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Abstract

The increasing volume of high-resolution multibeam bathymetry data collected along continental margins and adjacent deep seafloor regions is providing further opportunities to study new morphological seafloor features in deep water environments. In this paper, seafloor mounds have been imaged in detail with multibeam echosounders and parametric sub-bottom profilers in the deep central area of the Canary Basin (~350–550 km west off El Hierro Island) between 4800 and 5200 mbsl. These features have circular to elongated shapes with heights of 10 to 250 m, diameters of 2–24 km and with flank slopes of 2–50°. Based on their morphological features and the subsurface structures these mounds have been classified into five different types of mounds that follow a linear correlation between height and slope but not between height and size. The first, second (Subgroup A), and third mound-types show heights lower than 80 m and maximum slopes of 35° with extension ranging from 2 to 400 km2 and correspond to domes formed at the surface created by intrusions located at depth that have not outcropped yet. The second (Subgroup B), fourth, and fifth mound-types show higher heights up to 250 m high, maximum slopes of 47° and sizes between 10 and 20 km2 and are related to the expulsion of hot and hydrothermal fluids and/or volcanics from extrusive deep-seated systems. Based on the constraints on their morphological and structural analyses, we suggest that morphostructural types of mounds are intimately linked to a specific origin that leaves its footprint in the morphology of the mounds. We propose a growth model for the five morphostructural types of mounds where different intrusive and extrusive phenomena represent the dominant mechanisms for mound growth evolution. These structures are also affected by tectonics (bulge-like structures clearly deformed by faulting) and mass movements (slide scars and mass transport deposits). In this work, we report how intrusive and extrusive processes may affect the seafloor morphology, identifying a new type of geomorphological feature as ‘intrusive’ domes that have, to date, only been reported in fossil environments but might extend to other oceanic areas.

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... Here, an intensive research has been carried out in fields such as the Quaternary or current volcanic and hydrothermal activity (Klügel et al., 2020;van den Bogaard, 2013;Medialdea et al., 2017;Somoza et al., 2017), gravitational instabilities and mass-transport deposits (MTDs) (Georgiopoulou et al., 2009(Georgiopoulou et al., , 2010Hunt et al., 2011Hunt et al., , 2013Hunt et al., , 2014Palomino et al., 2016;León et al., 2019), the relationship between MTDs and volcanic activity (Hunt et al., 2014;Hunt and Jarvis, 2017;León et al., 2019) and the presence of critical metallic elements (Marino et al., 2017). However, only a few of these studies have focused on the relationship between tectonics, volcanic activity and seafloor morphology Sánchez-Guillamón et al., 2018a, 2018b. ...
... The magnetic anomalies derived from the Atlantic spreading fabric outline a regional trend of WNW-ESE oceanic fracture zones (Klitgord and Schouten, 1986;Roest et al., 1992) with depressions and highs. Numerous volcanic seamounts related to recent magmatic and hydrothermal activity are reported on the Western Canary Slope (WCS) and the distal part of the Madeira Distributary Channel System (MDCS) (Fig. 1B) Sánchez-Guillamón et al., 2018a, 2018b. ...
... This long tectonic history has built specific geomorphological features and domains in the eastern Canary Basin, which is characterized by two main geomorphological features: (i) the smooth nature of its slope gradient (0.1 • -6 • ) and the presence of large MTDs coming from the CIVP and the African continental margin (Hunt et al., 2013(Hunt et al., , 2014Krastel et al., 2001;León et al., 2019;Masson et al., 1998Masson et al., , 2002Talling et al., 2007;Urgeles et al., 2001;Wynn et al., 2000); and (ii) the presence of seamounts (>1000 m high; IHO, 2013), hills (<1000 m high; IHO, 2013) and mounds (<500 m high; IHO, 2013), on a seafloor with slopes of up to 25 • Palomino et al., 2016;Sánchez-Guillamón et al., 2018b;van den Bogaard, 2013) (Fig. 1B). ...
Article
This paper integrates sedimentary, tectonic and volcanic geological processes inside a model of volcano-tectonic activity in oceanic intraplate domains related to rifted continental margins. The study case, the eastern Canary Basin (NE Atlantic), is one of the few places in the world where giant MDTs and Quaternary volcanic and hydrothermal edifices take place in intraplate domains. In this paper, we analyse how two structural systems (WNW-ESE and NNE-SSW) matching with the oceanic fabric control the location of volcanic systems, seafloor tectonic reliefs and subsequently the distribution of main sedimentary systems. Linear turbidite channels, debris flow lobes and the lateral continuity of structural and volcanic reliefs follow a WNW-ESE trend matching the tracks of the oceanic fracture zones. Furthermore, escarpments, anticline axes and volcanic ridges follow a NNE-SSW trend matching normal faults delimiting blocks of oceanic basement. The morpho-structural analysis of all the above geomorphological features shows evidence of a volcanic and tectonic activity from the middle–upper Miocene to the Lower–Middle Pleistocene spread over the whole of the eastern Canary Basin that reached the western Canary Islands. This reactivation changes the paradigm in the seamount province of Canary Islands reported inactive since Cretaceous. A tecto-sedimentary model is proposed for this period of time that can be applied in other intraplate domains of the world. A tectonic uplift in the study area with a thermal anomaly triggered volcanic and hydrothermal activity and the subsequent flank collapse and emplacement of mass transport deposits on the Western Canary Slope. Furthermore, this uplift reactivated the normal basement faults, both trending WNW-ESE and NNE-SSW, generating folds and faults that control the location of turbidite channels, escarpments, mass transport deposits and volcanic edifices.
... level and related to hydrothermal-volcanic activity (Medialdea et al., 2017;Sanchez-Guillamón et al., 2018). Magma injection causes differential uplifting, forced folding and faulting of the overlying sedimentary layers, and can induce the transport of hot fluids to the surface. ...
... Magma injection causes differential uplifting, forced folding and faulting of the overlying sedimentary layers, and can induce the transport of hot fluids to the surface. According to the morphostructural classification of Sanchez-Guillamón et al. (2018) based on the Canary Basin, the Comorian mound morphologies are similar to the subcircular mounds corresponding to the morphostructural type 2 (group A), in good agreement with a sill-like intrusion into the sedimentary cover (Sanchez-Guillamón et al., 2018). We identify small-scale faulting along the summit and the flanks of the mounds (Fig. 8A, B), which can be interpreted as vertical fracture pipes and hence fluid migration pathways, and forced folds near the base that may result from the elevation generated by the intrusive bodies (Sanchez-Guillamón et al., 2018). ...
... According to the morphostructural classification of Sanchez-Guillamón et al. (2018) based on the Canary Basin, the Comorian mound morphologies are similar to the subcircular mounds corresponding to the morphostructural type 2 (group A), in good agreement with a sill-like intrusion into the sedimentary cover (Sanchez-Guillamón et al., 2018). We identify small-scale faulting along the summit and the flanks of the mounds (Fig. 8A, B), which can be interpreted as vertical fracture pipes and hence fluid migration pathways, and forced folds near the base that may result from the elevation generated by the intrusive bodies (Sanchez-Guillamón et al., 2018). However, lower-resolution seismic profiles or multichannel profiles are needed to better characterize the magmatic activity and to image the internal structure of the mounds, to confirm the presence of sill intrusions and the location of hydrothermal vent complexes, as well as their age. ...
Article
A detailed morpho-bathymetric study of the Comoros archipelago, based on mostly unpublished bathymetric data, provides a first glimpse into the submarine section of these islands. It offers a complete view of the distribution of volcanic structures around the archipelago, allowing to discuss the origin and evolution of this volcanism. Numerous volcanic cones and erosional-depositional features have been recognized throughout the archipelago. The magmatic supply is focused below one or several volcanoes for each island, but is also controlled by lithospheric fractures evidenced by volcanic ridges, oriented along the supposed Lwandle-Somali plate boundary. Massive mass-wasting morphologies also mark the submarine flanks of each island. Finally, the submarine geomorphological analysis made possible to propose a new scheme for the succession of the island's growth, diverging from the east-west evolution previously described in the literature.
... Analysis of seafloor geomorphologic features can provide a hint for understanding subsurface structures and processes developed in many sedimentary basins. This is the case of seafloor domes related to igneous sills emplaced at depth along magma-rich margins (Hansen and Cartwright, 2006;Sánchez-Guillamón et al., 2018a, 2018b. These magmatic intrusions have important implications for hydrocarbon exploration (Hansen et al., 2008;Holford et al., 2012), metal mineralization (Nelson, 2000), global climate change , and basin-scale processes . ...
... These magmatic intrusions have important implications for hydrocarbon exploration (Hansen et al., 2008;Holford et al., 2012), metal mineralization (Nelson, 2000), global climate change , and basin-scale processes . Examples of sill-dome structures have been well described in the southern Australian margin , the Norwegian Sea (Planke et al., 2005;Omosanya et al., 2017), the eastern central Atlantic (Medialdea et al., 2017;Sánchez-Guillamón et al., 2018a, 2018b and other worldwide magma-rich margins. Igneous intrusions are also widely distributed among the South China Sea (SCS) basins and continental slopes (Yan et al., 2006;Song et al., 2017;Wang et al., 2019). ...
... Emplacement of igneous sills within sediments can result in the development of forced folds (Hansen and Cartwright, 2006;Jackson et al., 2013;Sun et al., 2014;Omosanya et al., 2017;Zhang et al., 2017) and/or formation of hydrothermal vent complexes Svensen et al., 2004;Planke et al., 2005;Hansen et al., 2008;Magee et al., 2015;Medialdea et al., 2017;Omosanya et al., 2018;Wang et al., 2019). These sill-related forced folds typically manifest as domes on the seafloor (Sánchez-Guillamón et al., 2018a, 2018b, and some may be overlain by younger strata, which will date the timing of intrusion event (Trude et al., 2003;Hansen and Cartwright, 2006;Jackson et al., 2013). ...
Article
Magmatism can exert significant impact on sedimentary basins such as the Zhongjiannan Basin (ZJNB), western South China Sea. We have evaluated multibeam bathymetric and multichannel seismic reflection data acquired by the Guangzhou Marine Geological Survey in recent years, in order to investigate the distribution, the characteristics and the subsurface structures related to seafloor domes found in the northeastern ZJNB. Our data revealed forty-two domes at water depths between 2312 m and 2870 m, which are clustered around volcanic mounds, large seamounts and along the edge of the central depression in the study area. These domes are generally circular to elongate or irregular in plan view with large basal areas, and they also have gentler flanks (dips of 1.46°–7.73°) with vertical reliefs ranging from tens to hundreds of meters. In seismic sections, majority of the domes are underlain by variably shaped and complex magmatic sills, which provide a cause-effect relationship between domes formation and igneous intrusions. These intrusions heat surrounding organic-rich sediments, release hydrocarbons, fluidize sediment pore waters and form gas-rich fluids, which fill in sediment and uplift overlying strata immediately above the sills to form forced folds, which are manifested as seafloor domes. These sill-folds-dome structures have important implications for understanding geomorphologic features caused by sills emplaced at depth.
... The advent of new echo-sounding techniques is improving traditional geomorphometric techniques, Finally, we have used these quantitative characteristics to establish a morphometric growth model that includes the complexity of size and shape of the different seafloor edifices. [42]. The highlighted mounds are categorized according to their origin following [41]. ...
... Nevertheless, in the central area of this basin, known as the Subvent Area, these seafloor mounds are hydrothermal domes and scattered volcanoes related to Quaternary intrusive activity that gave rise to a huge magmatic sill complex together with volcanic activity [41]. Indeed, [42]. The highlighted mounds are categorized according to their origin following [41]. ...
... The Canary Basin is located in an intraplate setting over Jurassic to present oceanic crust [43]. This basin has been characterized as having a heterogeneous distribution of various volcanic elevations including seamounts, hills, and seafloor mounds [42,44]. Nevertheless, in the central area of this basin, known as the Subvent Area, these seafloor mounds are hydrothermal domes and scattered volcanoes related to Quaternary intrusive activity that gave rise to a huge magmatic sill complex together with volcanic activity [41]. ...
Article
Derived digital elevation models (DEMs) are high-resolution acoustic technology that has proven to be a crucial morphometric data source for research into submarine environments. We present a morphometric analysis of forty deep seafloor edifices located to the west of Canary Islands, using a 150 m resolution bathymetric DEM. These seafloor structures are characterized as hydrothermal domes and volcanic edifices, based on a previous study, and they are also morphostructurally categorized into five types of edifice following an earlier classification. Edifice outline contours were manually delineated and the morphometric variables quantifying slope, size and shape of the edifices were then calculated using ArcGIS Analyst tools. In addition, we performed a principal component analysis (PCA) where ten morphometric variables explain 84% of the total variance in edifice morphology. Most variables show a large spread and some overlap, with clear separations between the types of mounds. Based on these analyses, a morphometric growth model is proposed for both the hydrothermal domes and volcanic edifices. The model takes into account both the size and shape complexity of these seafloor structures. Grow occurs via two distinct pathways: the volcanoes predominantly grow upwards, becoming large cones, while the domes preferentially increase in volume through enlargement of the basal area.
... EF-1: Bedded echofacies consisting of continuous and regular subbottom reflectors with high penetration reaching approximately 80-120 m. This echofacies is frequently attributed to well-bedded sedimentary units, such as hemipelagic deposits, distal turbidites (Damuth, 1980;Sandstrom and Elliott, 1984;Lee et al., 2002;Domzig et al., 2009;Loncke et al., 2009;Sanchez-Guillamón et al., 2018), or contourite drifts (Faugères and Stow, 2008). Cores IG-KSF-19 and IG-KSF-02, collected, respectively, on the upper marginal plateau and in the abyssal plain (Figures 1b and 6), display relatively homogeneous gray muds (silt and clays) with poorly visible bioturbation structures. ...
... Contrary to the previous echofacies, this one is not associated with a wavy seafloor on the bathymetry. These diffraction hyperbolas can be induced by steep slopes or by the seafloor irregularity associated with complex structures, such as depressions, seamounts, faults (Damuth, 1980;Lee et al., 2002;Loncke et al., 2009;Sanchez-Guillamón et al., 2018), or MTD-related seafloor roughness (Damuth, 1980(Damuth, , 1994Domzig et al., 2009;Loncke et al., 2009). These artifacts make the subbottom profiles unusable in places and cannot be eliminated by processing, due to the acquisition configuration of the hullmounted SBP data. ...
... EF-1: Bedded echofacies consisting of continuous and regular subbottom reflectors with high penetration reaching approximately 80-120 m. This echofacies is frequently attributed to well-bedded sedimentary units, such as hemipelagic deposits, distal turbidites (Damuth, 1980;Sandstrom and Elliott, 1984;Lee et al., 2002;Domzig et al., 2009;Loncke et al., 2009;Sanchez-Guillamón et al., 2018), or contourite drifts (Faugères and Stow, 2008). Cores IG-KSF-19 and IG-KSF-02, collected, respectively, on the upper marginal plateau and in the abyssal plain (Figures 1b and 6), display relatively homogeneous gray muds (silt and clays) with poorly visible bioturbation structures. ...
... Contrary to the previous echofacies, this one is not associated with a wavy seafloor on the bathymetry. These diffraction hyperbolas can be induced by steep slopes or by the seafloor irregularity associated with complex structures, such as depressions, seamounts, faults (Damuth, 1980;Lee et al., 2002;Loncke et al., 2009;Sanchez-Guillamón et al., 2018), or MTD-related seafloor roughness (Damuth, 1980(Damuth, , 1994Domzig et al., 2009;Loncke et al., 2009). These artifacts make the subbottom profiles unusable in places and cannot be eliminated by processing, due to the acquisition configuration of the hullmounted SBP data. ...
Article
Off French Guiana and Surinam, NADW and AABW oceanic currents contour the Demerara marginal plateau, which promotes the formation of contourites. We have studied these contourites thanks to a new compilation of high-resolution sub-bottom profiles calibrated by sedimentary cores. The echo-facies and isopach maps we constructed highlight a sedimentary distribution parallel to the isobaths. The presence of moats along the slope is confirmed by the observation of parallel, elongated, sedimentary depleted zones and echo-facies strongly affected by diffraction hyperbola and transparent echo-facies. We interpret these features to be related to eroded slopes and Mass Transport Deposits. In contrast, the sedimentary drifts we mapped are characterized by elongated and thick slope-parallel depocentres displaying bedded echo-facies with wave-like bedforms. According to our interpretation, they result from interactions between currents and the seafloor. Seismic wipe-outs frequently affect those drifts, possibly resulting from high water or organic contents.
... Alternatively, the acoustically transparent mounds off Madeira could be interpreted as structures linked to the migration and escape of over-pressurized fluids within the sedimentary column, such as mud volcanoes and domes. Mud volcanoes, domes and associated pockmarks are widely recognized features in continental margins (e.g., Judd and Hovland, 2007) and deep-water environments (e.g., Medialdea et al., 2017;Sánchez-Guillamón et al., 2018a;Sánchez-Guillamón et al., 2018b). The isolated mounds recognized off Madeira Island share some characteristics with acoustically transparent mounded features described by Rebesco et al. (2007) in Antarctica's distal sediment drifts. ...
Article
The deep-water sedimentary processes and morphological features offshore Madeira Island, located in the Central-NE Atlantic have been scantly studied. The analysis of new multibeam bathymetry, echo-sounder profiles and few multichannel seismic reflection profiles allowed us to identify the main geomorphologies, geomorphic processes and their interplay. Several types of features were identified below 3800 m water depth, shaped mainly by i) the interplay between northward-flowing Antarctic Bottom Water (AABW) and turbidity currents and ii) interaction of the AABW with oceanic reliefs and the Madeira lower slope. Subordinate and localized geomorphic processes consist of tectono-magmaticslope instability,turbidity currents and fluid migration, . The distribution of the morphological features defines three regional geomorphological sectors. Sector 1 represents a deep-seafloor with its abyssal hills, basement highs and seamounts inherited from Early Cretaceous seafloor spreading. Sector 2 is exclusively shaped by turbidity current flows that formed channels and associated levees. Sector 3 presents a more complex morphology dominated by widespread depositional and erosional features formed by AABW circulation, and localized mixed contourite system developed by the interplay between the AABW circulation and WNW-ESE-flowing turbidite currents. The interaction of the AABW with abyssal hills, seamounts and basement ridges leads to the formation of several types of contourites: patchdrifts, double-crest mounded bodies, and elongated, mounded and separated drifts. The patch drifts formed downstream of abyssal hills defining an previously unknown field of relatively small contourites. We suggest they may be a result of localized vortexes that formed when the AABW's flow impinges these oceanic reliefs producingthe erosional scours that bound these features. The bottom currents in the area are known to be too weak (1–2 cm s⁻¹) to produce the patch drifts and scours. Therefore, we suggest that these features could be relics at present, having developed when the AABW was stronger than today, as during glacial/end of glacial stages.
... Seafloor terrain classification is an especially active development direction in the fields of submarine mapping, ocean geophysics, ocean acoustics and submarine ecology. As an important way to characterize the seafloor, seabed sediments and the seabed ecological environment, classification of different seafloor terrain types that contain submarine characters link the classification zone with the physical, geological, chemical or biological characteristics of the seafloor [8,2,6,5,3,7,4]. Therefore, seafloor terrain classification is an important research content of ocean spatial information. ...
... Alternatively, the acoustically transparent mounds off Madeira could be interpreted as structures linked to the migration and escape of over-pressurized fluids within the sedimentary column, such as mud volcanoes and domes. Mud volcanoes, domes and associated pockmarks are widely recognized features in continental margins (e.g., Judd and Hovland, 2007) and deep-water environments (e.g., Medialdea et al., 2017;Sánchez-Guillamón et al., 2018a;Sánchez-Guillamón et al., 2018b). The isolated mounds recognized off Madeira Island share some characteristics with acoustically transparent mounded features described by Rebesco et al. (2007) in Antarctica's distal sediment drifts. ...
... It is found that mud volcanoes in the Gulf of Cadiz were formed through the diapir piercing the seafloor (Palomino et al., 2016;Somoza et al., 2003). Based on the seismic profiles, the formation of the seafloor domes in the Atlantic Canary Basin is inferred to be associated with mud eruption induced by the compression of the magma system (Sanchez-Guillamón et al., 2018). ...
Article
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Multibeam echo‐sounding and seismic data were obtained during three geophysical surveys by Guangzhou Marine Geological Survey (GMGS) on the north‐eastern slope of the South China Sea (SCS). Nineteen seafloor domes and numerous elongated pockmarks were distinguished in the study area. The seafloor domes vary on number of summits, shape, and spatial interrelation, based on which they were classified into four types which are single‐summit, double‐summits, elongated, and connected. Bottom simulating reflectors and blanking zone underneath were observed in the seismic profiles, indicating the potential existence of gas hydrate and free gas. Multiple faults and diapirs exist beneath the seafloor domes and pockmarks, which were inferred to provide efficient pathways for fluid migrating from deep subsurface. The seafloor domes exhibits high backscatter feature, which is potentially associated with the seep‐related gas hydrate and authigenic carbonate in the shallow sediment. It is speculated that fault and diapir activity, fault sliding caused by region extension and the interaction between neighbour domes control and affect the activity and formation of the domes. However, further investigations, including geological sampling and seafloor observation, are still needed to confirm whether they are mud volcanoes.
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The study presents the insights of the tectonic development and geological settings of the Atlantic Ocean supported by cartographic visualization in Generic Mapping Tools (GMT). The aim is to study geologic situation and trends in the tectonic development of the Mid-Atlantic Ridge and Atlantic Ocean seafloor. The objective is to find out impact of various factors (such as volcanic, tectonic, hydrothermal and sedimentary processes) that sculpt seafloor geomorphology, and correlation between early history of crust formation, geological processes and present submarine landforms. Other assignments in this work refer to mutual comparison of raster grids on sedimentation, topography, geology, seafloor fabric and highlighting similarities among the landforms and sediment thickness. Asymmetry in crustal accretion is explained by the tectonic history of the lithosphere formation. Correlation between plate subduction and development of the submarine landforms is explained by the Earth's crust extension resulting in formation of cracks, elongations, faults, rifts. Ocean seafloor geomorphology is shaped by a variety of factors that impact its form at different scales. These drivers (tectonic evolution, oceanic currents, hydrology, sedimentation) have effects on geomorphic landforms of the seafloor in context of historic geological development and during Quaternary. Technical part of this work was performed by GMT scripting toolset with all maps plotted in American polyconic projection. The results are received by overlay, cartographic analysis and synthesis of the multi-source geodata through mapping and interpreting grids (ETOPO1, EGM96, GlobSed, crustal age). This work contributes to expand the knowledge on geological and tectonic development of the Atlantic Ocean seabed in order to complete the view of its submarine geomorphology.
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High-resolution single channel and multichannel seismic reflection profiles and multibeam bathymetric and backscatter data collected during several cruises over the period 1999 to 2007 have enabled characterising not only the seabed morphology but also the subsurface structural elements of the Yuma, Ginsburg, Jesús Baraza and Tasyo mud volcanoes (MVs) in the Gulf of Cádiz at 1,050–1,250 m water depth. These MVs vary strongly in morphology and size. The data reveal elongated cone-shaped edifices, rimmed depressions, and scarps interpreted as flank failures developed by collapse, faulting, compaction and gravitational processes. MV architecture is characterised by both extrusive and intrusive complexes, comprising stacked edifices (including seabed cones and up to four buried bicones) underlain by chaotic vertical zones and downward-tapering cones suggesting feeder systems. These intrusive structures represent the upper layer of the feeder system linking the fluid mud sources with the constructional edifices. The overall architecture is interpreted to be the result of successive events of mud extrusion and outbuilding alternating with periods of dormancy. Each mud extrusion phase is connected with the development of an edifice, represented by a seabed cone or a buried bicone. In all four MVs, the stacked edifices and the intrusive complexes penetrate Late Miocene–Quaternary units and are rooted in the Gulf of Cádiz wedge emplaced during the late Tortonian. Major phases of mud extrusion and outbuilding took place since the Late Pliocene, even though in the Yuma and Jesús Baraza MVs mud volcanism started in the Late Miocene shortly after the emplacement of the Gulf of Cádiz wedge. This study shows that fluid venting in the eastern sector of the Gulf of Cádiz promoted the outbuilding of large long-lived mud volcanoes active since the Late Miocene, and which have been reactivated repeatedly until recent times.
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Seabed fluid flow involves the flow of gases and liquids through the seabed. Such fluids have been found to leak through the seabed into the marine environment in seas and oceans around the world - from the coasts to deep ocean trenches. This geological phenomenon has widespread implications for the sub-seabed, seabed, and marine environments. Seabed fluid flow affects seabed morphology, mineralization, and benthic ecology. Natural fluid emissions also have a significant impact on the composition of the oceans and atmosphere; and gas hydrates and hydrothermal minerals are potential future resources. This book describes seabed fluid flow features and processes, and demonstrates their importance to human activities and natural environments. It is targeted at research scientists and professionals with interests in the marine environment. Colour versions of many of the illustrations, and additional material - most notably feature location maps - can be found at www.cambridge.org/9780521819503.
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Alkali pillow basalts were collected from the toe of the oceanward slope of the northern Japan Trench. These alkali-basalts formed as a result of a low degree of partial melting of Pacific Ocean mantle and rapid rise of the magma (no fractionation in shallow magma chambers). Reconstructing Pacific Plate motion based on 40Ar-39Ar age dates of 5.95+/-0.31Ma for these basalts indicates that they erupted outboard of outer swell or forebulge of the Japan Trench in the NW Pacific. We suggest that these alkali-basalts represent a new form of intra-plate volcanism, whereby magmatic activity occurs off the forebulge of the downgoing Pacific slab, perhaps using conduits related to fracturing of the slab during bending prior to subduction.
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The Canary Island Seamount Province forms a scattered hotspot track on the Atlantic ocean floor ~1300 km long and ~350 km wide, perpendicular to lithospheric fractures, and parallel to the NW African continental margin. New (40)Ar/(39)Ar datings show that seamount ages vary from 133 Ma to 0.2 Ma in the central archipelago, and from 142 Ma to 91 Ma in the southwest. Combining (40)Ar/(39)Ar ages with plate tectonic reconstructions, I find that the temporal and spatial distribution of seamounts is irreconcilable with a deep fixed mantle plume origin, or derivation from passive mantle upwelling beneath a mid-ocean ridge. I conclude that shallow mantle upwelling beneath the Atlantic Ocean basin off the NW African continental lithosphere flanks produced recurrent melting anomalies and seamounts from the Late Jurassic to Recent, nominating the Canary Island Seamount Province as oldest hotspot track in the Atlantic Ocean, and most long-lived preserved on earth.
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Seamounts can provide conduits for the entry and exit of hydrothermal fluids in ocean basins. However, only a few ridge flank hydrothermal systems that discharge through seamounts have been discovered, all located on relatively young crust. We have retrieved samples from 126 m.y. old Henry Seamount, an extinct volcano near the youngest Canary island of El Hierro, that provide evidence for Holocene low-temperature hydrothermal fluid discharge. This is the first documented finding of such activity at the Canary archipelago. The samples include shells from vesicomyid clams <18.6 k.y. old, massive barite, and trachytes that are pervasively barite metasomatized. Sulfur, oxygen, and strontium isotope ratios of barite indicate that the fluid contained residual sulfate from microbial reduction at the recharge site and reacted with basement rocks. Recharge probably occurred at basement outcrops of El Hierro's submarine flank at 25-30 km distance, the driving force for hydrothermal circulation through old crust being provided by increased basal heat flow from Canary magmatism. The data show that island flanks may provide important recharge sites for seawater circulation and that even old and small seamounts can contribute to heat and mass exchange between ocean crust and seawater.
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Structure of mud volcano systems and pockmarks in the region of the Ceuta Contourite Depositional System (Western Alborán Sea), Marine Geology (2012), doi: 10.1016/j.margeo.2012.06.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Pockmarks form where fluids discharge through seafloor sediments rapidly enough to make them quick, and are common where gas is present in near-seafloor sediments. This paper investigates how gas might lead to pockmark formation. The process is envisioned as follows: a capillary seal traps gas beneath a fine-grained sediment layer or layers, perhaps layers whose pores have been reduced in size by hydrate crystallization. Gas accumulates until its pressure is sufficient for gas to invade the seal. The seal then fails completely (a unique aspect of capillary seals), releasing a large fraction of the accumulated gas into an upward-propagating gas chimney, which displaces water like a piston as it rises. Near the seafloor the water flow causes the sediments to become “quick” (i.e., liquefied) in the sense that grain-to-grain contact is lost and the grains are suspended dynamically by the upward flow. The quickened sediment is removed by ocean-bottom currents, and a pockmark is formed. Equations that approximately describe this gas–piston–water-drive show that deformation of the sediments above the chimney and water flow fast enough to quicken the sediments begins when the gas chimney reaches half way from the base of its source gas pocket to the seafloor. For uniform near-surface sediment permeability, this is a buoyancy control, not a permeability control. The rate the gas chimney grows depends on sediment permeability and the ratio of the depth below seafloor of the top of the gas pocket to the thickness of the gas pocket at the time of seal failure. Plausible estimates of these parameters suggest gas chimneys at Blake Ridge could reach the seafloor in less than a decade or more than a century, depending mainly on the permeability of the deforming near-surface sediments. Since these become quick before gas is expelled, gas venting will not provide a useful warning of the seafloor instabilities that are related to pockmark formation. However, detecting gas chimney growth might be a useful risk predictor. Any area underlain by a gas chimney that extends half way or more to the surface should be avoided.
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Chapter
Recent advances in seafloor imagery systems have enabled the extensive mapping of submarine volcanic areas, depicting with unprecedented detail a large spectrum of landforms. They can be grouped in two main types: volcanic and erosive-depositional landforms, reflecting the interplay between constructive and destructive forces that control the growth and morphological evolution of volcanic edifices. Volcanic landforms mainly include primary volcanic constructs (cones, lava flows and delta, undifferentiated bedrock outcrops), but also volcano-tectonic features, such as caldera collapses. Erosive-depositional landforms typically cover most part of the volcanic flanks and include features related to wave erosion and sea-level fluctuations (insular shelves and guyots), gravity-driven instability processes (landslide scars), and density gravity flows (gullies, canyons, fan-shaped features, sediment waves). However, despite the large number of marine studies realized until now, we are still far from having a complete mapping of these areas as well as reliable models for the correct interpretation of several landforms, mainly because of the paucity of direct observations. A systematic monitoring of active processes is essential to understand the genesis and evolution of such phenomena; repeated multibeam surveys are playing a key role in this regard. Also the comparison and parameterization of these landforms in different settings can provide insights on the main factors controlling their genesis. But it is necessary to set shared and standardized protocols for the interpretation and analysis of morpho-bathymetric data, also in consideration of the exponential increase in data availability from these areas, whose study is becoming crucial for several disciplines.
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The Lost City Hydrothermal Field on the Atlantis Massif is an off-axis hydrothermal system, hosted in serpentinized ultramafic and metagabbroic rocks, that discharges through high permeability faults and fractures. High pH fluids vent through carbonate chimneys with temperatures ranging between 24 °C and 91 °C; and vent fluid geochemistry indicates quantitative removal of magnesium and water-rock reactions consistent with heating to between 250 °C and 300 °C, conductive cooling, and mixing with seawater. This paper develops a relatively simple, steady state, fault-controlled fluid circulation and crustal heat transfer model that is consistent with these observations. The model results predict that the (1) heat output at Lost City is ~ 4 x 104 W, (2) circulation depth is approximately 2.7 km, and (3) permeability of the fault zone is ~ 10-14 m2. The model assumes that the hydrothermal system mines heat from the adjacent country rock, and it does not appear that heat from serpentinization reactions or cooling intrusions is needed to drive the system. Because it takes ~ 105a for the current model to reach steady state, earlier episodes of hydrothermal activity at LCHF may have operated differently and vent temperatures may have been higher.
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Seafloor mounds are potential geohazards to offshore rig emplacement and drilling operations and may contain evidence of underlying petroleum systems. Therefore, identifying and mapping them is crucial in de-risking exploration and production activities in offshore domains. A 738 km² high resolution three-dimensional seismic dataset was used to investigate the occurrence, seismic characteristics and distribution of features interpreted as seafloor and buried sediment mounds, at water-depths of 800–1600 m, on the western Niger Delta slope. Fifteen seafloor mounds and eighteen shallowly buried mounds were identified. The seafloor mounds are characterised by lower seismic amplitude anomalies than the surrounding seabed sediments, and overlie vertical zones of acoustic blanking. The buried mounds in contrast are characterised by high amplitude anomalies; they also directly overlie sub-vertical zones of acoustic blanking. Seismic evidences from the features, their distribution patterns and tectono-stratigraphic associations suggest that their formation was controlled by the juxtaposition of buried channels and structural highs and their formation caused by focused fluid flow and expulsion of entrained sediments at the seabed. Considering the acoustic and geometrical characteristics of the mounds and comparing them with mound-shaped features from around the world, we conclude that the mounds most likely comprise heterolithic seafloor extrusions of muds and sands from the Agbada Formation with gas and possibly oil in some of the pore space giving rise to the acoustic characteristics.
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The margin of the continental slope of the Volcanic Province of Canary Islands is characterised by seamounts, submarine hills and large landslides. The seabed morphology including detailed morphology of the seamounts and hills was analysed using multibeam bathymetry and backscatter data, and very high resolution seismic profiles. Some of the elevation data are reported here for the first time. The shape and distribution of characteristics features such as volcanic cones, ridges, slides scars, gullies and channels indicate evolutionary differences. Special attention was paid to recent geological processes that influenced the seamounts. We defined various morpho-sedimentary units, which are mainly due to massive slope instability that disrupt the pelagic sedimentary cover. We also studied other processes such as the role of deep bottom currents in determining sediment distribution. The sediments are interpreted as the result of a complex mixture of material derived from a) slope failures on seamounts and submarine hills; and b) slides and slumps on the continental slope.
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The Beebe Vent Field (BVF) is the world's deepest known hydrothermal system, at 4960 m below sea level. Located on the Mid-Cayman Spreading Centre, Caribbean, the BVF hosts high temperature (∼401°C) “black smoker” vents that build Cu, Zn and Au-rich sulfide mounds and chimneys. The BVF is highly gold-rich, with Au values up to 93 ppm and an average Au:Ag ratio of 0.15. Gold precipitation is directly associated with diffuse flow through “beehive” chimneys. Significant mass-wasting of sulfide material at the BVF, accompanied by changes in metal content, results in metaliferous talus and sediment deposits. Situated on very thin (2–3 km thick) oceanic crust, at an ultraslow spreading centre, the hydrothermal system circulates fluids to a depth of ∼1.8 km in a basement that is likely to include a mixture of both mafic and ultramafic lithologies. We suggest hydrothermal interaction with chalcophile-bearing sulfides in the mantle rocks, together with precipitation of Au in beehive chimney structures, has resulted in the formation of a Au-rich volcanogenic massive sulfide (VMS) deposit. With its spatial distribution of deposit materials and metal contents, the BVF represents a modern day analogue for basalt hosted, Au-rich VMS systems.
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A detailed analysis of chirp (3–7 kHz) subbottom profiles and bathymetry was performed on data collected from seamounts near the Ogasawara Fracture Zone (OFZ) in the western Pacific. The OFZ, which is a 150 km wide rift zone showing 600 km of right-lateral movement in a NW–SE direction, is unique among the fracture zones of the Pacific in that it includes many old seamounts (e.g., Magellan Seamounts and seamounts on Dutton Ridge). Sub-seafloor acoustic echoes on the seamounts are classified into nine specific types based on the nature and continuity of the echoes, subbottom structure, and morphology of the seafloor: (1) distinct echoes (types I-1, I-2, I-3), (2) indistinct echoes (types II-1, II-2, II-3), and (3) hyperbolic echoes (types III-1, III-2, III-3). Type I-2 pelagic sediments, characterized by thin and intermittent coverage, were probably deposited in topographically sheltered areas when bottom currents were strong, whereas type I-1 pelagic sediments accumulated during continuous and widespread sedimentation. Development of seamount flank rift zones in the OFZ may have been influenced by preexisting structures in the transform fracture zone at the time of volcanism, whereas those on Ita Mai Tai seamount in the Pigafetta Basin originated solely by edifice-building processes. Flank rift zones that formed by dike intrusions and eruptions played an important role in mass wasting. Mass-wasting processes included block faulting or block slides around the summit margin, sliding/slumping, debris flows, and turbidites, which may have been triggered by faulting, volcanism, dike injection, and weathering during various stages in the evolution of the seamounts.
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The northern California continental margin contains evidence of abundant subsurface gas and numerous seafloor features that suggest a causative link between gas expulsion and geomorphology. Analyses of seismic reflection, sidescan sonar, and high-resolution multibeam bathymetric data show that the occurrence of subbottom gas and the migration processes beneath the shelf differ from those beneath the slope. Subsurface gas, inferred from enhanced reflectors and other geophysical indicators, is spatially variable and related more to total depth and stratigraphy than to underlying structure, with the exception of one band of gas that follows the regional structural trend. Shallow depressions on the seafloor (pockmarks) are used to infer expulsion sites. The largest zone of acoustically impenetrable subsurface gas occurs between water depths of 100 m and 300 m, where expulsion features are rare. The upper slope (water depths 400–600 m) has a high concentration of pockmarks (diameter 10–20 m), in contrast to a near-absence of pockmarks at water depths shallower than 400 m. Of nearly 4000 pockmarks observed on sidescan sonar records, more than 95% are located in water depths deeper than 400 m. Bottom simulating reflectors (BSRs) on some seismic reflection profiles indicate the possible presence of gas hydrate. We find that gas and pore-fluid migration in the offshore Eel River Basin is: (1) correlated to surface morphology; (2) a contributor to seabed roughness; (3) a significant mode of sediment redistribution on the upper slope; and (4) potentially a factor in large slope failures.
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A number of enigmatic, km-scale mound structures have been discovered in the Baiyun Sag of the Pearl River Mouth Basin using high-resolution 2D and 3D seismic data combined with multi-beam bathymetry. Based on detailed seismic characterization, we interpret these as sill-fed volcanic mounds. Associated structures include igneous sills, trans-tensional faults, lava flows and compactional drape folds. The igneous intrusions and associated mounds were emplaced in the late Early Miocene (c. 18.5 Ma or shortly thereafter), constrained by the onlap relationship between the mounds and their overburden. The volcanic mounds are preferentially developed above or adjacent to basement highs, indicating a structural control on the igneous plumbing system. We propose that the fractures and tectonic faults above the raised basement follow pre-existing zones of weakness and may have acted as feeding conduits for the magmatic material. The discovery of the Early Miocene igneous province has important implications for the understanding of the evolution of the Pearl River Mouth Basin and for regional hydrocarbon prospectivity.
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This study presents a systematic analysis and interpretation of autonomous underwater vehicle-based microbathymetry combined with remotely operated vehicle (ROV) video recordings, rock analyses and temperature measurements within the PACManus hydrothermal area located on Pual Ridge in the Bismarck Sea of eastern Manus Basin. The data obtained during research cruise Magellan-06 and So-216 provides a framework for understanding the relationship between the volcanism, tectonism and hydrothermal activity. PACManus is a submarine felsic vocanically-hosted hydrothermal area that hosts multiple vent fields located within several hundred meters of one another but with different fluid chemistries, vent temperatures and morphologies. The total area of hydrothermal activity is estimated to be 20,279 m2. The microbathymetry maps combined with the ROV video observations allow for precise high-resolution mapping estimates of the areal extents of hydrothermal activity. We find the distribution of hydrothermal fields in the PACManus area is primarily controlled by volcanic features that include lava domes, thick and massive blocky lava flows, breccias and feeder dykes. Spatial variation in the permeability of local volcanic facies appears to control the distribution of venting within a field. We define a three-stage chronological sequence for the volcanic evolution of the PACManus based on lava flow morphology, sediment cover and lava SiO2 concentration. In Stage-1, sparsely to moderately porphyritic dacite lavas (68 - 69.8 wt. % SiO2) erupted to form domes or cryptodomes. In Stage-2, aphyric lava with slightly lower SiO2 concentrations (67.2 – 67.9 wt. % SiO2) formed jumbled and pillowed lava flows. In the most recent phase Stage-3, massive blocky lavas with 69 to 72.5 wt. % SiO2 were erupted through multiple vents constructing a volcanic ridge identified as the PACManus neovolcanic zone. The transition between these stages may be gradual and related to progressive heating of a silicic magma following a recharge event of hot, mantle-derived melts.
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The recent marine carbonate world comprises two major compartments: (1) the surface domain of the photozoan carbonates, confined in space by water depth and by the penetration of light, and (2) a deep domain, where heterozoan mound-builder guilds directly forage on fluxes of nutrients, which primarily percolate from the photic zone and/or are generated by in situ benthic processes. Locally, giant cold-water coral mounds tower up to heights of 150 to 250 m above the sea floor, in general between 500 and 1300 m water depth and within sharply delineated provinces. Some 15 years of research on these giant mound provinces conveys a picture of their distribution in space and possibly sheds light on controls, acting in concert. Globally, there is no counterpart for the prolific North Atlantic Mound Basin (NAMB). A chemical control is seen by an overlay of the mound provinces on a map of the aragonite saturation horizon (ASH). An external physical control is inferred from the position of the mound provinces, girdling a vigorous North Atlantic subtropical gyre system and clustering close to the roof of the intermediate to deep water masses of a dynamically stratified ocean. On the eastern boundary of the NAMB, nutrient fluxes are enhanced by mixing processes, driven either by internal waves between Galicia and the Shetlands, or by the vast and heterogeneous Eastern Boundary Upwelling System along the Iberian/African margins down to 10°N. Early diagenesis by carbonate dissolution and re-precipitation driven by convecting or advecting internal fluids can contribute to stabilize such constructions, facilitating an exuberant vertical accretion. It is speculated that in the North Atlantic Ocean, the deep-water carbonate factory outclasses in size the shallow water coral reefs.
Article
The Nile deep turbiditic system displays many fluid-releasing structures on the seabed: mud volcanoes reassembling small cones (100–900 m in diameter), mud pies (5 km in diameter), and pockmarks. The cones are restricted to the western province. Mud ‘pies’ delineate a belt of apparently very active gas chimneys along the upper continental slope. Pockmarks are associated either with strongly destabilized sedimentary masses or with gas chimneys. We distinguish five main controlling parameters interacting in fluid release locations: (1) the presence, at depth, of potential source rocks and reservoirs; (2) the distribution of Messinian evaporites preventing upward fluid migration; (3) the distribution of sedimentary overloading inducing localized overpressures on under-compacted and fluid-rich sediments; (4) the presence of syn-sedimentary faults acting as potential conduits for fluid migration; and finally (5) chiefly for pockmarks and mounds, the occurrence of large-scale sedimentary instabilities.
Article
Multibeam bathymetric charts (100 fm = 183 m contour interval) are presented and analyzed for the approx 110-km-long, 120o trending Geisha chain of eight guyots and at least 13 other seamounts (>1 km height) in the NW Pacific; these guyots are compared with 23 others in the North Pacific. Published radiometric dates from the two ends of the Geisha chain are consistent with rapid (14 cm/yr) plate motion over a fixed hotspot, but other mechanisms cannot be excluded. The evolution of the guyots and their flank rift zones is considered. -after Authors
Article
Major landslides in volcanic islands on old oceanic lithosphere, such as in the Canaries, are revealed by deep embayments in their subaerial slopes, and below sea level by chutes and broad debris lobes at the base of their edifices. With a view to addressing whether volcanic islands growing on young oceanic lithosphere show a different character or incidence of landsliding to those on old lithosphere, a database was created from hydrographic soundings, radar images and multibeam and sidescan sonar data from Ascension, Bouvet, Guadalupe, Jasper Seamount and several of the Galapagos and Azores islands. The data suggest the following: 1) Major landslides on these islands are less common and are not obviously associated with multiple cycles of island growth and collapse as they are in the Canaries. 2) Landslides appear different from analogous Canary Island landslides, for example they do not clearly exhibit extensive chutes. 3) Landslides range in size from minor submarine slope failures (e.g., Ascension), to failures involving lower subaerial slopes (e.g., Tristan da Cunha), and only rarely to extensive events affecting the volcano summit or caldera (e.g. Guadalupe). 4) The Azores islands of Terceira, Flores, Corvo and Sao Jorge reach no more than 2500 m above the surrounding seafloor and show no landslides of significant size. This presentation will review the data and discuss possible origins of the contrasting incidence and character of landsliding between islands on young and old lithosphere.
Article
Temporal and spatial changes in volcano morphology and internal architecture can determine eruption style and location. However, the relationship between the external and internal characteristics of volcanoes and sub-volcanic intrusions is often difficult to observe at outcrop or interpret uniquely from geophysical and geodetic data. We use high-quality 2D seismic reflection data from the Ceduna Sub-basin, offshore southern Australia, to quantitatively analyse 56, pristinely-preserved, Eocene-age volcanogenic mounds, and a genetically-related network of sub-volcanic sills and laccoliths. Detailed seismic mapping has allowed the 3D geometry of each mound to be reconstructed and distinct seismic facies within them to be recognised. Forty-six continental, basaltic shield volcanoes have been identified that have average flank dips of <12°, basal diameters of 1.94–18.89 km, central summits that are 0.02–1 km high and volumes that range from 0.06 to 57.21 km3. Parallel seismic reflections within the shield volcanoes are interpreted to represent interbedded volcanic and clastic material, suggesting that a series of temporally separate eruptions emanated from a central vent. The shield volcanoes typically overlie the lateral tips of sills and we suggest that the intermittent eruption phases correspond to the incremental emplacement of discrete magma pulses within the laterally extensive sill-complex. Eight volcanogenic hydrothermal vents, which are also associated with the lateral tips of sills, were also recognised, and these appear to have formed from the seepage of intrusion-related hydrothermal fluids onto the seafloor via emplacement-induced fractures. This work highlights that deformation patterns preceding volcanic eruptions may (i) be offset from the eruption site; (ii) attributed to intrusions with complex morphologies; and/or (iii) reflect magma movement along pre-existing fracture systems. These complexities should therefore be considered in eruption forecasting models that link pre-eruption ground deformation to subterranean magma emplacement depth and volume. More generally, our study highlights the key role that seismic reflection data can play in understanding the geometry, distribution and evolution of ancient and modern volcanic systems.
Article
Over a decade of research on recent cold-water coral mounds in various oceans has set the stage for comparative studies between recent and ancient carbonate mound systems, with the aim to unravel generic processes and reveal the “red thread” in a fundamental strategy of Life building Geology — a strategy nearly as ancient as Life itself. Natural laboratories have been identified in the present ocean, which provide new insights in oceanographic controls on species migration and settlement, in the interaction of currents and carbonate build-up, in the earliest diagenesis which overprints environmental signals and shapes the template of compartmentalization of carbonate build-ups, and in so many other processes and factors ultimately shaping carbonate bodies, comparable in size and properties to the large-scale carbonate hydrocarbon reservoirs in the geological record. Ocean drilling and coring is an essential component of this research.
Article
The North Faroe-Shetland Basin (NFSB) Sill Complex is of late Paleocene/earliest Eocene age and was emplaced within Cretaceous and Paleocene sedimentary rocks, in places to depths as shallow as a few hundred metres below the contemporaneous basin floor. Intersections of the Complex occur in exploration wells drilled by the oil industry and indicate tholeiitic basaltic compositions. High quality 3D seismic data, obtained during hydrocarbon exploration along the NE Atlantic Margin, provide a unique view of an uneroded suite of these sheet-like intrusions in UK Quadrants 218 and 219 and indicate the multi-centred nature of the NFSB Sill Complex, with upward-fingering terminations from broad bowl-shaped foci of intrusion. Where the intrusion depth is very shallow, depending upon the host lithology, sill emplacement has lead to the development of structures on the contemporaneous basin floor interpreted as submarine hyaloclastite-dominated vents, up to c. 2 km across and with heights of up to c. 100 m. Where intrusion depth is greater, 'seismic chimney' structures are interpreted as the fluid-escape feeders of sedimentary-hydrothermal mounds. Subsequent differential compaction of sedimentary sections, with and without shallow-emplaced sills, has given rise to distinctive 'eye' structures, as seen in seismic sections.
Article
Seismic surveys carried out in the Madeira Abyssal Plain have revealed patterns of faulting in abyssal-plain sediments. Shallow faults are concentrated within linear bands which run parallel to the trend of underlying basement topography. The faults are believed to be caused by differential compaction over the basement topography. The reverse throw of many faults is thought to be a result of sediment dewatering. A recent model has invoked preferential dewatering along faults. An alternative mechanism is proposed in which faults act as seals. Overpressurized pore fluids are a feature of both models.
Article
We use 3D seismic data to describe the 3D geometry of a large igneous intrusion, the Solsikke Compound Sill, and address a number of issues related to sill emplacement. The Solsikke Compound Sill formed by amalgamation of a number of sills and exhibits a complex internal morphology dominated by saucer-shaped depressions and linear discontinuities. One of the saucer-shaped sub-elements of the Compound sill, the Solsikke Lobate Sill, has a previously unrecognized morphology It has a basal feeder and consists of a bifurcating network of interlinked lobe-shaped sill segments. We propose two models for the development of this intrusive style based on analogues from igneous systems and hydrofracturing experiments. The lobate pattern indicates that the Solsikke Lobate Sill was fed at its deepest point and adopted its geometry through outwards and upwards propagation. The feeder location is coincident with a fault intersection, suggesting that magma transport from the underlying source exploited the zone surrounding the intersection.
Article
Multichannel seismic reflection and gravity data define the structure of Mesozoic ocean crust of the Canary Basin, formed at slow spreading rates. Single and multichannel seismics show a transition from smooth to rough basement topography from Jurassic to Cretaceous crust and a coeval change in crustal structure. Internal reflectivity of the rough basement area comprises upper, upper middle or whole crust cutting discrete dipping reflections. Lower-crustal reflectivity is almost absent and reflections from the crust-mantle transition are short and discontinuous or absent for several kilometers. In contrast, crust in the smooth basement area is characterized by sparse lower crustal events and common reflections from the crust-mantle boundary. The crustal structure of fracture zones in the rough basement area is associated with depressions in the basement top and in most cases with thin crust. In the smooth basement area, fracture zones exhibit neither a clear topographic expression nor crustal thinning. We interpret these characteristics as indicative of an increase in extensional tectonic activity and decrease in magmatic activity at the spreading ridge associated with a general decrease of spreading rate from Jurassic to Cretaceous times. In addition, the crust imaged across the path of the Cape Verde Hot Spot in the Canary Basin exhibits a widespread lower crustal reflectivity, very smooth topography and apparently thick crust. Our data document significant changes in the structure of crust formed at slow spreading rates which we attribute to thermal changes in the lithosphere due either to variations in spreading rate or to the presence of a hot spot beneath the Mesozoic Mid-Atlantic Ridge.
Article
Surficial sediments and sea-floor features of the northern Gulf of Mexico continental slope record dramatic episodic venting and slower seepage of formation fluids, brines, crude oil, gases, and fine-grained sediments. Faults, activated by massive sediment input during periods of Plio-Pleistocene falling to low sea levels accompanied by compensating salt movement, provide avenues of vertical transport to the continental slope surface. Many of these faults cut thick sedimentary sequences that frequently contain geopressured zones which provide the driving force for fluid and gas expulsion. Flux rate and fluid characteristics are interpreted as important determinants of modern sea-floor geology and biology. Qualitatively, under conditions of rapid flux of sediment-rich fluids, mud volcanoes (up to 1 km wide and 50 m high) and extensive sheets of extruded mud result. These sediments contain old and displaced microfaunas. Muds are frequently extruded with large volumes of crude oil and gas (both biogenic and thermogenic). In water depths greater than ∼500-m rapid to moderate vertical flux of hydrocarbon gases and fluids results in the construction of relief features composed of gas hydrates, ejected mud, and authigenic carbonates. Areas with near-surface hydrates frequently support complex chemosynthetic communities with associated hardgrounds containing calcareous remains of mussels, clams, and associated gastropods. Slow flux (seeps) promotes the formation of 13C-depleted carbonate hardgrounds, stacked carbonate slabs, and moundlike carbonate buildups (frequently >20-m relief). Barite along with carbonate has been found in areas where rapid flux of sediment-carrying fluids has waned in the recent past and mineral-rich brines are now being ejected. Crusts, chimneys, and cones of these minerals are found in such settings. Both recent numerical simulations of fluid release from geopressured zones and direct observations at the sea floor support a pattern of episodic venting. Short-term episodes of venting are probably regulated by fault movement, perhaps controlled by local salt adjustment. Destabilization of gas hydrates by oceanographic processes also causes short-term episodic gas expulsion. These events occur with interannual to intra-annual frequencies. Longer gas hydrate stabilization-destabilization cycles in continental margin settings are forced by hydrostatic loading and unloading at the frequency of sea-level change. Radiometrie dating suggests that other long-term episodes of venting and major venting expulsion events are also probably modulated by sea-level change, time scales of thousands of years in response to low-stand sedimentary loading. Results of this summary of vent-seep-related phenomena suggest that sediment input from the shelf margin associated with cyclic Plio-Pleistocene falling to low sea-level periods, followed by major listic fault and salt adjustments to a new sediment load have activated venting-seepage throughout this period and perhaps longer.
Article
Seafloor features identified in multibeam bathymetry data from the Capel and Faust basins are correlated with sub-surface features interpreted from seismic data. Seismic interpretation has revealed major depocentres within the study area, and that potential source, reservoir and seal rocks are likely to be present. However, volcanic activity has to be considered when assessing the petroleum prospectivity of the Capel and Faust basins and elsewhere on the Lord Howe Rise. Analysis of multibeam data suggests that seafloor deformation is linked to the underlying basement architecture. Most seafloor features are related to igneous intrusions and fluid flow features located within depocentre megasequences or along basin bounding faults. The geological relationships of these features indicate that fluid flow appears to be mainly driven by igneous activity. Three major magmatic events have been identified: Maastrichtian to Paleocene, Eocene to Late Oligocene and Middle Miocene to Early Pliocene. Extant fluid flow and modern bathymetric features appear to be related to after-effects of Pliocene igneous activity. Several bypass systems have been identified that breach the regional seal to the seafloor. Each magmatic pulse appears to have generated fluid flow that has re-utilised pre-existing fluid conduits. The magmatism and fluid flow activity have implications for petroleum prospectivity. In particular, the timing, driver and spatial distribution of the fluid flow features show that hydrocarbon accumulations potentially formed within the deeper syn-rift and the pre-rift sections. They are less likely to have been affected by the Cenozoic magmatic and fluid flow processes and, therefore, may represent lower-risk exploration targets. (C) 2012 Published by Elsevier Ltd.
Article
The morphology of the source area of the Canary Debris Flow has been mapped using both GLORIA reconnaissance and TOBI high-resolution sidescan sonar systems. West of ≈19°W, the seafloor is characterized by a strongly lineated downslope-trending fabric. This fabric can be interpreted as being caused by streams of debris separated by longitudinal shears. Multiple flow pulses are indicated by a series of asymmetrical lateral ridges which mark the northern boundary of the flow. East of ≈19°W, GLORIA data show only a weak fabric of irregular patches and alongslope lineaments. The TOBI data show the patches to be coherent sediment blocks up to 10 km across, surrounded by debris flow material. These are interpreted as in situ areas of seafloor sediment which have survived the slope failure and debris flow event rather than transported fragments of a failed sediment slab. TOBI data from the best developed area of alongslope lineaments show a series of small faults downstepping to the west. This area of seafloor is interpreted as one of partial sediment failure, where the failure process became ‘frozen’ before total mobilization of the seafloor sediments could occur. The overall morphology of the failure area indicates removal of a slab-like body of sediment, although we cannot distinguish between retrogressive and slab-slide failure mechanisms. If the latter mechanism is applicable, fragmentation of the failing ‘slab’ must have commenced concurrently with the onset of downslope transport. Immediately upslope from the debris flow source area, a seafloor of characteristic rough blocky texture is interpreted as the surface of a debris avalanche derived from the slopes of the island of El Hierro. The debris flow and avalanche appear to be simultaneous events, with failure of the slope sediments occurring while the avalanche deposits were still mobile enough to fill and disguise the topographic expression of the debris flow headwall. Loading of the slope sediments by the debris avalanche most probably triggered the Canary Debris Flow.
Article
Large volcanic islands and guyots have stellate forms that reflect the relief of radiating volcanic rift zones, multiple volcanic centers, and embayments due to giant flank failures. Small mid-ocean ridge volcanoes, in contrast, are commonly subcircular in plan view and show only embryonic rift zones. In order to characterize the transition between these two end-members the morphology of 141 seamounts and guyots was studied using the shape of the depth contour at half the height of each edifice. Irregularity was characterized by measuring perimeter distance, elongation, and moment of inertia of the contours, assuming an "ideal" edifice is circular. The analysis reveals a general transition over 2-4 km edifice height (best transition estimate 3 km), while some large edifices 4-5 km high show no major embayments or ridges, suggesting considerable variation in the effectiveness of mechanisms that cause flank instability and growth of rift zones. The various origins of the transition are discussed, and the upper limit of magma chambers, many of which lie above the basement of the larger edifices, is proposed to affect the morphologic complexity via a number of mechanisms and is an important factor affecting the mode of growth. The origins of the truncated cone shape of mid-ocean ridge volcanoes are also discussed. Of the eruption mechanisms that have been proposed to explain their flat summits, the most likely mechanisms involve eruption from small ephemeral magma bodies lying within the low-density upper oceanic crust. The discussion includes speculations on factors affecting the depths of magma chambers beneath oceanic volcanoes. Supporting table is available via Web browser or via Anonymous FTP from ftp://kosmos.agu.org, directory "append" (Username = "anonymous", Password ="guest"); subdirectories in the ftp site are arranged by paper number. Information on searching and submitting electronic supplements is found at http://www.agu.org/pubs/csupp_about.html.
Article
The origin of mantle hotspots is a controversial topic. Only seven ('primary') out of 49 hotspots meet criteria aimed at detecting a very deep origin (three in the Pacific, four in the Indo-Atlantic hemisphere). In each hemisphere these move slowly, whereas there has been up to 50 mm/a motion between the two hemispheres prior to 50 Ma ago. This correlates with latitudinal shifts in the Hawaiian and Reunion hotspots, and with a change in true polar wander. We propose that hotspots may come from distinct mantle boundary layers, and that the primary ones trace shifts in quadrupolar convection in the lower mantle.
Article
On the western part of the Pacific Plate most seamounts formed during the Cretaceous period in the so-called West Pacific Seamount Province (WPSP). On the northwestern part of the same plate, the Joban and Japanese Seamount Trail (JJST) are also composed of Early Cretaceous seamounts. However, two new groups of knolls were recently discovered during multibeam surveys on the Pacific Plate along the Japan Trench. One group consists of circular knolls that are flat-topped in shape and correspond to eruptive ages of approximately 75 Ma. The other group consists of irregularly shaped knolls, also called petit-spot volcanoes, that are found on the outer-rise systems of the subducting Pacific Plate. These petit-spots seem much younger and available age data suggest that they only formed in the last few million years. Acoustic reflective data, which are simultaneously obtained with bathymetrical data, are a most powerful tool to distinguish the petit-spots from the Cretaceous edifices in the WPSP and JJST. In this paper, we present the results of an exploratory search for these new kind of petit-spot volcanoes along the trenches in the Pacific Ocean, with an emphasis on the Japan and Tonga trenches. The sizes of these irregularly shaped petit-spot volcanoes are several orders of magnitude less than the Cretaceous seamounts and circular knolls, yet they appear to be ubiquitous on the ocean floor, in particular, where incipient melts in the asthenosphere can be squeezed out by tectonic forces.
Article
Submersible observations and seafloor mapping over areas of mud volcanism in the eastern Mediterranean Sea reveal an abundance of methane-rich fluid emissions, as well as specific seep-associated fauna (e.g. tubeworms, bivalves and chemosynthetic bacteria) and diagenetic deposits (i.e. carbonates crusts). Cold seeps characterized by abundant chemosynthetic bivalves, bacterial mats, and reduced sediments are the surface manifestations of present-day expulsion of methane-rich fluids, whereas massive carbonate crusts with clusters of tubeworms indicate long-term seeping that might be dormant at present. The surface characteristics of the mud flows (roughness of the surface, gas escape features, presence of hemipelagic dust, colonization by benthic fauna, development of carbonate crusts) present a high variability related to the intensity and age of seepage, and can be correlated to the observed backscatter variations. The repartition of the mud flows and active seeps is organized concentrically around the centre of the mud volcanoes and potential parasitic cones. Sidescan sonar backscatter patterns are thus indirectly related to spatial and temporal variation of mud volcanic activity.
Article
Three newly identified submarine caldera volcanoes (Brothers, Healy, and Rumble II West), of the southern Kermadec arc (within water depths <1500 m), provide new insights into modes of submarine arc caldera formation. Swath MR1 data, seafloor photography, and rock dredge sampling establish the edifice and caldera morphology, outcrop volcanology, magma compositions, and petrography and rock properties of erupted components for each volcano. These data reveal the Brothers and Rumble II West volcanoes have: (1) concentric, steep and 50–450 m high escarpments, that are interpreted as caldera wall ring-faults, encircling ca. 3-km wide calderas and associated central cones, (2) heterogeneous flank-draping lava flows, and reworked pyroclastic deposits, and (3) the predominance of incipiently to poorly vesiculated (mostly <40%), moderately dense (1.6–2.4 g cm−3) eruptive flows and clasts. Erupted lavas from Brothers and Rumble II West are predominantly dacitic (64–67 wt.% SiO2), and basalt–basaltic–andesite (64–67 wt.% SiO2), respectively. Morphology of the Brothers and Rumble II West volcanoes are interpreted (although the former less confidently) as evidence of incremental caldera collapse formed by prolonged, episodic, effusive magma withdrawal from summit and rift flank vents. In contrast, the Healy volcano has: (1) a flat ca. 2.5-km wide “caldera” floor flanked by a 250–400 m high, and a relatively shallowly inclined “caldera” wall, (2) the presence of a near ubiquitous mantle of relatively well sorted, blocks and lapilli over the edifice flanks and caldera, with rare seafloor outcrop, and no obvious evidence of lava flows, (3) the predominance of rhyodacitic compositions (66–70 wt.% SiO2) contents, and (4) extremely vesiculated (>80%) and low density (0.3–0.5 g cm−3) erupted pumiceous clasts. These observations are interpreted as evidence of subaqueously quenched, pyroclastic eruption(s), sheathed by ambient water flashing to steam, discharging some 5 km3 of pyroclastic material from Healy volcano. Caldera collapse is inferred to be penecontemporeous with the pyroclastic eruption(s), although it is possible formation as an explosive crater occurred, but we consider the latter unlikely given the volume of erupted material and water depth. Extrapolation of these observations to other submarine arcs indicates pyroclastic eruption of silicic magmas, and syn-eruptive caldera collapse, is possible within water depths of <1500–1000 m. In contrast, incremental, post-eruptive caldera collapse is more likely to occur with repetitive effusive eruptions of more mafic compositions, at both summit and rift flank sites, within water depths of >500–1000 m.
Article
Travel-time inversion of wide-angle ocean-bottom seismic (OBS) data results in detailed P-wave velocity models of the shallow sub-seabed beneath the Nyegga pockmark field. The area lies on the northern flank of the Storegga Slide on the mid-Norwegian margin. Velocity anomalies indicate two low P-wave velocity zones (LVZs) providing evidence for the presence of gas-rich fluids in the subsurface at Nyegga. Integrating the velocity results with 2D and 3D reflection seismic data demonstrates that LVZs coincide with zones of high-amplitude reflections that allow mapping the extent of the fluids in the subsurface. The upper fluid accumulation zone corresponds to a velocity inversion of ∼250 m/s and occurs at a depth of about 250 mbsf. The lateral extent is documented in two distinct areas. The westward area is up to 40 m thick where gas-rich fluids beneath a bottom-simulating reflection indicate that fluids may be trapped by gas hydrates. The eastward zone is up to 60 m thick and comprises a contourite deposit infilling a paleo-slide scar. On top, glacigenic debris flow deposits provide a locally effective seal for fluids. The second velocity inversion of ∼260 m/s extends laterally at about 450 mbsf with decreasing thickness in westward direction. Based on effective-medium theory the gas saturation of pore space in both layers is estimated to be between 0.5 and <1% assuming a homogeneous distribution of gas. Fluids probably originate from deeper strata approximately at the location of the top of the Helland-Hansen Arch. Fluids migrate into the second LVZ and are distributed laterally. Fluids migrate into shallower strata or are expelled at the seabed through the formation of vertical fluid migration features (VFMFs), so-called chimneys. The distribution of the chimneys is clearly linked to the two fluid accumulation zones in the subsurface. A conceptual model draws on the major controlling factors for fluid migrations at specific locations within Nyegga. Fluid migrations vary according to their actual position with respect to the prograding Plio–Pleistocene sedimentary wedge.
Article
This study presents seismic observation of pipe anomalies from offshore Nigeria, outcrops of blow-out pipes from Rhodes, Greece, and geophysical modelling of an acoustic pipe. The studies give insight into how pipes form, their internal structure, the seismic image and geophysical artefacts related to the pipes. Over one hundred seafloor craters, 100 m–700 m wide and up to 30 m deep, have been observed on the seafloor offshore Nigeria. They are underlain by interpreted cones and seismic pipe anomalies that can be traced down to reservoir zones at 1000 m–1300 m below the seafloor. The seismic pipe anomalies are 50 m–150 m wide and almost vertical. They are interpreted as up-scaled pipes found in outcrops on Rhodes, Greece. The outcrops show pipe-related structures at three levels. Lowest, the reservoir rock contains metre-sized cavities which are filled with a mixture of clay derived from the overlying cap rock. In the middle, several circular to oval structures in plane view of pipes are observed in the lowest part of the cap rock. Highest, 15 m into the clay cap rock, strongly sheared country rock forms circular structures with a core of structureless clay. Based on outcrop observation on Rhodes we constructed an acoustic model of a 50 m wide and 1000 m long pipe. Seismic modelling proves that such pipes would be expressed in seismic data, that they are similar to the seismic pipe anomalies offshore Nigeria but this study also revealed that prominent intra-pipe reflections are artefacts. A formation model for the pipes is suggested: High fluid overpressure in the reservoir generated hydro fractures from the reservoir to seafloor where a mixture of gas and fluid flowed at high speed to form pipes, cones and seafloor craters. After hours to weeks of gas and fluid flow through the pipe the pore pressure in the reservoir dropped and the blow-out terminated. Muddy slurry fell back and plugged the cavity in the reservoir and the pipe.