the rift system evolution through time and space, it is clear that the role of plume impacts is determinant. The main phenomenon is formation of domes related to plume effect, weakening the lithosphere and, long after, failure inducing focused upper mantle thinning, asthenospheric intrusion and related thermal uplift of shoulders. The plume that had formed first at around 30 Ma was not in the Afar but likely in Lake Tana region (Ethiopia), its almost 1000 km diameter panache weakening the lithosphere and preparing the later first rifting episode along a preexisting weak zone, a Pan-African suture zone bordering the future Afar region. From the Afar, the rift propagated afterward from north to south on the whole, with steps of local lithospheric failure nucleations along preexisting weak zones. These predisposed lines are mainly suture zones, in which partial activation of low angle detachment faults reworked former thrust faults verging in opposite directions, belonging to double verging ancient belts. This is responsible for eventual reversal in rift asymmetry from one basin to the next. Supposing the plume migrated southward, or other plumes emplaced, the rift could propagate following former weaknesses, even outside areas influenced by plumes. This view of rift formation reconciles the classical models: active plume effect triggered the first ruptures; passive propagations of failure along lithospheric scale weak zones were responsible for the onset of the main rift segments. Various other aspects are shortly considered, such as tectonics and sedimentation, and relationships of the ‘cradle of Mankind’ with human evolution. By its size, structure and occurrence of oceanic lithosphere in the Afar, the EARS can be taken as a model of the prelude of oceanic opening inside a continent.
Bukavu, on the south coast of Lake Kivu in the Democratic Republic of the Congo, suffers from slow but never ending landsliding. This leads to the continuous degradation and destruction of houses, buildings and of the roads, waterworks and sewerage infrastructure in several districts of the town.Research of mass-wasting processes is hampered by the limited geotechnical and hydrogeological information concerning the deeply weathered Tertiary and Quaternary lavas. There is also a disagreement about the exact location of active faults, believed to play a role in the mass wasting. Additionally, there is little information about the exact location and the rate of soil movements and whether ground instability is caused by tectonics and seismics or by increase in hydrostatic pressure.Based on aerial photographic interpretation, landslides cover more than 15% of the town of Bukavu.Thirty-one landslides occur outside the Bukavu “microrift” and do not contact active faults. Rather, they occur in actively incising river basins, ostensibly caused by neotectonic activity.Four from the six landslides within the “micrograben” are bigger and wider than the others, apparently governed by pre-existing tectonically induced landforms. They are adjacent to or crossed by active faults. They also fall far below the envelope of topographic thresholds for landslides established for North America and verified in Rwanda.Therefore, this threshold, a combination of slope at the head of the slide and surface drained to it, seems a promising tool to discriminate hydrologically from tectonically seismically induced landslides.Recent soil movements in Bukavu generally correspond to landslide distribution portrayed on the geomorphologic map.Most Bukavu soil movements occur on previous slides. Therefore, from an engineering–geologic standpoint, old landslides should be avoided, or, if economically feasible, be mitigated.
Detailed historical records of rift volcanism are scarce. Such information is, however, of a great importance to study the role of magmatism and volcanism as forces contributing to continental rifting. Nyamulagira (or Nyamuragira) volcano, located in the western branch of the East African Rift System, is one of the most active volcanoes on Earth and, hence, represents a good case study to investigate the spatial and temporal evolution of eruptive activity in a youthful extensional setting. Since 1882, Nyamulagira produced at least 42 eruptions, offering a dense historical record for that time period. Using scientific literature, including colonial scientific documents archived at the Royal Museum for Central Africa in Belgium, field data and a detailed mapping of the volcanic field, the spatial and temporal evolution of the eruptive activity of Nyamulagira is studied in order to 1) review the current state of knowledge on eruption history and dynamics at that volcano, and 2) infer the influence of the rift on factors con- trolling the location, occurrence and characteristics of Nyamulagira eruptions. The results show that the duration and location of eruptions allow the distinction of four eruption groups: summit/upper-flank, classical flank, long- lived flank and remote eruptions. The interpretation of these groups suggests that Nyamulagira eruptions are re- lated to the main crustal magmatic system and are strongly influenced by the gravitational stress field induced by the main central edifice. However, remote eruptions (1904, 1912, 1948, 1991–1993) are not influenced by this ed- ifice loading and seem directly related to deeper magma reservoirs. The 1938–1940 eruption is also atypical, but is related to an important drainage of the upper magma plumbing system, which triggered a caldera collapse. No tec- tonic trigger of eruption is evidenced, but cone and eruptive fissure alignments suggest that rift structures can also influence the eruption location. In addition, the present study suggests that the major eruptions of Nyamulagira along structural axes close to Nyiragongo can induce a change in the eruptive activity of the latter hazardous vol- cano. Hence, the historical eruptive activity of Nyamulagira highlights the complex interplay between the magmat- ic system and the local and regional stress fields in controlling the location and characteristics of eruptions, in this youthful extensional tectonic setting where main volcanic edifices developed.
The seismically and volcanically active Kivu Rift, in the western branch of the East African RiftSystem, is a type locale for studies of high-elevation, humid-climate rift basins, as well as magmatic basin development. Interpretations of offshore multi-channel seismic (MCS) reflection data, terrestrial radar topography, lake bathymetry, and seismicity data recorded on a temporary array provide new insights into the structure, stratigraphy and evolution of the Kivu rift. The Kivu rift is an asymmetric graben controlled on its west side by a ~110 km-long, N-S striking border fault. The southern basins of the lake and the upper Rusizi river basin are an accommodation zone effectively linking 1470 m-high Lake Kivu to770 m-high Lake Tanganyika. MCS data in the eastern Kivu lake basin reveal a west-dipping half graben with at least 1.5 km of sedimentary section; most of the ~ 2 km of extension in this sub-basin is accommodated by the east-dipping Iwawa normal fault, which bounds an intrabasinal horst. Lake Kivu experienced at least three periods of near desiccation. The two most recent of these approximately correlate to the African Megadrought and Last Glacial Maximum. There was a rapid lake level transgression of at least 400 m in the early Holocene. The line load of the Virunga volcanic chain enhances the fault-controlled basin subsidence; simple elastic plate models suggest that the line load of the Virunga volcanic chain depresses the basin by more than 1 km, reduces flank uplift locally, and broadens the depocentre. Not only do the voluminous magmatism and degassing to the lake pose a hazard to the riparian population, but our studies demonstrate that magmatism has important implications for short-term processes such as lake levels, inflow and outlets, as well as long term modification of classic half-graben basin morphology.This article is protected by copyright. All rights reserved.
Observations of active dike intrusions provide present day snapshots of the magmatic contribution to continental rifting. However, unravelling the contributions of upper crustal dikes over the timescale of continental rift evolution is a significant challenge. To address this issue, we analyzed the morphologies and alignments of >1,500 volcanic cones to infer the distribution and trends of upper crustal dikes in various rift basins across the East African Rift (EAR). Cone lineament data reveal along-axis variations in the distribution and geometries of dike intrusions as a result of changing tectono-magmatic conditions. In younger (<10 Ma) basins of the North Tanzanian Divergence, dikes are largely restricted to zones of rift-oblique faulting between major rift segments, referred to here as transfer zones. Cone lineament trends are highly variable, resulting from the interplay between (1) the regional stress field, (2) local magma-induced stress fields, and (3) stress rotations related to mechanical interactions between rift segments. We find similar cone lineament trends in transfer zones in the western branch of the EAR, such as the Virunga Province, Democratic Republic of Congo. The distributions and orientations of upper crustal dikes in the eastern branch of the EAR vary during continental rift evolution. In early-stage rifts (<10 Ma), upper crustal dikes play a limited role in accommodating extension, as they are confined to areas in and around transfer zones. In evolved rift basins (>10 Ma) in Ethiopia and the Kenya Rift, rift-parallel dikes accommodate upper crustal extension along the full length of the basin.
Nyamulagira and Nyiragongo are two of the most active volcanoes in Africa, but their eruptive histories are poorly known. Assessing lava flow volumes in the region remains difficult, as field surveys are often impossible and available Digital Elevation Models (DEMs) do not have adequate spatial or temporal resolutions. We therefore use TerraSAR-X add-on for Digital Elevation Measurement (TanDEM-X) interferometry to produce a series of 0.15 arc sec (∼5 m) DEMs from between 2011 and 2012 over these volcanoes. TanDEM-X DEMs have an absolute vertical accuracy of 1.6 m, resulting from the comparison of elevation with GPS measurements acquired around Nyiragongo. The difference between TanDEM-X-derived DEMs from before and after the 2011–2012 eruption of Nyamulagira provides an accurate thickness map of the lava flow emplaced during that activity. Values range from 3 m along the margins to 35 m in the middle, with a mean of 12.7 m. The erupted volume is 305.2 ± 36.0 × 106 m3. Height errors on thickness depend on the land covered by the flow and range from 0.4 m in old lavas to 5.5 m in dense vegetation. We also reevaluate the volume of historical eruptions at Nyamulagira since 2001 from the difference between TanDEM-X and SRTM 1 arc sec DEMs and compare them to previous work. Planimetric methods used in literature are consistent with our results for short-duration eruptions but largely underestimate the volume of the long-lived 2011–2012 eruption. Our new estimates of erupted volumes suggest that the mean eruption rate and the magma supply rate were relatively constant at Nyamulagira during 2001–2012, respectively, 23.1 m3 s−1 and 0.9 m3 s−1.
The Lake Kivu catchment in the East African Rift is subject to various geologic hazards, including frequent volcanic eruptions, earthquakes, and potential limnic overturns and degassing events. Integration of high-resolution seismic reflection data, 14C dated sediment cores, and lake-floor bathymetry reveals large axial and transverse turbidite systems in the eastern basin of the lake. The turbidites were sourced by hyperpycnal river flows during exceptional floods, and the temporal occurrence of the turbidites was climatically controlled. The turbidite record over the past ~12 k.y. is correlated with the regional paleohydrologic records from tropical East Africa. Our study suggests that flood-introduced turbidites preserved in deep lakes are indicators of hydrological changes, and that extreme floods in Lake Kivu's recent history may have triggered deep mixing events. This study also has implications for the current degassing efforts in Lake Kivu; potential geologic hazards may be triggered by extraordinary turbidity currents, and need to be considered in the design and deployment of gas extraction facilities.
The history of Lake Kivu is strongly linked to the activity of the Virunga volcanoes. Subaerial and subaquatic volcanoes, in addition to lake-level changes, shape the subaquatic morphologic and structural features in Lake Kivu's Main Basin. Previous studies revealed that volcanic eruptions blocked the former outlet of the lake to the north in the late Pleistocene, leading to a substantial rise in the lake level and subsequently the present-day thermohaline stratification. Additional studies have speculated that volcanic and seismic activities threaten to trigger a catastrophic release of the large amount of gases dissolved in the lake. The current study presents a bathymetric mapping and seismic profiling survey that covers the volcanically active area of the Main Basin at a resolution that is unprecedented for Lake Kivu. New geomorphologic features identified on the lake floor can accurately describe related lake-floor processes for the first time. The late Pleistocene lowstand is observed at 425 m depth, and volcanic cones, tuff rings, and lava flows observed above this level indicate both subaerial and subaquatic volcanic activities during the Holocene. The geomorphologic analysis yields new implications on the geologic processes that have shaped Lake Kivu's basin, and the presence of young volcanic features can be linked to the possibility of a lake overturn.
Three representative basins in the Western rift system of East Africa
are bordered along one side by high-angle normal faults with 2- to 5-km
throws (border faults). In plan view ˜100-km-long systems of
linear border faults form curvilinear border fault segments bounding the
East Kivu, West Kivu, and Rusizi basins. The opposite sides of these
asymmetric basins are bounded by lower relief faulted monoclines or en
echelon ramps. The largely unfaulted rift flanks have been uplifted 2 km
above the 1.3-km-high East African plateau, with uplift narrowing basins
during Quaternary time. Maximum estimates of ˜E-W crustal
extension within basins are less than 25% (< 16 km), and planar
border faults may penetrate the crust. The East Kivu and West Kivu
basins are linked across the rift valley by a horst that serves as a
hinge for subsidence in both basins. The westward tilted East Kivu and
eastward tilted Rusizi border fault segments are linked along the rift
by oblique-slip transfer faults that also accommodate along-axis
differences in elevation. Upper Miocene-Recent eruptive volcanic centers
within the comparatively high-strain interbasinal region (accommodation
zone) generally coincide with the tips of border fault segments and
transfer faults. The orientations of Miocene-Recent dip-slip and
oblique-slip faults show little correlation with Precambrian shear zones
or foliations in metamorphic basement. Differences between the East
Kivu, West Kivu, and Rusizi basins in the age of initial faulting,
subsidence, and age/composition of volcanic products suggest that border
fault segments developed diachronously and propagated along the length
of the rift. This along-axis border fault propagation and the
crosscutting geometry of transfer faults contribute to the segmentation
of the Western rift valley.
Plots of extension versus maximum synrift sediment thickness for rifts
display a wide range of values. Some rifts display almost equal amounts
of vertical displacement and horizontal extension, while others display
considerably more extension than vertical displacement. The former
appear to be associated with deep depths to detachment and/or steeply
dipping boundary faults, while the latter tend to correspond with
shallower depths to detachment and/or more gently dipping boundary
faults. Boundary fault shape is inferred to exert an important control
on relative amounts of extension and maximum sediment thickness and in
sedimentation style. The deep anoxic lakes of the Western Branch, East
African Rift system are associated with relatively steeply
(40°-70°) dipping faults and depths to detachment ranging
between 15 and 30 km. The more coarse clastic-dominated Miocene half
grabens in Northern Kenya are associated with relatively gently dipping
boundary faults (50°-20°) and shallower depths to detachment
(less than 15 km).
Using the record of 30 flank eruptions over the last 110years at Nyamuragira, we have tested the relationship between the eruption dynamics and the local stress field. There are two groups of eruptions based on their duration () that are also clustered in space and time. We find that the eruptions fed by dykes parallel to the East African Rift Valley have longer durations (and larger volumes) than those eruptions fed by dykes with other orientations. This is compatible with a model for compressible magma transported through an elastic-walled dyke in a differential stress field from an over-pressured reservoir (Woods et al., 2006). The observed pattern of eruptive fissures is consistent with a local stress field modified by a northwest-trending, right-lateral slip fault that is part of the northern transfer zone of the Kivu Basin rift segment. We have also re-tested with new data the stochastic eruption models for Nyamuragira of Burt et al. (1994). The time-predictable, pressure-threshold model remains the best fit and is consistent with the typically observed declining rate of sulphur dioxide emission during the first few days of eruption with lava emission from a depressurising, closed, crustal reservoir. The 2.4-fold increase in long-term eruption rate that occurred after 1977 is confirmed in the new analysis. Since that change, the record has been dominated by short-duration eruptions fed by dykes perpendicular to the Rift. We suggest that the intrusion of a major dyke during the 1977 volcano-tectonic event at neighbouring Nyiragongo volcano inhibited subsequent dyke formation on the southern flanks of Nyamuragira and this may also have resulted in more dykes reaching the surface elsewhere. Thus that sudden change in output was a result of a changed stress field that forced more of the deep magma supply to the surface. Another volcano-tectonic event in 2002 may also have changed the magma output rate at Nyamuragira.
The city of Bukavu, on the south coast of Lake Kivu in the Democratic Republic of Congo, suffers from frequent landsliding, which leads to continual damage and destruction of buildings, roads, waterworks and sewerage infrastructure. Thirty-one landslides identified on aerial photographs of 1954 and 1973 are located outside the Bukavu micro-rift and are related to actively incising rivers. Their origin is thought to be due to increases in hydrostatic pressure. Six landslides occur within the Bukavu micro-rift, four of which are much larger and wider than other slope failures in the study region. These large landslides are adjacent to, or crossed by, active faults. They fall far below the topographic threshold envelope, a relationship of slopes at the head of the slide and the surface area drained into it, indicating seismic or anthropomorphic triggering. The Bukavu landslides still reactivate occasionally. Deforestation, followed by a large increase in the population, have been indirect causes of the reactivated mass wasting processes. On the steep slopes in the south of the city the high density of newly built houses has led to reduced water infiltration and enhanced runoff, causing landslides and mudflows. Very high spatial resolution IKONOS satellite images have recently been used as base maps for geohazard assessments of Bukavu. A geographical information system (GIS) has been developed for Bukavu's planners: this shows the locations of buildings, roads and tracks, the river network, the water distribution system and the sewerage infrastructure, as well as areas of slope instability.
In previous literature, the “Kibara belt” has often been portrayed as a Mesoproterozoic belt trending NE over 1300 km across the Central African Congo craton, from the Angola-Zambia-D.R.Congo border triple-junction in the SW, through Katanga and Kivu-Maniema (DRC), Rwanda and Burundi, up to SW Uganda and NW Tanzania in the NE. However, north of Katanga in the DRC, there is a clear break in continuity of the thus-defined “Kibara belt”, cross-cut by Palaeoproterozoic (Rusizian) terranes, in structural continuity with the NW-SE trending Ubende shear belt further south in Tanzania.
Lake Tanganyika is part of the east African rift system. The amount of
extension along 22 dip section seismic lines in the lake has been
measured. The maximum extension observed is 4.5 km (approximately 10%)
at the central part of the rift and decreases to about 0.5 to 1-km
extension at the north and south ends of the rift. Deformation style
changes with the amount of calculated extension. More complex rift
geometries (paired half grabens and full grabens) are found in the
central part where extension is greatest while single half grabens are
present toward the northern and southern ends of the lake where
extension is less. Local decreases in the amount of extension are
superimposed on this regional trend. These drops tend to occur in
regions defined as transfer zones and suggest a change from simple
extension to more complex three-dimensional strains (extension in more
than the dip direction). Implied in the relationship between rift
morphology and observed extension is the conclusion that Lake Tanganyika
is a single rift segment that did not propagate with time but grew as a
fixed length segment from day 1. Lake Tanganyika is a
subsidence-dominated rift, with up to 7-km vertical displacement along
major boundary faults for only 4.5-km extension. The fairly steep,
weakly listric boundary faults indicate that they sole out at deep
(20-30 km) crustal levels.
This study presents new major and trace element and Sr-Nd isotopic results for a suite of Miocene-Recent mafic lavas from the Kivu volcanic province in the western branch of the East African Rift. These lavas exhibit a very wide range in chemical and isotopic characteristics, due to a lithospheric mantle source region that is heterogeneous on a small scale, probably < 1 km. The chemical and isotopic variations are mostly geographically controlled: lavas from Tshibinda volcano, which lies on a rift border fault on the northwestern margin of the province, have higher values of 87Sr/86Sr, (La/Sm)n, Ba/Nb, and Zr/Hf than the majority of Kivu (Bukavu) samples. The range of 87Sr/86Sr at Tshibinda (0.70511–0.70514) overlaps some compositions found in the neighboring Virunga province, while Bukavu group lavas include the lowest 87Sr/86Sr (0.70314) and highest εNd (+7.6) yet measured in western rift lavas. The Tshibinda compositions trend towards a convergence for Sr-Nd-Pb isotopic values among western rift lavas. Among Kivu lavas, variations in 143Nd/144Nd correlate with those for certain incompatible trace element ratios (e.g., Th/Nb, Zr/Hf, La/Nb, Ba/Rb), with Tshibinda samples defining one compositional extreme. There are covariations of isotopic and trace element ratios in mafic lavas of the East African Rift system that vary systematically with geographic location. The lavas represent a magmatic sampling of variations in the underlying continental lithospheric mantle, and it appears that a common lithospheric mantle (CLM) source is present beneath much of the East African Rift system. This source contains minor amphibole and phlogopite, probably due to widespread metasomatic events between 500 and 1000 Ma. Lava suites which do not show a strong component of the CLM source, and for which the chemical constraints-also suggest the shallowest magma formation depths, are the Bukavu group lavas from Kivu and basanites from Huri Hills, Kenya. The inferred extent of lithospheric erosion therefore appears to be significant only beneath these two areas, which is generally consistent with lithospheric thickness variations estimated from gravity and seismic studies.
The Toro-Ankole volcanics in the northern sector of the Western Branch of the East African Rift are generally assumed to be of Quaternary age. New field observations and comparative KAr and ArAr analyses on whole rock samples and phenocrysts of perpotassic lavas reveal the presence of excess Ar and led to the conclusion that apparently no volcanics have been erupted prior to 50 ka.The present geochronological data, compared with previous ones for the Virunga and South Kivu volcanic fields, suggest that a Late Quaternary pulse of volcanic activity occured in the three areas, with the emission of perpotassic, mildly potassic or sodic alkaline magmas. The genetic implications of such a synchronous activity with wide compositional variation are briefly discussed with respect to basement age and assumed lithosphere characteristics.
This study aims at showing how far pre-existing crustal weaknesses left behind by Proterozoic mobile belts, that pass around cratonic Archean shields (Tanzania Craton to the southeast and Congo Craton to the northwest), control the geometry of the Albertine Rift. Focus is laid on the development of the Lake Albert and Lake Edward/George sub-segments and between them the greatly uplifted Rwenzori Mountains, a horst block located within the rift and whose highest peak rises to >5000m above mean sea level. In particular we study how the southward propagating Lake Albert sub-segment to the north interacts with the northward propagating Lake Edward/George sub-segment south of it, and how this interaction produces the structures and geometry observed in this section of the western branch of the East African Rift, especially within and around the Rwenzori horst. We simulate behaviour of the upper crust by conducting sandbox analogue experiments in which pre-cut rubber strips of varying overstep/overlap connected to a basal sheet and oriented oblique and/or orthogonal to the extension vector, are placed below the sand-pack. The points of connection present velocity discontinuities to localise deformation, while the rubber strips represent ductile domain affected by older mobile belts. From fault geometry of developing rift segments in plan view and section cuts, we study kinematics resulting from a given set of boundary conditions, and results are compared with the natural scenario. Three different basal model-configurations are used to simulate two parallel rifts that propagate towards each other and interact. Wider overstep (model SbR3) produces an oblique transfer zone with deep grabens (max. 7.0km) in the adjoining segments. Smaller overlap (model SbR4) ends in offset rift segments without oblique transfer faults to join the two, and produces moderately deep grabens (max. 4.6km). When overlap doubles the overstep (model SbR5), rifts propagate sub-orthogonal to the extension direction and form shallow valleys (max. 2.9km). Relative ratios of overlap/overstep between rift segments dictate the kind of transition zone that develops and whether or not a block (like the Rwenzoris) is captured and rotates; hence determining the end-member geometry. Rotation direction is controlled by pre-existing fabrics. Fault orientation, fault kinematics, and block rotation (once in play) reinforce each other; and depending on the local kinematics, different parts of a captured block may rotate with variable velocities but in the same general direction. Mechanical strength anisotropy of pre-structured crust only initially centres fault nucleation and propagation parallel to the grain of weakness of the basement, but at later stages of a protracted period of crustal extension, such boundaries are locally defied.
The western branch of the East African Rift is composed of an arcuate succession of elongate asymmetric basins, which differ in terms of interaction geometry, fault architecture and kinematics, and patterns of uplift/subsidence and erosion/sedimentation. The basins are located within Proterozoic mobile belts at the edge of the strong Tanzanian craton; surface geology suggests that the geometry of these weak zones is an important parameter in controlling rift development and architecture, although other processes have been proposed. In this study, we use lithosphere-scale numerical models and crustal-scale analogue experiments to shed light on the relations between preexisting structures and rift architecture. Results illustrate that on a regional scale, rift localization within the mobile belts at the curved craton's western border results in an arcuate rift system, which implies that under a constant extensional stress field, part of the western branch experienced orthogonal extension and part oblique extension. Largest depocenters are predicted to form mostly orthogonal to the extension direction, and smaller depocenters will form along the oblique parts of the rift. The varying extension direction along the rift zone furthermore results in lengthwise varying rift asymmetry, segmentation characteristics, and border fault architecture (trend, length, and kinematics). Analogue models predict that discrete upper crustal fabrics may influence the location of accommodation zones and control the architecture of extension-related faults at a local scale. Models support that fabric reactivation is responsible for the oblique-slip kinematics on faults and for the development of Z-shaped or arcuate normal faults typically documented in nature.
Continental extension may occur in two main different modes, narrow and wide rifting, which mainly differ in the width of the deformed region. A third mechanism, the core complex, has been considered either a distinct mode of extension or a local anomaly within wide rifts. In terms of causative processes, continental rifting may be explained by both active or passive mechanisms, which also differ in the volume of magmatic products and in the rheological properties and stratification of the extending lithosphere. Both numerical and analogue models have investigated the main parameters controlling the extension of a rheologically layered lithosphere. In particular, analogue models have highlighted that the style of deformation is mainly controlled by the competition between the total resistance of the lithosphere and the gravitational forces; this competition, in turn, is mainly controlled by boundary conditions, such as the applied strain rate and the rheological characteristics of the extending lithosphere.
The Mesoproterozoic Kibara Belt (also Kibaran Belt or Kibarides in some references) of Central Africa was often portrayed as a continuous, c. 1500 km long orogenic belt, trending NE to NNE from Katanga, Democratic Republic of Congo (DRC) in the south, up into SW Uganda in the north. Recently however, the Karagwe-Ankole Belt (KAB; formerly the NE Kibara Belt) has been redefined as the part north of a NW oriented Palaeoproterozoic basement high of the Ubende-Rusizi Belts, while the Kibara Belt (KIB) is now limited to the part south of this rise.
The Edward and George lake basins are located in the western branch of the East African Rift System (EARS). Both basins occupy half grabens with the main boundary fault located in the west and are separated by a high relief accommodation zone. This structural high formed where the NNE–SSW trend of the Miocene rifting and NW–SE oriented basement lineaments interfere. The Kazinga Channel, connecting Lake George to Lake Edward, cuts across this uplifted area. The course of the channel has been affected by syn- and antithetic faulting. Volcanism in this part of the EARS is related to high relief accommodation zones, and volcanic craters are either parallel to the NW–SE oriented Precambrian lineaments or the NNE–SSW oriented Miocene rift. Volcanism that initiated during the early stages of rifting commonly parallels the NW–SE oriented accommodation zones and basement lineaments, while craters that formed during the later stages of rifting tend to parallel the large border faults of the NNE–SSW oriented rift. Rifting processes in the Edward and George basins have been disturbed by uplift mechanisms related to the Rwenzori horst block formation. The Kasindi Fault Zone located in the central part of the Edward Basin is related to a low relief accommodation zone formed between two opposite dipping border faults. The Edward–George basin is tectonically active; the large boundary faults, the syn- and antithetic faults of the rift basin, and the accommodation zones all show signs of recent movement. Faulting related to the Kasindi Fault Zone suggests a >12 m fault movement during the Holocene, possibly related to periods of extensive volcanism within the Toro Ankole volcanic province.
The proposed high-level radioactive waste repository at Yucca Mountain, Nevada, is located within an active volcanic field. Probabilistic volcanic hazard models for future eruptions through the proposed repository depend heavily on our understanding of the spatial controls on volcano distribution at a variety of scales. On regional scales, Pliocene-Quaternary volcano clusters are located east of the Bare Mountain fault. Extension has resulted in large-scale crustal density contrast across the fault, and vents are restricted to low-density areas of the hanging wall. Finite element modeling indicates that this crustal density contrast can result in transient pressure changes of up to 7 MPa at 40 km depth, providing a mechanism to generate partial melts in areas where mantle rocks are already close to their solidus. On subregional scales, vent alignments, including one alignment newly recognized by ground magnetic mapping, parallel the trends of high– dilation tendency faults in the Yucca Mountain region (YMR). Forty percent of vents in the YMR are part of vent alignments that vary in length from 2 to 16 km. Locally, new geological and geophysical data show that individual vents and short vent alignments occur along and adjacent to faults, particularly at fault intersections, and left-stepping en echelon fault segments adjacent to Yucca Mountain. Conditions which formed these structures persist in the YMR today, indicating that volcanism will likely continue in the region and that the proposed repository site is within an area where future volcanism may occur. On the basis of these data the probability of volcanic disruptions of the proposed repository is estimated between 10 8 /yr and 10 7 /yr.
a b s t r a c t The volcano–tectonic architecture of the Rungwe Volcanic Province in SW Tanzania, part of the East African Rift System, was studied with integrated remote sensing imagery. A Shuttle Radar Topography Mission Digital Elevation Model was draped with geo-referenced geological and topographical maps and air photos. The entire RVP region was inspected systematically for tectonic lineaments and volcanic vents. Tectonic lineaments show two distinct directions, NW–SE and NNE–SSW, consistent with the idea of a current stress regime of local NE–SW compression. We find that there is tectonic control on the regional location for at least two of the three major volcanoes as well as for local distribution of eruptive vents on each of these three volcanoes. Field data show that major volcano instability events occurred in the Holocene for Ngozi caldera and Rungwe. These instability events are possibly associated with the faults controlling the loca-tion of both volcanoes. This study highlights the need for monitoring RVP tectonic and volcanic activity.
Source parameters for 38 African earthquakes have been determined using P and SH body-waveform inversion of analogue and digital waveforms. The results of this modelling are combined with 15 other earthquakes whose source parameters are also well constrained by body-waveform inversion. This data set shows that parts of East Africa have a seismogenic thickness of up to ∼ 35 km, and that seismicity occurs throughout the upper and lower crust. 1-D heat-flow calculations suggest temperatures of ∼ 325–475 ° C at 35 km depth in the Archaean and Proterozoic crust, requiring that the lower crust has a dry, mafic bulk composition in order to deform seismogenically. The slip vectors associated with these 53 earthquakes are combined with an additional 28 solutions from the Harvard CMT catalogue in order to investigate plate kinematics. In the Red Sea/Gulf of Aqaba the slip vectors agree closely with the Africa–Arabia extension direction predicted by the current best-fit plate model of Jestin, Huchon & Gaulier (1994). In the southern East African Rift System motion is split between a southeast direction in Zambia and a northeast direction in Malaŵ i and Rukwa, which combine to achieve the predicted Africa–Somalia east–west extension. Thus the current best-fit plate model of Jestin et al. (1994) adequately describes the active kinematics of Africa.
With heights of 4–5km, the topography of Rwenzori Mountains, a large horst of old crustal rocks located inside a young passive
rift system, poses the question “Why are the Rwenzori Mountains so high?”. The Cenozoic Western Rift branch of the East African
Rift System is situated within the Late Proterozoic mobile belts between the Archean Tanzania Craton and Congo Craton. The
special geological setting of the massif at a rift node encircled by the ends of the northern Western Rift segments of Lake
Albert and Lake Edward suggests that the mechanism responsible for the high elevation of the Rwenzoris is related to the rifting
process. Our hypothesis is based on the propagation of the rift tips, surrounding the stiff old lithosphere at Rwenzori region,
thereby triggering the delamination of the cold and dense mantle lithosphere (ML) root by reducing viscosity and strength
of the undermost lower crust. As a result, this unloading induces fast isostatic pop-up of the less dense crustal Rwenzori
block. We term this RID—“rift induced delamination of Mantle Lithosphere”. The physical consistency of the RID hypothesis is tested numerically. Viscous flow of
2D models is approximated by a Finite Difference Method with markers in an Eulerian formulation. The equations of conservation
of mass, momentum and energy are solved for a multi-component system. Based on laboratory data of appropriate rock samples,
a temperature-, pressure- and stress-dependent rheology is assumed. Assuming a simple starting model with a locally heated
ML, the ML block between the weakened zones becomes unstable and sinks into the asthenosphere, while the overlying continental
crust rises up. Thus, RID seems to be a viable mechanism to explain geodynamically the extreme uplift. Important conditions
are a thermal anomaly within the ML, a ductile lower crust with visco-plastic rheology allowing significant strength reduction
and lateral density variations. The special situation of a two-sided rifting or offset rift segments to decouple the ML laterally
from the surrounding continental lithosphere seems to be most decisive. Further support for the RID mechanism may come from
additional crustal thickness and an extensive stress field. Some parameters, such as the excess temperature and yield stress,
are very sensitive, small changes determine whether delamination takes place or not.
KeywordsEast African Rift System-Rwenzori Mountains-Isostasy-Uplift-Delamination-Lithospheric mantle-Numerical models-Viscous flow
Centrifuge analogue experiments are used to model the reactivation of pre-existing crustal fabrics during extension. The models reproduced a weakness zone in the lower crust whose geometry was varied in order to investigate its role in controlling the architecture of rift segments and related transfer zones. The typical rift system geometry was characterised by two offset rift segments connected by a major transfer zone in which boundary faults were oblique to the extension vector and displayed a significant transcurrent component of movement. The transfer zone was also characterised by cross-basin faults with both trend and strike-slip component of movement opposite to that displayed by the master faults. Typically, different structural patterns were obtained by changing the offset angle φ between the rift segments, supporting that the structural pattern at transfer zones is strongly influenced by the orientation of pre-existing discontinuities with respect to the stretching vector. In the models, the aspect ratio (ratio of length vs. width) of the transfer zone shows a positive correlation with the offset angle (i.e., the more the inherited fabric is parallel to the extension direction, the longer and narrower the transfer zones). In case of staircase offset of the rift segments (φ=90°), the structural pattern was characterised by two isolated rift depressions linked by a narrow transfer zone in which border faults with alternating polarity overlapped. Prominent rise of the ductile lower crust was also observed at the transfer zone. Many of these geometrical features display striking similarities with natural rift systems. The results of the current experiments provide useful insights into the mechanics of continental rift architecture, supporting that rift propagation, width and along-axis segmentation may be strongly controlled by the reactivation of pre-existing pervasive crustal fabrics.
The Cenozoic East African Rift System (EAR) displays strong interaction between rift faults and zones of weakness in Precambrian basement and Late Carboniferous–Permian Karroo rifts. Assuming E–W regional extension, NW–SE and NE–SW trending rift segments following older zones of weakness are expected to show oblique extensional or transtensional displacement. Yet, the earthquake focal mechanism-derived modern maximum horizontal stress (Shmax) pattern in the Western Branch of the EAR displays similar orientations to dominant foliation direction in Proterozoic orogenic belts. The oblique segments are undergoing almost pure extension. Localized stress deflections near zones of strength anisotropy are commonly described in studies of geomechanics, in-situ stresses and fracture patterns. The correspondence between foliation and Shmax direction in the Western Branch suggests re-orientation of stress at a larger scale than has previously been recognized. The stress guide effect appears best developed in the Ubendian Belt-Rukwa Rift where the Shmax direction from the regional N–S direction to a NW–SE trend and NW–SE trending foliations are consistently steeply dipping. However, this effect is considered only one of a number of possible causes of stress deflection in the East African Rift. Precambrian basement foliation is not oriented consistently enough to re-orient stress on a large-scale everywhere (particularly the Kenya Rift). The effects of stress rotation along oblique fabrics are: 1) faults normally predicted to exhibit oblique slip are actually near pure dip-slip, 2) faults along oblique trends can be optimally oriented with respect to the (local) stress field, and develop in a zone of reduced cohesive shear strength (with respect to isotropic rocks), 3) where foliations are inclined from vertical the maximum principal stress will be similarly inclined leading to initiation of extensional faults at angles considerably lower than 60°, and 4) faults will strike parallel to the foliation but cross-cut foliations in cross-section.
Geochemical and sedimentological data in a continuous 60,000-year sediment core record from the Kalya horst region of central Lake Tanganyika provide a detailed history of paleoclimate-mediated weathering and overflow events from upstream Lake Kivu. Univariate (elemental profiles), bivariate (elemental ratios) and multivariate analyses of chemical trends show variations between the dry Late Pleistocene (32–18 ka cal yr BP) and the wetter conditions that both preceded and post-date that interval. This record places important new constraints on the timing of aridity in East Africa during the high-latitude Last Glacial Maximum (LGM) based on significant decreases in magnetic susceptibility and soluble cation concentrations coinciding with biogenic silica. Elemental indicators in the oldest portion of the sedimentary record (60–50 ka cal yr BP) characterize this interval as a comparatively wet period, similar to modern conditions. Our record demonstrates that the ensuing transition toward arid conditions in tropical Africa during high-latitude glaciation was a two staged event with intermediate levels of aridity occurring from 50–32 ka cal yr BP followed by intense aridity from 32–18 ka cal yr BP.The initiation of inflow from upstream Lake Kivu into Lake Tanganyika is evidenced at 10.6 ka cal yr BP through its influence on both elemental profiles (Mg, Ca) and through its effect on 87Sr/86Sr. Increases in elemental (Mg, Ca, Sr) concentrations coincide with the timing of the Lake Kivu overflow. Metal geochemistry suggests that the overflow from Lake Kivu into Lake Tanganyika may have ceased between 8 and 6 ka cal yr BP, suggesting a period of Middle Holocene aridity in East Africa.
The kinematic models and the associated orientation of extensional stress of the East African Rift System have been subjected to much debate since a long time. In the past decades, the proposed models relied on the interpretation of the overall rift geometry, geological fault-slip data and the few focal mechanisms available. These models generally suffer of a poor time control and an underestimation of the possible changes in the stress field and geodynamic regime with time and space. In the recent years, there has been a significant increase in the number of focal mechanisms available for the entire rift system, and it is now possible to estimate the present-day stress field in relative detail based on seismotectonic data alone.