Article
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

On the basis of 2D multichannel and very-high-resolution seismic data and swath bathymetry, we report a sequence of giant mass-transport deposits (MTDs) in the Scan Basin (southern Scotia Sea, Antarctica). MTDs with a maximum thickness of c. 300 m extend up to 50 km from the Discovery and Bruce banks towards the Scan Basin. The headwall area consists of multiple U-shaped scars intercalated between volcanic edifices, up to 250 m high and 7 km wide, extending c. 14 km downslope from 1750 to 2900 m water depth. Seismic sections show that these giant MTDs are triggered by the intersection between diagenetic fronts related to silica transformation and vertical fluid-flow pipes linked to magmatic sills emplaced within the sedimentary sequence of the Scan Basin. This work supports that the diagenetic alteration of siliceous sediments is a possible cause of slope instability along world continental margins where bottom-simulating reflectors related to silica diagenesis are present at a regional scale.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... The influence of bottom currents on deep-water sedimentation patterns increased progressively over time due to the gradual opening of the different basins (e.g., Pérez et al., 2017Pérez et al., , 2021. Additionally, Mass Transport Deposits (MTDs) are known to be a main constituent of the long-term sedimentary record of the Scotia Sea basins Somoza et al., 2019), given their proximity to the Scotia Plate active tectonic boundaries and associated seismicity (Fig. 1). ...
Article
Multibeam bathymetric imagery and acoustic sub-bottom profiles are used to reveal distribution patterns of sub-surface sedimentation in Dove Basin (Scotia Sea). The goals of the study are to determine the imprint of the inflow of deep Antarctic water masses from the Weddell Sea into the Scotia Sea, to establish the factors driving the styles of contourite deposition and to discern the relative contribution of alongslope versus downslope processes to the construction of the uppermost late Quaternary sedimentary record in the basin. The most significant morpho-sedimentary features in Dove Basin are linked to contouritic processes and to mass movements. Plastered drifts on the flanks of the basin constitute the most common contouritic deposits. Basement-controlled drifts on top of structural elevations are common along the central ridge, the central basin plain and scattered along the basin flanks. Sheeted drifts occur on top of adjacent banks or are restricted to the deep basin. In contrast, mounded drifts are poorly represented in Dove basin. A laterally extensive contouritic channel runs along the central ridge. Contouritic channels are also identified in the upper parts of the lateral banks and slopes. Numerous slide scars along the upper parts of the slopes evolve downslope into semitransparent lens-shaped bodies, with occasional development of across-slope channels. Semitransparent lenses occur intercalated within stratified deposits in the slopes of the basin, in the central ridge and in the deepest abyssal plain. The spatial arrangement of contouritic morphologies points to the influence of the water column structure and the basin physiography. In the eastern sub-basin, two different fractions (lower and upper) of Weddell Sea Deep Water (WSDW) leave an imprint on contourite deposits owing to the sloping interface between the two fractions. Contouritic influence is more subdued in the western sub-basin, and limited to the imprint of the lower WSDW. The upper parts of the surrounding banks are under the influence of deep-reaching Circumpolar waters (i.e., Lower Circumpolar Deep Water), which develops both depositional and erosional morphologies. The cross-section V-shaped morphology of the basin and the common occurrence of structural highs drive the predominance of plastered and basement-controlled drifts in the sediment record. The frequent alternation between contourites and downslope gravity-flow deposits is likely due to different processes associated with over-steepening in the basin, such as basement-controlled steep slopes, deformed drifts atop basement elevations, and the development of thick contouritic piles. Dove Basin is an example of a basin without mounded, plastered or mixed hybrid drifts in the transition between the lower slope and the deep basin, because the upper boundary of the deepest water mass —the Weddell Sea Deep Water— flows shallower along the middle slope. This fact underlines the relevance of the position and depth of water masses in shaping the morphology of the feet of slopes along continental margins.
... We also notice some columnar blankings on the continental rise ( Fig. 9) that may correspond to fluid escape features (pipes). The MTDs that we observe along smooth slopes could have been favored by fluidification above the pipes similarly to what happened in the southern Scotia Sea (Somoza et al., 2019). The fact that the reflectors inside the undeformed blocks are sub-parallel to the seabed may indeed suggest sliding (creeping) within a fluidized mass and not rotation by gravitational falling. ...
Article
Full-text available
In this paper we analyze how oceanic circulation affects sediment deposition along a sector of the Ross Sea continental margin, between the Iselin Bank and the Hillary Canyon, and how these processes evolved since the Late Miocene. The Hillary Canyon is one of the few places around the Antarctic continental margin where the dense waters produced onto the continental shelf, mainly through brine rejection related to sea ice production, flow down the continental slope and reach the deep oceanic bottom layer. At the same time the Hillary Canyon represents a pathway for relatively warm waters, normally flowing along the continental slope within the Antarctic Slope Current, to reach the continental shelf. The intrusion of warm waters onto the continental shelf produces basal melting of the ice shelves, reduces their buttressing effect and triggers instabilities of the ice sheet that represent one of the main uncertainties in future sea level projections. For this study we use seismic, morpho-bathymetric and oceanographic data acquired in 2017 by the R/V OGS Explora. Seismic profiles and multibeam bathymetry are interpreted together with age models from two drilling sites (U1523 and U1524) of the Integrated Ocean Discovery Program (IODP) Expedition 374. Oceanographic data, together with a regional oceanographic model, are used to support our reconstruction by showing the present-day oceanographic influence on sediment deposition. Regional correlation of the main seismic unconformities allows us to identify eight seismic sequences. Seismic profiles and multibeam bathymetry show a strong influence of bottom current activity on sediment deposition since the Early Miocene and a reduction in their intensity during the mid-Pliocene Warm Period. Oceanographic data and modelling provide evidence that the bottom currents are related to the dense waters produced on the Ross Sea continental shelf and flowing out through the Hillary Canyon. The presence of extensive mass transport deposits and detachment scarps indicate that also mass wasting participates in sediment transport. Through this integrated approach we regard the area between the Iselin Bank and the Hillary Canyon as a Contourite Depositional System (ODYSSEA CDS) that offers a record of oceanographic and sedimentary conditions in a unique setting. The hypotheses presented in this work are intended to serve as a framework for future reconstructions based on detailed integration of lithological, paleontological, geochemical and petrophysical data.
... They indicate that the diagenetic alteration of siliceous deposits is a possible cause of slope instability along high-latitude continental margins and oceanic basin. [22]. Gas hydrate studies on the northern Cascadia margin indicate possible link between slope failure and the presence of hydrates in the seafloor sediments [23]. ...
Article
Full-text available
Integrated investigations have revealed abundant resources of gas hydrates on the northern slope of the South China Sea (SCS). Regarding the gas hydrate research of northern SCS, the gas hydrate related environment problem such as seabed landslides were also concentrated on in those areas. Based on 2D seismic data and sub-bottom profiles of the gas hydrate areas, submarine landslides in the areas of Qiongdongnan, Xisha, Shenhu, and Dongsha have been identified, characterized, and interpreted, and the geophysical characteristics of the northern SCS region investigated comprehensively. The results show 6 major landslides in the gas hydrate zone of the northern SCS and 24 landslides in the Shenhu and Dongsha slope areas of the northern SCS. The landslide zones are located mainly at water depths of 200–3000 m, and they occur on the sides of valleys on the slope, on the flanks of volcanoes, and on the uplifted steep slopes above magmatic intrusions. All landslides extend laterally towards the NE or NEE and show a close relationship to the ancient coastline and the steep terrain of the seabed. We speculate that the distribution and development of submarine landslides in this area has a close relationship with the tectonic setting and sedimentary filling characteristics of the slopes where they are located. Seismic activity is the important factor controlling the submarine landslide in Dongsha area, but the important factor controlling the submarine landslides in Shenhu area is the decomposition of natural gas hydrates.
Article
High-resolution multichannel seismic reflection profiles and multibeam mosaic maps of the seafloor are used to document the presence of two prominent regions representing major sediment failure(s) and the subsequent gravity-driven mass transport across the southwestern continental margin of Anatolia. These regions are characterized by very sugged morphology (referred to as Scars 1 and 2), where the upper slope regions include several concave, interconnected steep seafloor escarpments marked by semi-circular indentations that link with one another by cusp-like features creating a sharp and very narrow curvilinear zone. The slope face across the rugged region there are numerous sharply irregular pinnacles/protrusions on the seafloor, consisting of exposed older bedrock successions. Scars 1 and 2 occupy seafloor areas of 1947 km2 and 1350 km2, solid volumes of 214–257 km3 and 92–111 km3, and masses of 467–681 Gt and 245–294 Gt, respectively, with a total solid volume of 307–368 km3 and a mass of 812–975 Gt. Mass transport deposits are identified at various stratigraphic levels across the Rhodes Basin characterized by chaotic seismic reflector configurations with zones of contorted and convoluted reflector geometries. The base of this facies is characterized by erosional down-cutting. The thickest and the regionally most extensive such deposits are found at the base of Unit 1, immediately above the upper bounding surface of the Messinian evaporites (the Top Erosional Surface or the former M-reflector). The lower mass transport deposit (L–MTD) is calculated to have a volume 205–171 km3, or a solid mass of 543–452 Gt, assuming that porosities of 40–50% and average grain density of 2.67 t m−3. Comparisons between the total mass of the L–MTD and the estimated masses of sediments mobilized across Scars 1 and 2 (812–975 Gt) indicate that there is ~360–432 Gt deficit in the calculated mass of the L–MTD. The missing sediments represent 17.5–21.0% of the total mass contained within Unit 1 across the present-day Rhodes Basin. This mismatch is remarkably large: it may arise from the uncertainties involved in the estimations of the masses of sediments contained in Scars 1 and 2; however, it is also possible that some of the gravity driven mass transports transitioned into turbidity currents, thus travelled great distances across the Rhodes Basin, and that some of these turbidity currents crossed the basin longitudinally, and exited it at its southwestern deeper regions (i.e., the present-day Strabo Trench). This is particularly plausible because the physiography of the Rhodes Basin was dramatically different during the early Pliocene and the southern and southwestern portions of the basin provided a possible exit route.
Chapter
Gas hydrate systems in the Scotia Sea, the deep-water oceanic gateway between Antarctica and South America, exist in a unique polar setting characterized by: (1) Very low bottom water temperatures (e.g., as low as −0.5 °C at water depths of 2000-4000 m); (2) strong bottom currents sourced both from the ice stream discharge in the Weddell Sea and from the Antarctic Circumpolar Current that, combined with high sediment supply, generates giant contourite drifts; (3) high lateral variability of the geothermal gradient due to near-surface magmatism and hydrothermal activity, which is associated with a system of oceanic ridges and relict subduction arcs within the Scotia plate. In this area, the base of the gas hydrate stability zone as inferred from a gas hydrate-related bottom simulating reflector adapt to this changing environment, producing a peculiar association with gas-related structures in the subsurface. Abstract 9
Article
Full-text available
The architecture of subsurface magma plumbing systems influences a variety of igneous processes, including the physiochemical evolution of magma and extrusion sites. Seismic reflection data provides a unique opportunity to image and analyze these subvolcanic systems in three dimensions and has arguably revolutionized our understanding of magma emplacement. In particular, the observation of (1) interconnected sills, (2) transgressive sill limbs, and (3) magma flow indicators in seismic data suggest that sill complexes can facilitate significant lateral (tens to hundreds of kilometers) and vertical (<5 km) magma transport. However, it is often difficult to determine the validity of seismic interpretations of igneous features because they are rarely drilled, and our ability to compare seismically imaged features to potential field analogues is hampered by the limited resolution of seismic data. Here we use field observations to constrain a series of novel seismic forward models that examine how different sill morphologies may be expressed in seismic data. By varying the geologic architecture (e.g., host-rock lithology and intrusion thickness) and seismic properties (e.g., frequency), the models demonstrate that seismic amplitude variations and reflection configurations can be used to constrain intrusion geometry. However, our results also highlight that stratigraphic reflections can interfere with reflections generated at the intrusive contacts, and may thus produce seismic artifacts that could be misinterpreted as real features. This study emphasizes the value of seismic data to understanding magmatic systems and demonstrates the role that synthetic seismic forward modeling can play in bridging the gap between seismic data and field observations.
Article
Full-text available
RÉSUMÉ Perpétrations à l'équilibre thermodynamique de sédiments chargés d'hydrates peuvent induire la dissociation des hydrates gazeux et libérer de larges quantités d'eau et de méthane gazeux. Cette eau et ce gaz ainsi produits provoqueront, en fonction des sédiments environnants, une augmentation de pression des pores, une expansion volumétrique, et/ou un fluide permettant aux gaz de s'échapper, ce qui a pour effet de diminuer la stabilité du sol. Ce papier analyse les résultats des tests en laboratoire, tests utilisant un modèle physique à petite échelle de sols sous-marins avec inclusions d'hydrate. Les sols sont modélisés par la Laponite, une argile synthétique qui enfle dans l'eau et produit alors un gel thixotropic incolore et clair. Les inclusions d'hydrate disposées en strates et en nodules sont créées à partir d'un réfrigérant synthétique (R-11) d'hydrates. Il a été démontré que les hydrates R-11 ont des propriétés structurelles similaires à celles qui se produisent naturellement pour les hydrates de méthane. L'objectif de cette expérimentation est d'observer le chemin suivi par le fluide permettant aux gaz de s'échapper et le plan de glissement qui s'ensuit dû a la dissociation des hydrates R-11. Ces glissements sont observés par une caméra à haute vitesse et haute résolution. ABSTRACT Perpetrations to the thermodynamic equilibrium of hydrate-laden sediments can induce gas hydrate dissociation and result in the release of large quantities water and methane gas. Depending on the surrounding sediments, the produced gas and water will cause increased pore pressures, volumetric expansion, and/or fluid escape structures all of which have the effect of reducing the soil stability. This paper examines the results from laboratory tests performed using a physical, small-scale model of submarine soils with hydrate inclusions. The soils are modeled using Laponite, a synthetic clay which swells to produce a clear, colorless thixotropic gel when dispersed in water. Hydrate inclusions in the form of layers and nodules are created from R-11 refrigerant. R-11 hydrates, which form at low temperatures and atmospheric pressures, have been shown to have similar structural properties to naturally occurring methane hydrates. The objective of the experimental program is to observe the path of the fluid escape structures and the subsequent slip plane that develops due to dissociation of the R-11 hydrate. The slopes are examined using high speed, high resolution imaging.
Article
Full-text available
Ona Basin is a small intra-oceanic basin located in the southwestern corner of the Scotia Sea. This region is crucial for an understanding of the early phases of opening of Drake Passage, since it may contain the oldest oceanic crust of the entire western Scotia Sea, where conflicting age differences from Eocene to Oligocene have been proposed to date. The precise timing of the gateway opening between the Pacific and Atlantic oceans, moreover, has significant paleoceanographic and global implications. Two sub-basins are identified in this region, the eastern and western Ona basins, separated by the submarine relief of the Ona High. A dense geophysical data set collected during the last two decades is analyzed here. The data include multichannel seismic reflection profiles, and magnetic and gravimetric data.
Article
Full-text available
Natural gas hydrates occur worldwide in polar regions, normally associated with onshore and offshore permafrost, and in sediment of outer continental and insular margins. The total amount of methane in gas hydrates likely exceeds 1019 g of methane carbon. Three aspects of gas hydrates are important: their fossil fuel resource potential, their role as a submarine geohazard, and their effects on global climate change. Because gas hydrates represent a large amount of methane within 2000 m of the Earth's surface, they are considered to be an unconventional, unproven source of fossil fuel. Because gas hydrates are metastable, changes of pressure and temperature affect their stability. Destabilized gas hydrates beneath the seafloor lead to geologic hazards such as submarine slumps and slides, examples of which are found worldwide. Destabilized gas hydrates may also affect climate through the release of methane, a “greenhouse” gas, which may enhance global warming and be a factor in global climate change.
Article
Full-text available
A voluminous magmatic complex was emplaced in the Vøring and Møre basins during Paleocene/ Eocene continental rifting and break-up in the NE Atlantic. This intrusive event has had a significant impact on deformation, source-rock maturation and fluid flow in the basins. Intrusive complexes and associated hydrothermal vent complexes have been mapped on a regional 2D seismic dataset (c.150 000 km) and on one large 3D survey. The extent of the sill complex is at least 80 000 km2, with an estimated total volume of 0.9 to 2.8 × 104 km3. The sheet intrusions are saucer-shaped in undeformed basin segments. The widths of the saucers become larger with increasing emplacement depth. More varied intrusion geometries are found in structured basin segments. Some 734 hydrothermal vent complexes have been identified, although it is estimated that 2-3000 vent complexes are present in the basins. The vent complexes are located above sills and were formed as a direct consequence of the intrusive event by explosive eruption of gases, liquids and sediments, forming up to 11 km wide craters at the seafloor. The largest vent complexes are found in basin segments with deep sills (3-9km palaeodepth). Mounds and seismic seep anomalies located above the hydrothermal vent complexes suggest that the vent complexes have been re-used for vertical fluid migration long after their formation. The intrusive event mainly took place just prior to, or during, the initial phase of massive break-up volcanism (55.0-55.8Ma). There is also evidence for a minor Upper Paleocene volcanic event documented by the presence of 20 vent complexes terminating in the Upper Paleocene sequence and the local presence of extrusive volcanic rocks within the Paleocene sequence.
Article
New seismic profiles, bathymetric data and sediment-rock sampling document for the first time the discovery of hydrothermal vent complexes and volcanic cones at 4800-5200 m depth related to recent volcanic and intrusive activity in an unexplored area of the Canary Basin (Eastern Atlantic Ocean, 500 km west of the Canary Islands). A complex of sill intrusions is imaged on seismic profiles showing saucer-shaped, parallel or inclined geometries. Three main types of structures are related to these intrusions. Type I consists of cone-shaped depressions developed above inclined sills interpreted as hydrothermal vents. Type II is the most abundant and is represented by isolated or clustered hydrothermal domes bounded by faults rooted at the tips of saucer-shaped sills. Domes are interpreted as seabed expressions of reservoirs of CH4- and CO2-rich fluids formed by degassing and contact metamorphism of organic-rich sediments around sill intrusions. Type III are hydrothermal-volcanic complexes originated above stratified or branched inclined sills connected by a chimney to the seabed volcanic edifice. Parallel sills sourced from the magmatic chimney formed also domes surrounding the volcanic cones. Core and dredges revealed that these volcanoes, which must be among the deepest in the world, are constituted by OIB-type, basanites with an outer ring of blue-green hydrothermal Al-rich smectite muds. Magmatic activity is dated, based on lava samples, at 0.78±0.05 and 1.61±0.09 Ma (K/Ar methods) and on tephra layers within cores at 25-237 ky. The Subvent hydrothermal-volcanic complex constitutes the first modern system reported in deep-water oceanic basins related to intraplate hotspot activity.
Chapter
The potential of gas hydrate systems to play a role in submarine slope failure has been well-documented since the late 1970s. Several conceptual models exist for how the gas hydrate-free gas system might weaken submarine sediments, but there is no definitive evidence for gas hydrate-related processes being the primary cause of a particular submarine slope failure. We present a review of coincident gas hydrates and submarine slope instabilities on New Zealand’s active margins. The examples we show represent different failure modes in a range of slope environments, including the upper continental slope and tectonic ridges, with the common factor being that the base of gas hydrate stability approaches the seafloor in these regions. We synthesise several proposed sediment weakening mechanisms and draw comparisons to other global models for gas hydrate-related slope instability. This contribution highlights diverse influences that gas hydrate systems could have on submarine sediment strength, while acknowledging gaps in our understanding of the potential role of gas hydrates, free gas and fluid flow on slope stability.
Article
Geophysical traverses across flat or nearly flat mud bottom at depths between 2000 and 5500 meters off western Africa permitted the making of extensive shallow-penetration recordings at 3.5 kHz. The recordings reveal the common presence of alternating dark and light triangular features whose internal structure and acoustic properties may be due to local centers of cementation by gas hydrates, or clathrates. Probably the features are widely distributed in the fine-grained sediments of continental rises and abyssal plains of the world ocean, but are not detected by the usual echo sounding at 12 kHz or seismic reflection profiling at 20 to 100 Hz.
Article
The southern margin of the Scotia Sea hosts the convergent boundary between the Scotia and Antarctic plates where a number of small basins are sitated. Mass transport deposits (MTDs) within two of these small basins, Dove and Scan basins, reveal the importance of seismicity, slope instabilities and depositional processes in their growth patterns. Swath-bathymetry and very high-resolution seismic data show that there are over 200 MTDs in these basins in the last 100 ky record. MTD characterizations are determined on the basis of their regional distribution, shape, apparent size and depth. Their sedimentary and tectonic implications are discussed, as well as the evidence of different triggering mechanisms in this region, which is characterized at present by moderate-to-high magnitude, shallow to intermediate earthquakes. MTDs are more abundant in Dove Basin (with lenticular and wedge shapes), suggesting that this basin was affected by active tectonics to a greater degree than Scan Basin. This finding is significant in the overall evolutionary context of the Scotia Sea region and Scotia-Antarctic plate geodynamics. Nevertheless, other factors —volcanic activity, vigorous bottom-currents, and/or higher sedimentation rates — must also be considered for the generation of MTDs in the Scan Basin, where a variety of processes generated more diverse MTD morphologies. Paleoseismological estimations of the repeated occurrence of wedge shaped MTDs in contact with fault scarps point to potential sources of large magnitude (Mw ~ 7.2-7.3) paleoearthquakes in several sites, in agreement with the present high magnitudes of regional seismicity. This study shows MTDs to be appropriate as paleoearthquake indicators in active tectonic settings. The distribution of MTDs in the southern Scotia Sea has important implications for geodynamic and geohazard research. They may prove to be unmistakable stratigraphic markers for future basin analysis.
Article
The N-S trending Scan Basin is the easternmost deep basin north of the South Scotia Ridge, which is a geologically complex structural elevation that hosts the strike-slip boundary between the Scotia and Antarctic plates. We characterized the main morpho-structural features of the basin by analyzing the available multichannel seismic reflection profiles. The reconstruction of the seismo-stratigraphy reveals the growth patterns of the Scan Basin. Seismic data and gravity modeling support the interpretation that the basin is mainly floored by oceanic crust, however its northern and southern provinces exhibit different seismic attributes. Stratigraphic calibrations with adjacent regions together with the distribution of sedimentary units indicate that this basin was formed by rifting processes and subsequent spreading accretion from the Oligocene to the Miocene. This age attribution suggests that the Scan Basin might be one of the oldest oceanic basins of the southern Scotia Sea—possibly coeval with the Eocene-Oligocene opening of the Drake Passage. The basin is the most direct connection between the Weddell Sea and the Scotia Sea, whereas the stratigraphic features reveal the occurrence of major paleoceanographic changes. The initial phases of the evolution were influenced by mass-transport and turbidite processes of sediment supply from the nearby continental margins of the eastern tip of the Antarctic Peninsula. From the Middle Miocene to the Present-day, the eastward motion of the basin due to plate tectonic and the connection with the Weddell Sea through gateways enabled instauration of the overflow of Weddell Sea Deep Water (WSDW) into the Scan Basin. The WSDW forced the northward migration of the Circumpolar Deep Water (CDW) and became progressively dominant, controlling depositional patterns. The results that we report here should prove essential for understanding the formation of the Scotia Sea, the beginning of the Scotia Arc fragmentation, and the increasing role played by Weddell Sea/Scotia Sea water-mass exchange.
Article
The central Scotia Sea, located between the South American and Antarctic plates, is an inte- gral part of the marine conduit that permits eastward deep-water flow from the Pacific Ocean to the Atlantic Ocean. The geologic history of the central Scotia Sea is therefore critical for a full understanding of the initiation and subsequent evolution of the complete, deep Antarctic Circumpolar Current, widely believed to have been a key factor in the history of Antarctic glaciation. Here, we present new evidence on the nature and age of the central Scotia Sea floor. Multibeam surveys and the first dredged samples indicate that a now-submerged remnant volcanic arc may have formed a barrier to deep eastward oceanic circulation until after the mid-Miocene climatic optimum. Inception and development of a full deep Antarctic Circum- polar Current may therefore have been important, not in the drop in global temperatures at the Eocene-Oligocene boundary as long surmised, but in the subsequent late Miocene global cooling and intensification of Antarctic glaciation.
Article
Three classes of bottom simulating reflectors (BSR) cross-cut the post-breakup sediments of the mid-Norwegian margin. The first class is caused by free gas at the base of the pressure- and temperature-dependent gas hydrate stability zone. The second class of BSR is caused by the diagenetic transition from opal A to opal CT. The third class of BSR is always observed underneath the opal A/opal CT transition, but heat flow data and the amplitude characteristics of this arrival exclude one of the known silicate diagenetic transitions or gas hydrates as the explanation for this reflector. ODP Site 643 drilling results from the Vøring Plateau suggest two possible processes as the reason for this third BSR: (a) smectite illite conversion or (b) a sudden increase in the abundance of authigenic carbonates. The genesis of both is pressure- and temperature-dependent and could potentially result in a cross-cutting seismic reflector. The data are not conclusive as to which process is causing the third class of observed BSR.
Article
Polygonal fault arrays have been documented in sedimentary basins from around the world and several theories exist as to how they initiate and propagate. Three-dimensional seismic data from polygonal fault arrays from offshore Norway are used to develop a new process model for polygonal fault development. We propose that in siliceous sediment, polygonal fault arrays can be triggered thermally, due to the conversion of opal-A to opal-CT at depths of 100–1000m. This conversion causes differential compaction and shear failure and therefore fault initiation. The location of the earliest faults is dependent on where opal-A to opal-CT conversion and compaction occur first. This is controlled by which strata have a favourable bed composition, local fluid chemistry and temperature or because the strata reach the depth of the reaction front first due to the presence of pre-existing structural relief (folds or faults). Subsidence of biosiliceous sediment through the opal-A to opal-CT reaction front causes fault propagation because of continued localised differential compaction. Fault initiation and propagation due to silica conversion generate polygonal fault arrays at significantly deeper burial depths than previously thought possible.
Book
This paper is part of the special publication Gas hydrates: relevance to world margin stability and climatic change (eds J.P. Henriet and J. Mienert). Reflection tomography techniques have been applied to two multi-channel seismic profiles, acquired across the accretionary prism of the South Shetland margin, in order to reconstruct the velocity field associated with gas hydrate and free gas layers in the sedimentary sequence. Data show the presence of a strong bottom simulating reflector (BSR), running along the slope in water depths ranging from 1000 to 4600 m, locally underlain by a weak normal polarity reflector about 80 ms deeper in the section. The analysis indicates a velocity trend from the sea floor to the BSR generally consistent with that of normally compacted marine sediments, with an abrupt decrement between the BSR and the underlying reflector, indicating the presence of free gas in the sediment pore spaces. The calculated thickness of this gas-bearing layer is approximately 50 m. Local increments of tomographic velocity above the BSR can be related either to gas hydrate abundances in normally compacted slope basin sediments or to overcompaction in accreted sediments, as imaged by the pre-stack depth migrated sections. We conclude that clathrates and free gas distribution on the South Shetland continental slope are strongly controlled by the structural setting of the accretionary prism, where faults act as conduits for migration of natural gas towards the surface. A brief description of the adopted tomography method is also presented.
Article
Weddell Sea Deep Water influences the thermohaline circulation of the world ocean directly as a component of the deep western boundary current in the South Atlantic Ocean and indirectly by cooling and freshening Circumpolar Deep Water. Because it is filled with recently ventilated Weddell Sea Deep Water, the Scotia Sea is important to both influences. The main component of the abyssal waters renewing most of the world oceans via deep boundary currents is the Circumpolar Deep Water of the Antarctic Circumpolar Current. Weddell Sea Deep Water is recognized as the main source of cold, fresh waters to Circumpolar Deep Water, and we show that Weddell Sea Deep Water is incorporated into the Antarctic Circumpolar Current within the Scotia Sea. As a result of this ventilation, the Scotia Sea provides an effective link between the deep waters of the Weddell Sea and the rest of the world abyssal ocean. Some of the Weddell Sea Deep Water filling the Scotia Sea leaves as a westward flow via the southern Drake Passage. Weddell Sea Deep Water also enters the Georgia Basin directly from the Scotia Sea and flows beneath the Antarctic Circumpolar Current to contribute to the deep western boundary current of the Argentine Basin. In most previous studies, a deep spreading route from the Weddell Sea over the South Sandwich Trench east of the Scotia Sea had been considered the only source of Weddell Sea Deep Water for this deep western boundary current.
Article
Editor: R.D. van der Hilst Keywords: Scotia–Antarctica plate boundary seismic profiles fault-plane solutions tectonic development A compilation of available multichannel seismic profiles acquired along the southern margin of the Scotia Sea east of the South Orkney microcontinent has allowed identifying and mapping the main morphological and structural features of the central segment of the Scotia–Antarctica plate boundary. This margin is composed by several bathymetric highs of variable size and uncertain crustal nature, separated by deep troughs and restricted oceanic basins. Some of these troughs represent pull-apart basins. Three main segments oriented WNW–ESE (the western sector), ENE–WSW (the central sector, here named Bruce Deep), and NE–SW (the eastern sector), have been described. These segments are separated by NNW–SSE-trending release zones, disposed in an en-echelon geometry, which represent mostly strike–slip faults. The western segment corresponds to the northern margin of the South Orkney microcontinent, where a subduction zone seems to be present, even if its present-day activity is unclear. The segment further to the east corresponds to an ENE-oriented basin (Bruce Deep), which separates the Bruce Bank from the eastern promontory of the South Orkney continental platform. To the south of the Bruce Deep, a wide deformation zone with N-verging folds and thrusts (here named Jane Thrust Belt), has been identified from seismic data. The eastern segment of the plate boundary is structurally the less constrained, and may be composed by a series of tectonic lineaments of different lengths. From the Bruce Bank to the east, focal mechanisms maintain a prevalent left-lateral strike–slip motion combined with an extensional component. In this sector, earthquakes are located in a 150 km wide area and on a local scale are difficult to follow unambiguously at the plate boundary. Lithologic analyses on dredged material recovered along a flank of one of the morphological relieves present south of the Discovery Bank to 35°W (here collectively named Irizar Highs), yielded a dominant granitic composition. A similar composition characterizes the rocks collected in the southern flank of the south-easternmost Jane Bank. This suggests a continental crust nature for these bathymetric highs, now dispersed along this sector of the Scotia–Antarctica plate boundary. We propose here a tectonic evolution for this margin, dominated since the Early Miocene by the northward subduction of the Weddell Sea oceanic crust. The development of a dextral, en-echelon transform fault system facilitated the process of fragmentation and dispersion of the crustal blocks, dismembered the subduction zone, and possibly inverted the direction of convergence: Therefore, the Scotia plate would subduct beneath the Antarctic plate, in the western sector, and Weddell Sea would subduct beneath Scotia plate, in the eastern sector. Finally, the activation of left-lateral transtensional strike–slip lineaments generated narrow pull-apart basins in the fore-arc sectors of the convergent zones.
Article
Diatom ooze and diatomaceous mudstone overlie terrigenous mudstone beds at Leg 19 Deep Sea Drilling Project sites. The diatomaceous units are 300-725 m thick but most commonly are about 600 m. Diagenesis of diatom frustules follows a predictable series of physical and chemical changes that are related primarily to temperature (depth of burial and local geothermal gradient). During the first 300-400 m of burial frustules are fragmented and undergo mild dissolution. By 600 m dissolution of opal-A (biogenic silica) is widespread. Silica reprecipitates abundantly as inorganic opal-A between 600 and 700 m sub-bottom depth. Inorganic opal-A is rapidly transformed by crystal growth to opal-CT. The result is formation of silica cemented mudstone and porcelanite beds.
Article
Analysis of physical properties measured on cores and on discrete samples collected by the Ocean Drilling Programme (ODP) Leg 178 on the Pacific margin of the Antarctic Peninsula reveals anomalous down-hole curves of porosity, density, water content, and P-wave velocity. These indicate an overall trend of increasing porosity with depth and suggest that the drifts are mostly undercompacted. In one of the two boreholes analysed, a sharp decrease in porosity, matching increasing bulk sediment density and increasing compressional velocity occurs towards the base of the hole, which corresponds to a bottom-simulating reflector in the seismic section. Analysis of seismic reflection, down-hole logging, geotechnical and mineralogical data from two drilling sites indicates that the observed anomalous consolidation trends are a consequence of the presence of biogenic silica (diatom and radiolarian skeletons) even with a small to moderate amount. Above the bottom-simulating reflector, intergranular contacts among whole or broken siliceous microfossils prevent normal sediment consolidation. Diagenetic alteration of biogenic opal-A to opal-CT causes a dramatic reduction of intra- and interskeletal porosity allowing sediments to consolidate at depth. This results in overpressuring and a decrease in the effective stress. Excess fluids are expelled towards the sediment surface through near vertical, small throw normal faults extending from the diagenetic front to the seafloor and affecting the stability of the submarine slope in the form of gravitational creep along a weakened surface. This work shows how physical properties of shallow fine-grained marine sediments can be analysed as basin-wide indicators of biogenic silica abundance. The diagenetic alteration of siliceous microfossils is a possible cause of slope instability along world continental margins where bottom-simulating reflectors related to silica diagenesis are present at a regional scale.
Article
Processing and interpretation of a grid of intermediate-resolution multichannel seismic reflection profiles collected on the NE sector of the South Shetland continental margin, allowed us to map the lateral extent of a Bottom Simulating Reflector (BSR). The margin, an accretionary wedge located off the northern tip of the Antarctic Peninsula, consists of two distinct and superimposed tectonic regimes: an older regime is related to Mesozoic–Middle Cenozoic subduction-related tectonism; a younger one is associated with a mainly extensional tectonic phase, and related to the Oligocene development of the western Scotia Sea. The occurrence of the BSR appears to be controlled by the geological structure of the margin. The BSR lacks continuity near basement structures, main geological discontinuities and faults. On the other hand, the amplitude and continuity of the BSR are not affected by the presence of folded structures and undeformed sedimentary layering. We found that the BSR is mostly confined to the NE sector of the South Shetland Margin, where propagation of faulting associated with the Shackleton Fracture Zone may have driven migration of natural gas towards the surface and created the conditions for a BSR to appear. The application of reflection tomography techniques allowed us to reconstruct the averaged seismic velocity field between the seafloor and BSR in order to map the depth of BSR. By averaging the observed velocity structure above and below the BSR, and applying a theoretical model of elastic wave propagation in porous media, we attempted as rigorous a quantitative assessment as possible of the natural gas present as gas hydrate above the BSR and as free gas between the BSR and the Base of Gas Reflector (BGR).
Article
The distribution of seismic units in deposits of the basins near the Antarctic–Scotia plate boundary is described based on the analysis of multichannel seismic reflection profiles. Five main seismic units are identified. The units are bounded by high-amplitude continuous reflectors, named a to d from top to bottom. The two older units are of different age and seismic facies in each basin and were generally deposited during active rifting and seafloor spreading. The three youngest units (3 to 1) exhibit, in contrast, rather similar seismic facies and can be correlated at a regional scale. The deposits are types of contourite drift that resulted from the interplay between the northeastward flow of Weddell Sea Bottom Water (WSBW) and the complex bathymetry in the northern Weddell Sea, and from the influence of the Antarctic Circumpolar Current and the WSBW in the Scotia Sea. A major paleoceanographic event was recorded by Reflector c, during the Middle Miocene, which represents the connection between the Scotia Sea and the Weddell Sea after the opening of Jane Basin. Unit 3 (tentatively dated ∼Middle to Late Miocene) shows the initial incursions of the WSBW into the Scotia Sea, which influenced a northward progradational pattern, in contrast to the underlying deposits. The age attributed to Reflector b is coincident with the end of spreading at the West Scotia Ridge (∼6.4Ma). Unit 2 (dated ∼Late Miocene to Early Pliocene) includes abundant high-energy, sheeted deposits in the northern Weddell Sea, which may reflect a higher production of WSBW as a result of the advance of the West Antarctic ice-sheet onto the continental shelf. Reflector a represents the last major regional paleoceanographic change. The timing of this event (∼3.5–3.8Ma) coincides with the end of spreading at the Phoenix–Antarctic Ridge, but may be also correlated with global events such as initiation of the permanent Northern Hemisphere ice-sheet and a major sea level drop. Unit 1 (dated ∼Late Pliocene to Recent) is characterized by abundant chaotic, high-energy sheeted deposits, in addition to a variety of contourites, which suggest intensified deep-water production. Units 1 and 2 show, in addition, a cyclic pattern, more abundant wavy deposits and the development of internal unconformities, all of which attest to alternating periods of increased bottom current energy.
Article
Multibeam echosounder data and TOPAS seismic reflection profiles collected during the AntPac 1997, Scan 2004, and Scan 2008 cruises aboard the RV Hespérides reveal a host of surficial geomorphological features as yet poorly investigated in the Scan Basin, south-central Scotia Sea. This area represents one of the deep gateways between the Weddell Sea and the Scotia Sea, since it enables the northward flow of a branch of the Weddell Sea Deep Water (WSDW). Analysis of the data identifies numerous elongated depressions interpreted as furrows in the southernmost sector of the basin. These furrows show two main trends, i.e., either N–NNW parallel to, or NE oblique to regional bathymetric contours. These trends plausibly reflect a tectonic influence on the bottom-flow distribution, conditioned by a set of recent, conjugate strike-slip faults that developed on the seafloor under dominant NNE–SSW compression and orthogonal extension. The furrows exhibit distinct geomorphological patterns at either side of the basin, which can be related to west–east asymmetry in the WSDW flow direction. Consistent with existing knowledge of regional WSDW dynamics, northward WSDW overflows would be channeled along the western part of the basin at higher bottom-current velocities, thereby generating more erosional-type furrows that are straighter, more elongated, and have more abrupt sidewalls than their eastern counterparts. In contrast, weaker southward WSDW would flow along the eastern part of the basin, resulting in more depositional-type furrows that are more curved, less elongated, and have gentler sidewalls.
Article
New swath bathymetry with multichannel and high resolution seismic profiles shows a variety of contourite drift, sediment wave morphologies, and seismic facies in the central Scotia Sea. The deposits are to be found at the confluence between the two most important bottom current flows in the southern ocean: the eastward flowing Antarctic Circumpolar Current (ACC) and the northward outflow of the Weddell Sea Deep Water (WSDW). The contourite drifts are wedge-like deposits up to 1 km thick, that exhibit aggradational reflectors along axis thinning towards the margins. The contourite drifts occur in areas of weaker flows along the margins of contourite channels and in areas protected by obstacles. The elongate-mounded drifts are best developed along the left-hand margins of channelized bottom current flows, due to the Coriolis force. A contourite fan has a main channel and two distributary channels that expand over a gentle seafloor. The proximal fan exhibits sediment waves with the distal fan incised by furrows. Sediment wave fields are well developed in areas of intensified bottom flows without channels, particularly at the confluence of the ACC and the WSDW. Small sediment waves occur where unidirectional bottom current flows predominate. Sediment waves may develop under the influence of internal waves produced by the interaction of the flows and sea-bottom relief. The stratigraphic sequence above the oceanic crust of Early to Middle Miocene age contains six seismic units separated by major reflectors. All the units were shaped under the influence of strong bottom current flows, although they exhibit distinct seismic facies changes that record the variations of the bottom current pathways over time. The age of the units was calculated based on the age of the oceanic crust and sedimentation rates of deep-sea deposits in the region. The oldest, Units VI–IV, are of Early to Middle Miocene age and developed under the influence of the ACC. They are characterized by a southward progradational pattern of the seismic units and sedimentation rates of 5–8 cm/ky. Unit III, with an estimated Middle Miocene age, evidences the first incursion of WSDW into the central Scotia Sea, when plate movement caused openings in the South Scotia Ridge and allowed the connection with the northern Weddell Sea through Jane Basin and gaps in the ridge. Unit II, estimated to be of Late Miocene to Early Pliocene age, extends over the area and is characterized by internal unconformities. A major unconformity at the base of Unit II records an important reorganization of bottom current flows that may predate the onset of grounded ice sheets on the Antarctic Peninsula shelf. Unit I, of Late Pliocene to Quaternary age, shows intensified bottom currents. The unconformity at the base of Unit I probably predates the onset of major Northern Hemisphere glaciations and the greater expansion of Antarctic ice sheets during the Late Pliocene. The extensive distribution of contourite deposits above the oceanic crust testifies to the long-term production of Antarctic Bottom Water. Cold, deep water was swept northward from the Weddell Gyre, interacting with the ACC, and possibly exerting profound influences on the global circulation system and the onset of major glaciations.
Article
New multibeam (swath) bathymetric sonar data acquired using an EM120 system on the RRS James Clark Ross, supplemented by sub-bottom profiling, reveals the underwater morphology of a ∼ 12,000 km2 area in the northern part of the mainly submarine South Sandwich volcanic arc. The new data extend between 55° 45′S and 57° 20′S and include Protector Shoal and the areas around Zavodovski, Visokoi and the Candlemas islands groups. Each of these areas is a discrete volcanic center. The entirely submarine Protector Shoal area, close to the northern limit of the arc, forms a 55 km long east–west-trending seamount chain that is at least partly of silicic composition. The seamounts are comparable to small subaerial stratovolcanoes in size, with volumes up to 83 km3, indicating that they are the product of multiple eruptions over extended periods. Zavodovski, Visokoi and the Candlemas island group are the summits of three 3–3.5 km high volcanic edifices. The bathymetric data show evidence for relationships between constructional volcanic features, including migrating volcanic centers, structurally controlled constructional ridges, satellite lava flows and domes, and mass wasting of the edifices. Mass wasting takes place mainly by strong erosion at sea level, and dispersal of this material along chutes, probably as turbidity currents and other mass flows that deposit in extensive sediment wave fields. Large scale mass wasting structures include movement of unconsolidated debris in slides, slumps and debris avalanches. Volcanism is migrating westward relative to the underlying plate and major volcanoes are asymmetrical, being steep with abundant recent volcanism on their western flanks, and gently sloping with extinct, eroded volcanic sequences to their east. This is consistent with the calculated rate of subduction erosion of the fore-arc.
Article
A survey of the current field over the South Scotia Ridge, obtained with a lowered Acoustic Doppler Current Profiler (LADCP), is presented. There is a pattern of northward (southward) flow on the western (eastern) side of each of four deep passages in the ridge, which is supported by tracer measurements. The net full-depth LADCP-referenced geostrophic transport over the ridge is 22±7 Sv (1 Sv=106 m3 s−1) northward, with the jets on either side of the passages transporting 5–10 Sv in alternating directions. The corresponding Weddell Sea Deep Water (WSDW) transport over the ridge is 6.7±1.7 Sv. This is a factor of 4 larger than the only previous estimate in the literature, and suggests that a significant proportion of the Antarctic Bottom Water (AABW) invading the world ocean abyss escapes the Weddell Sea via the Scotia Sea.
Article
Large volumes of greenhouse gases such as CH4 and CO2 form by contact metamorphism of organic-rich sediments in aureoles around sill intrusions in sedimentary basins. Thermogenic gas generation and dehydration reactions in shale are treated numerically in order to quantify basin-scale devolatilization. We show that aureole thicknesses, defined as the zone of elevated metamorphism relative to the background level, vary within 30–250% of the sill thickness, depending on the temperature of the host-rock and intrusion, besides the sill thickness. In shales with total organic carbon content of >5 wt.%, CH4 is the dominant volatile (85–135 kg/m3) generated through organic cracking, relative to H2O-generation from dehydration reactions (30–110 kg/m3). Even using conservative estimates of melt volumes, extrapolation of our results to the scale of sill complexes in a sedimentary basin indicates that devolatilization can have generated ∼2700–16200 Gt CH4 in the Karoo Basin (South Africa), and ∼600–3500 Gt CH4 in the Vøring and Møre basins (offshore Norway). The generation of volatiles is occurring on a time-scale of 10–1000 years within an aureole of a single sill, which makes the rate of sill emplacement the time-constraining factor on a basin-scale. This study demonstrates that thousands of gigatons of potent greenhouse gases like methane can be generated during emplacement of Large Igneous Provinces in sedimentary basins.
Article
In an active petroleum system the amount of trapped hydrocarbons is the difference between the volumes charged and the volumes that have leaked or are otherwise destroyed. This paper focuses on the leakage processes taking place above a hydrocarbon-filled trap and how leakage is expressed on seismic data. A variety of seismic anomalies related to hydrocarbon leakage are interpreted and illustrated.A three step workflow is suggested for hydrocarbon leakage interpretation.First, all anomalies related to hydrocarbon leakage in the study area should be observed, described and mapped. The description should focus on both simple reflection amplitude and patterns or groups of anomalies. Geographical distribution and 3D shapes should also be revealed. Second, each anomaly should be interpreted individually. This paper presents several seismic examples of leakage anomalies and their interpretations are discussed. The interpreted leakage-related anomalies imaged on seismic data are subdivided into two categories: (1) permanent deformation of the primary bedding post-deposition and/or build up of new “syn-leakage” features, and (2) changes in seismic expression and/or secondary effects caused by continuous or discontinuous change in formation fluid from formation water to oil or gas. Third, genetically related leakage anomalies should be grouped into a leakage zone. The leakage zone has a root where the leakage from the reservoir initiates, a body or the zone itself where vertical movements of hydrocarbons occur and a top where the leakage terminates. Seismic data often image only parts of the leakage in the rocks and hence there may be significant differences between the real leakage zones in nature and that imaged on seismic data. The seismic observations in the leakage zone, at the top and the root may help to reveal if hydrocarbons are preserved in the underlying trap.
Evidence of a modern deep-water magmatic hydrothermal system in the Canary Basin (Eastern Central Atlantic Ocean). Geochemistry, Geophysics, Geosystems
  • T Medialdea
  • L Somoza
  • Al
MEDIALDEA, T., SOMOZA, L. ET AL. 2017. Evidence of a modern deep-water magmatic hydrothermal system in the Canary Basin (Eastern Central Atlantic Ocean). Geochemistry, Geophysics, Geosystems, 18, 3138-3164, https://doi.org/10.1002/2017GC006889
Tectonic development, sedimentation and paleoceanography of the Scan Basin (southern Scotia Sea, Antarctica). Global and Planetary Change
  • L F Pérez
  • E Lodolo
  • Al
PÉREZ, L.F., LODOLO, E. ET AL. 2014. Tectonic development, sedimentation and paleoceanography of the Scan Basin (southern Scotia Sea, Antarctica). Global and Planetary Change, 123, 344-358.
Are buried hydrothermal systems fueling sub-seafloor gas hydrate mounds in deep-water sub-polar oceanic basins? Examples from the Scotia and Weddell Sea
  • Somozal
  • Medialdeat
  • . J Gonzálezf
  • Maldonadoa
  • Somoza L.
SOMOZA, L., MEDIALDEA, T., GONZÁLEZ, F.J. & MALDONADO, A. 2016. Are buried hydrothermal systems fueling sub-seafloor gas hydrate mounds in deep-water subpolar oceanic basins? Examples from the Scotia and Weddell Sea, Antarctica. In: MIENERT, J. (ed.) 13th International Conference on Gas in Marine Sediments GIMS13, 19-22 September 2016, Tromso, Norway. Abstracts Book Part 1, 45.
Gas hydrate dissociation structures in submarine slopes
  • I Gidley
  • J Grozic
  • J Locat
  • D Perret
  • D Turmel
  • D Demers
  • S Leroueil
GIDLEY, I. & GROZIC, J. 2008. Gas hydrate dissociation structures in submarine slopes. In: LOCAT, J., PERRET, D., TURMEL, D., DEMERS, D. & LEROUEIL, S. (eds) Proceedings of the 4th Canadian Conference on Geohazards: From Causes to Management. Presse de l'Université Laval, Québec, Canada, 81-88.
Seismic characteristics and distribution of volcanic intrusions and hydrothermal vent complexes in the Vøring and Møre basins
  • Plankes
  • Rasmussent
  • . S Reys
  • Maklebustr
  • Planke S.
Gas hydrate dissociation structures in submarine slopes
  • Gidleyi
  • Grozicj
  • Gidley I.