Multichannel Seismic Lines Research Articles

Characteristics of the Fault Damage Zone From High‐Resolution Seismic Imaging Along the Palos Verdes Fault, California

AbstractThe distribution and intensity of fault damage zones provides insight into fault activity and its relationship to fluid flow in the crust. Presently, measures of the in‐situ distribution of fault damage remain limited and along‐strike studies are rare. This study focuses on an offshore section Palos Verdes Fault damage zone that spans 28 km, near Los Angeles, California. To investigate the previously unresolved shallow (∼400 m below the seafloor) fault damage zone we use densely spaced (∼500 m line separation) newly collected sparker multichannel seismic lines and sub‐bottom profiles. The combination of high‐resolution acquisition methods and specialized seismic processing workflows provide improved imaging of shallow faulting. We apply a multi‐trace similarity technique to identify discontinuities in the seismic data that may be attributed to faults and fractures. This fault detection approach reveals diverse fault damage patterns on adjacent seismic profiles. However, a discernible damage zone pattern emerges by stacking multiple damage detection profiles along strike. We find that peak damage identified in this way corresponds to the active main fault strand, confirmed in this study, and thus the technique may be useful for identifying active fault strands elsewhere. Additionally, we observe that the variable width of the damage zone along strike is controlled by fault obliquity. Furthermore, our observations reveal a correlation between fault damage and seafloor fluid seeps visible in the water column, suggesting that damage plays a role in controlling fluid flow around the fault.

Asymmetric magma plumbing system beneath Axial Seamount based on full waveform inversion of seismic data

The architecture of magma plumbing systems plays a fundamental role in volcano eruption and evolution. However, the precise configuration of crustal magma reservoirs and conduits responsible for supplying eruptions are difficult to explore across most active volcanic systems. Consequently, our understanding of their correlation with eruption dynamics is limited. Axial Seamount is an active submarine volcano located along the Juan de Fuca Ridge, with known eruptions in 1998, 2011, and 2015. Here we present high-resolution images of P-wave velocity, attenuation, and estimates of temperature and partial melt beneath the summit of Axial Seamount, derived from multi-parameter full waveform inversion of a 2D multi-channel seismic line. Multiple magma reservoirs, including a newly discovered western magma reservoir, are identified in the upper crust, with the maximum melt fraction of ~15–32% in the upper main magma reservoir (MMR) and lower fractions of 10% to 26% in other satellite reservoirs. In addition, a feeding conduit below the MMR with a melt fraction of ~4–11% and a low-velocity throat beneath the eastern caldera wall connecting the MMR roof with eruptive fissures are imaged. These findings delineate an asymmetric shallow plumbing system beneath Axial Seamount, providing insights into the magma pathways that fed recent eruptions.

Origin of a high-velocity layer: Insights from seismic reflection imaging (South China Sea)

Magmatism and crustal structural characteristics are well-known phenomena that support understanding the marginal sea's tectonic evolution. The South China Sea (SCS) has become a natural laboratory for researchers to investigate the evolution of the marginal sea because of its complex tectonic background and multiple phases of magmatic activities. However, due to limitations in resolution and depth of observations, the evolutionary process of the SCS is still controversial. We use the latest processed multi-channel seismic reflection profile Line AB to evaluate the sedimentary sequences, crustal tectonics, and the Cenozoic magmatism in the southern SCS. The gravity modeling results indicate a reduction in crustal thickness from 18 km in the proximal domain to 7 km in the Continent-Ocean Transition (COT). The COT is roughly 35 km wide, representing a rapid break up process of the SCS. A high-velocity layer (HVL) of about 3 km thickness was formed in the continental slope crust, interpreted as post-spreading magmatic underplating. In addition, four sedimentary sequences, post-spreading magmatic intrusion, and the possible magma conduits were identified in the profile. Combining with previous ocean bottom seismometer (OBS) data, multi-channel seismic data, fault system data, and petrologic observations, we indicate the deep mantle magmatic material upwelled to the crust and further intruded into sediments along the hyper-extended crust or detachment faults during the post-spreading stage. We contend that cooling and heat shrinking within the SCS oceanic lithosphere have the potential to initiate decompression melt deformation in the continental slope. In summary, we construct a model of the origin and migration mechanism of extensive post-spreading magmatism in the SCS. Our research reveals the Cenozoic evolution and the HVL of the southern SCS, and provides a possible genesis for the anomalously young magmatism observed at the continental margin.

Geological setting of the Phu Khanh Basin based on updated geological-geophysical data

The Phu Khanh Basin is a Tertiary sedimentary basin located in deep a water area adjacent to the continental shelf of Central Vietnam, which has been begun to form since the Eocene(?)/Oligocene to the Quaternary. This area has previously only been briefly studied based on satellite magnetic, gravity data and some 2D multichannel seismic lines. In this article, the authors present the most recently interpreted and merged gravity data to update the research results on geological structure, tectonics and the history of geological evolution of the Phu Khanh Basin, which are considered significant contributions for hydrocarbon exploration campaigns in the future. Our new results show that mechanism of the the Phu Khanh Basin formation was started by a rifting process with most of it located in the central basin, controlled by four main fault systems, including the NW - NE system, the NE - SW system, sub-meridian system and less active is the sub-latitude system. The evolution of the basin has undergone through four main phases of different tectonic deformation, which are characterized by its typical geodynamic regimes and tectonic activities. These evolutionary phases can be briefly described as follows: (i) Paleocene peneplanation phase, (ii) The active rifting phase in Eocene/Oligocene-Early Miocene, (iii) The Middle Miocene thermal subsidence phase and finally (iv) The Late Miocene-Quaternary shelf building phase. However, the issue of determining the Eocene age at the bottom of the basin has so far remained unclear and elucidated by the lack of deep wells penetrating the deepest region of the basin.

Reflection tomography by depth warping: a case study across the Java trench

Abstract. Accurate subsurface velocity models are crucial for geological interpretations based on seismic depth images. Seismic reflection tomography is an effective iterative method to update and refine a preliminary velocity model for depth imaging. Based on residual move-out analysis of reflectors in common image point gathers, an update of the velocity is estimated by a ray-based tomography. To stabilize the tomography, several preconditioning strategies exist. Most critical is the estimation of the depth error to account for the residual move-out of the reflector in the common image point gathers. Because the depth errors for many closely spaced image gathers must be picked, manual picking is extremely time-consuming, human biased, and not reproducible. Data-driven picking algorithms based on coherence or semblance analysis are widely used for hyperbolic or linear events. However, for complex-shaped depth events, purely data-driven picking is difficult. To overcome this, the warping method named non-rigid matching is used to estimate a depth error displacement field. Warping is used, for example, to merge photographic images or to match two seismic images from time-lapse data. By matching a common image point gather against its duplicate that has been shifted by one offset position, a locally smooth-shaped displacement field is calculated for each data sample by gather matching. Depending on the complexity of the subsurface, sample tracking through the displacement field along predefined horizons or on a simple regular grid yields discrete depth error values for the tomography. The application to a multi-channel seismic line across the Sunda subduction zone offshore Lombok island, Indonesia, illustrates the approach and documents the advantages of the method to estimate a detailed velocity structure in a complex tectonic regime. By incorporating the warping scheme into the reflection tomography, we demonstrate an increase in the velocity resolution and precision by improving the data-driven accuracy of depth error picks with arbitrary shapes. This approach offers the possibility to use the full capacities of tomography and further leads to more accurate interpretations of complex geological structures.

Subduction related faults and sedimentary basins: The Western Ionian Sea case

The Pliocene-Quaternary evolution of the western margin of the Calabrian Arc has been investigated by analysing the configuration and distribution of local sedimentary basins and their relationship with shallow and deep tectonic structures. Research focussed on the evolution of the more recent offshore sedimentary basins located to the east of Sicily, along the Alfeo Fault System, using multichannel seismic lines, high-resolution bathymetry and newly acquired single-channel seismic profiles. The chosen study area encompasses a crustal sector where external thrusts of the Calabrian Arc branch into the Malta Escarpment, and this zone has been interpreted as a Subduction Transform Edge Propagator (STEP) Fault. The Western Ionian Sea is bounded by regional faults that controlled the deposition of the Pliocene-Quaternary sequences and define the present-day bathymetry. In this work we identified four main seismic sequences and three main unconformities in the basins, to better understand the geometry and the depositional setting of the basins. In addition, lateral correlation and 3D reconstruction of the sedimentary basins allows us to establish the direct relationship between the Alfeo Fault System activity and the geometry, the thickness, the evolution and filling phases of the sedimentary basins in this area. Results presented show how the Alfeo Fault drove the development of separate basin depocenters in the early stages of tectonic activity, resulting in a single elongated basin in the latest phases. The inferred fault kinematics, the analysis of displacements along strike and sediment thickening suggest a link with the slab geometry, as occurs in other subduction systems that develop subduction related faults. The results and the method applied in this study can also be applied for the interpretation of similar systems with similar geological settings in other areas of the world.

The sines contourite depositional system along the SW Portuguese margin: Onset, evolution and conceptual implications

The Sines Contourite Depositional System, located in the Southwest Portuguese Margin, is a central segment of the Iberian Contourite Depositional Complex, built under the influence of the Mediterranean Outflow Water (MOW). This work presents the onset and evolution of this system using multibeam bathymetry, multichannel seismic reflection lines, sediment cores and well data. Six seismic units of Late Miocene through Holocene age have been identified, which defined three evolutionary stages for drift construction: a) an onset and initial stage, from the Latest Miocene to the Late Pliocene (5.33–2.5 Ma), where a sheeted drift was built under a weak flowing MOW; b) a growing stage from the Early to Middle Quaternary (2.5–0.7 Ma), characterized by the deposition of a mounded drift in the south and a sheeted drift in the north, which is correlated with a strengthening of the MOW; and c) a maintenance stage between the Middle Pleistocene and Holocene (0.7 Ma – present), where the modern depositional and erosional contourite features developed associated with three intensification periods of the MOW at 0.7 Ma, 0.4 Ma and 20 ka. The development of the Sines Contourite Depositional System was constrained in the long-term by seafloor paleomorphologies inherited from the Mesozoic rifting phases of the Southwest Portuguese Margin. The NNW-SSE horsts and grabens built during the Mesozoic rifting provided accommodation for drift growth, with limited lateral migration, and locally enhanced the alongslope bottom-current. In short-term, environmental (climate and sea-level) fluctuations modulated changes in bottom-current density and intensity, determining sedimentary cycles in the depositional record, especially during the Quaternary. Our findings emphasize the occurrence of three main plastered morphologies throughout the seismic record, which were compared with other plastered drifts in different continental margins to establish a conceptual model.

Structural Evolution at the Northeast North German Basin Margin: From Initial Triassic Salt Movement to Late Cretaceous‐Cenozoic Remobilization

AbstractIn this study, we investigate the regional tectonic impact on salt movement at the northeastern margin of the intracontinental North German Basin. We discuss the evolution of salt pillows in the Bay of Mecklenburg in the light of thick‐ and thin‐skinned tectonics, including gravity gliding, and differential loading using seismic imaging. Stratigraphic and structural interpretation of a 170 km long, multichannel seismic line, extending from the Bay of Mecklenburg to northeast of Rügen Island, incorporates well information of nearby onshore wells. This new high‐resolution seismic line completely images the stratigraphic and tectonic pattern of the subsurface, from the base of the Zechstein to the seafloor. Our analysis reveals that subsidence during Late Triassic to Early Cretaceous at the northeastern basin margin was associated with transtensional dextral strike slip movement within the Trans‐European Suture Zone. We reinterpret the Werre and Prerow Fault Zones west of Rügen Island as an inverted, thin‐skinned normal fault system associated with the formation of the Western Pomeranian Fault System. Salt movement in the Bay of Mecklenburg was initiated in the Late Triassic and lasted until the Early Jurassic. A second phase of salt pillow growth occurred during the Coniacian until Cenozoic and correlates with compression‐related regional basin inversion due to the onset of the Africa‐Iberia‐Europe convergence. Thin‐skinned extensional initialization of salt pillow growth and compressional salt remobilization explains salt pillow evolution in the Bay of Mecklenburg. Additionally, we discuss an impact of gravity gliding on salt pillow evolution induced by basin margin tilt.

Basement structural control in the Magallanes-Malvinas Fold and Thrust Belt, offshore Argentina

In this study, we examine the external part of the Magallanes-Malvinas Fold and Thrust Belt in offshore areas, including a portion in Tierra del Fuego onshore, in southern South America. Our investigations focus on the interaction between a thin-skinned fold and thrust belt and the basement in that deformation, considering pre-existing structures acting as stress risers and localizing a possible tectonic inversion. The data is composed of around 14,000 km of 2-D multichannel seismic lines and three exploratory wells, used to analyse the anticlines of the fold and thrust belt in the area. The strike of these folds progressively evolves from NW-SE trending in western onshore regions, to WSW-ENE trending in eastern offshore areas. The fold and thrust belt shows a buttressing effect against the Río Chico Arch. The most shortening is seen in the apex of the Río Chico Arch around 67° W, in the coast of Tierra del Fuego, and gradually decreases eastwards in offshore regions. A connection between basement-involved faults and folds offshore is inferred from the analysis of the Géminis and Ciclón anticlines, developed during the N–S last compressional stage of deformation in the late Oligocene/early Miocene. The Géminis anticline is a fault propagation fold with a total shortening of 205 m with a piggyback basin developed over its backlimb. The location and ENE-WSW strike-direction of the fold have been controlled by a basement-involved fault. The Ciclón anticline is a subtle fold trending WNW-ESE developed as a result of the slight tectonic inversion of a negative flower structure.

Investigating the Goban Spur rifted continental margin, offshore Ireland, through integration of new seismic reflection and potential field data

The Goban Spur, offshore Ireland, is a magma-poor rifted continental margin conjugate to the well-studied Newfoundland margin, offshore Canada. Published studies demonstrated that a 70-km-wide zone of exhumed serpentinized mantle lies between oceanic crust and stretched continental crust at the seaward limit of Goban Spur. However, the along-strike extent of this serpentinized zone has, until now, been unknown due to insufficient data coverage. The crustal architecture of the margin is complicated due to its multi-staged tectonic history. Here, six newly acquired multi-channel seismic reflection lines are processed and interpreted, along with vintage seismic profiles, to characterize its structure and evolution. These seismic profiles reveal significant along-strike structural variations along the Goban Spur margin, and allow us to delimit five distinct crustal zones related to different rifting stages and their regional extents. The geometries of each crustal domain are variable along the margin strike, probably suggestive of different extension rates during the evolution of the margin or inherited variations in crustal composition and rheology. The transitional zone between oceanic crust and stretched continental crust consists of both shallow peridotite ridges and deeper exhumed serpentinized mantle, much like the conjugate Iberian and Newfoundland margins. Above the top basement in the exhumed domain, the syn-exhumed sediments show strikingly weak reflectivity, rarely seen at other magma-poor margins. Magmatic events occur coincident with each rifting stage, and the volume of magmatic accretions increases from NW to SE, more than previously interpreted. Plate reconstruction of the Goban Spur and its possible conjugate – the Flemish Cap, shows asymmetry in the crustal architectures, likely due to rift evolution involving more 3-D complexity than can be explained by simple 2-D extensional kinematics.

High-resolution seismic imaging reveals infill history of a submerged Quaternary fjord system in the subantarctic Auckland Islands, New Zealand

AbstractQuaternary processes and environmental changes are often difficult to assess in remote subantarctic islands due to high surface erosion rates and overprinting of sedimentary products in locations that can be a challenge to access. We present a set of high-resolution, multichannel seismic lines and complementary multibeam bathymetry collected off the eastern (leeward) side of the subantarctic Auckland Islands, about 465 km south of New Zealand's South Island. These data constrain the erosive and depositional history of the island group, and they reveal an extensive system of sediment-filled valleys that extend offshore to depths that exceed glacial low-stand sea level. Although shallow, marine, U-shaped valleys and moraines are imaged, the rugged offshore geomorphology of the paleovalley floors and the stratigraphy of infill sediments suggests that the valley floors were shaped by submarine fluvial erosion, and subsequently filled by lacustrine, fjord, and fluvial sedimentary processes.

Evolution of heat flow, hydrothermal circulation and permeability on the young southern flank of the Costa Rica Rift

SUMMARYWe analyse 67 new conductive heat-flow measurements on the southern flank of the Costa Rica Rift (CRR). Heat-flow measurements cover five sites ranging in oceanic crustal age between approximately 1.6 and 5.7 Ma, and are co-located with a high-resolution multichannel seismic line that extends from slightly north of the first heat-flow site (1.6 Ma) to beyond ODP Hole 504B in 6.9 Ma crust. For the five heat-flow sites, the mean observed conductive heat flow is ≈85 mW m−2. This value is approximately 30 per cent of the mean lithospheric heat flux expected from a half-space conductive cooling model, indicating that hydrothermal processes account for about 70 per cent of the heat loss. The advective heat loss fraction varies from site to site and is explained by a combination of outcrop to outcrop circulation through exposed basement outcrops and discharge through faults. Supercritical convection in Layer 2A extrusives occurs between 1.6 and 3.5 Ma, and flow through a thinly sedimented basement high occurs at 4.6 Ma. Advective heat loss diminishes rapidly between ≈4.5 and ≈5.7 Ma, which contrasts with plate cooling reference models that predict a significant deficit in conductive heat flow up to ages ≈65 ± 10 Ma. At ≈5.7 Ma the CRR topography is buried under sediment with an average thickness of ≈150 m, and hydrothermal circulation in the basement becomes subcritical or perhaps marginally critical. The absence of significant advective heat loss at ≈5.7 Ma at the CRR is thus a function of both burial of basement exposure under the sediment load and a reduction in basement permeability that possibly occurs as a result of mineral precipitation and original permeability at the time of formation. Permeability is a non-monotonic function of age along the southern flank of the CRR, in general agreement with seismic velocity tomography interpretations that reflect variations in the degree of ridge-axis magma supply and tectonic extension. Hydrothermal circulation in the young oceanic crust at the southern flank of CRR is affected by the interplay and complex interconnectedness of variations in permeability, sediment thickness, topographical structure, and tectonic and magmatic activities with age.

Seismic Evidence for Tectonically Dominated Seafloor Spreading in the Southwest Sub‐basin of the South China Sea

AbstractSeafloor spreading can be explained by different dynamic mechanisms, magmatically or tectonically dominated. The Southwest Sub‐basin, located at the southwest tip of the propagating seafloor spreading of the South China Sea, remains unclear for its spreading regime owing to poor and debated knowledge of the crustal structures. Here two multichannel seismic lines and one oceanic bottom seismometer line across this subbasin are reprocessed and analyzed with focus on crustal imaging. The sediments are usually 0.5–1.0 km thick over the abyssal basin and slightly thicker in the few grabens. The thickest (3.3 km) sediments are found in the fossil spreading center. The basement is fairly rough and highly faulted by ubiquitous crustal faults. The fossil spreading center is characterized by a deep median valley, similar to that of magma‐poor spreading cases. Both multichannel seismic lines show a few intermittent and diffusive reflectors at 1.5‐ to 3.6‐km depth below the fragmented basement. These reflectors, correlating well with the velocities of 6.8–7.2 km/s obtained from velocity inversion of oceanic bottom seismometer data, are interpreted as Moho reflections. Thus, the inferred crustal thickness is only 1.5–3.6 km excluding the sediments, which is much thinner than usual. The underlying mantle with velocities lower than 8.0 km/s might be serpentinized with water infiltration along these crustal faults. Therefore, it is proposed that the spreading of the Southwest Sub‐basin was tectonically dominated.

Thermal environment of the Southern Washington region of the Cascadia subduction zone

AbstractEleven recently collected multichannel seismic (MCS) profiles from the Cascadia Open‐Access Seismic Transects experiment offshore Washington State are used to characterize the distribution of bottom‐simulating reflectors (BSRs) from seaward of the deformation front onto the continental shelf of the Cascadia Subduction Zone. The 11 MCS lines consisted of nine lines perpendicular and two lines parallel to the Cascadia margin covering a 100 km along‐strike region of the accretionary wedge. From these MCS profiles we generated a 3‐D view of the Cascadia margin thermal structure by interpreting 40,232 individual BSR picks in terms of temperature and heat flow. Overall BSR‐derived heat flow values decrease from approximately 95 mW m−2 10 km east of the deformation front to approximately 60 mW m−2 located 60 km landward of the deformation front. Anomalously low heat flow values near 25 mW m−2 on a prominent midmargin terrace indicate recent sediment failure within the accretionary prism. Localized differences between BSR heat flow and numerical models reflect an estimated regional mean vertical fluid flow of +0.53 cm yr−1 for the survey area, with localized fluid flow approaching a maximum of +3.8 cm yr−1. Distinct finite element models for the nine MCS profiles perpendicular to the deformation front reproduce BSR heat flow values, producing an overall root‐mean‐square misfit of 10.2 mW m−2. At the deformation front, the incoming oceanic sediment/crust interface temperatures vary from 164°C to 179°C, indicating the updip limit of the Cascadia seismogenic zone.

Subducted oceanic relief locks the shallow megathrust in central Ecuador

AbstractWhether subducted oceanic reliefs such as seamounts promote seismic rupture or aseismic slip remains controversial. Here we use swath bathymetry, prestack depth‐migrated multichannel seismic reflection lines, and wide‐angle seismic data collected across the central Ecuador subduction segment to reveal a broad ~55 km × 50 km, ~1.5–2.0 km high, low height‐to‐width ratio, multipeaked, sediment‐bare, shallow subducted oceanic relief. Owing to La Plata Island and the coastline being located, respectively, ~35 km and ~50–60 km from the trench, GPS measurements allow us to demonstrate that the subducted oceanic relief spatially correlates to a shallow, ~80 km × 55 km locked interplate asperity within a dominantly creeping subduction segment. The oceanic relief geometrical anomaly together with its highly jagged topography, the absence of a subduction channel, and a stiff erosive oceanic margin are found to be long‐term geological characteristics associated with the shallow locking of the megathrust. Although the size and level of locking observed at the subducted relief scale could produce an Mw >7+ event, no large earthquakes are known to have happened for several centuries. On the contrary, frequent slow slip events have been recorded since 2010 within the locked patch, and regular seismic swarms have occurred in this area during the last 40 years. These transient processes, together with the rough subducted oceanic topography, suggest that interplate friction might actually be heterogeneous within the locked patch. Additionally, we find that the subducted relief undergoes internal shearing and produces a permanent flexural bulge of the margin, which uplifted La Plata Island.

An offshore-onland transect across the north-eastern Black Sea basin (Crimean margin): Evidence of Paleocene to Pliocene two-stage compression

The tectonic evolution of the Black Sea (BS) is a subject of debate, there are several unsolved questions: 1) the timing and the spatial progression of the BS basin opening and 2) the timing of Cenozoic shortening along the northern margin of the Eastern BS basin. The timing of the main compressional deformations, related to the inversion of the Greater Caucasus (GC) basin, is assumed to be Oligo-Miocene. However, Late Cretaceous/Early Paleocene shortening, linked to the final closure of the northern branch of the Neotethys, is also suggested. The Crimean Mountains (CM), to the north of the Eastern BS, is one of the key areas to investigate in order to fix the tectonic evolution of the BS. To precise the timing of the Cenozoic shortening of the Eastern BS, we focus on an integrated onshore/offshore transect from the Eastern CM to the Sorokin Trough (north of Eastern BS). We use newly collected stratigraphic and structural data from the Eastern CM, and a new interpretation of multichannel seismic lines. We define 1) the offshore seismic stratigraphy and constrain the relative chronology of deformations, 2) the age of seismic units by correlation of the seismic data with the Subbotina-403 well log, and 3) we construct an on-to-off shore transect of Eastern CM - northern Eastern BS region. Our results evidence a polyphased Cenozoic compression in the northern part of the Eastern BS: 1) Paleocene-Earliest Eocene and 2) Oligocene-Miocene (Maikopian). Normal faults appear to be related to a formation of the foreland basin, instead of evidencing the Eastern BS Cretaceous rifting. Finally, this study allows precising the shortening phases within Eastern CM and Eastern BS since the Early Paleocene, linking them to the Neotethys closure and the GC tectonic evolution.

Seismic stratigraphy, structure and morphology of Makarov Basin and surrounding regions: tectonic implications

The tectonic history of Amerasia Basin, Arctic Ocean, is not well known because of a paucity of data and complexities introduced by the Alpha–Mendeleev Ridge large igneous province. Makarov Basin, at the northern limit of Amerasia Basin and adjacent to Lomonosov Ridge, may provide a window into understanding the larger tectonic framework. The objective of this study is to decipher the sedimentary and tectonic history of northern Amerasia Basin by analysing the seismic stratigraphy, structure and morphology of Makarov Basin and surrounding regions (Alpha and Lomonosov ridges) of the central Arctic Ocean. The principal data sources for this study are a 400km long multi-channel seismic line, extending from Alpha Ridge to the crest of Lomonosov Ridge via central Makarov Basin, and the Arctic bathymetric chart.The seismic record within Makarov Basin is divided into five seismic units. The first unit overlying basement hosts Late Cretaceous (minimum age) slope to base of slope sediments. Some of these sediments are interbedded with volcanic or volcanoclastic rocks with a minimum age of 89Ma. Makarov Basin becomes isolated from proximal sources of sediments after the onset of rifting that separated Lomonosov Ridge from the Barents Shelf, which may have occurred as early as the mid-Late Cretaceous, and led to the creation of Eurasia Basin. Sediments are largely pelagic to hemipelagic as a result of this isolation. This deposition style also applies to the draped sedimentary strata on Alpha and Lomonosov ridges. The uppermost seismic units within Makarov Basin arejump-correlatedto the stratigraphic record of the ACEX drill site on top of Lomonosov Ridge to provide age control. This correlation shows that the 44.4–18.2Ma hiatus documented in the drill core is not apparent in the basin. Inter-ridge correlations and the absence of an obvious planate surface on Alpha Ridge also suggest that sedimentation was uninterrupted on this ridge during the hiatus.Seismic data reveal a deep subbasin (~5km thick) within Makarov Basin. This subbasin is immediately adjacent to Lomonosov Ridge within major bends in the general strike orientation of the ridge. The rhomboid shape of the deep subbasin, the straight and steep morphology of the Amerasian flank of Lomonosov Ridge and the presence of numerous sub-parallel ridges (e.g. Geophysicists and Marvin spurs) created by splay faulting are evidence of strike-slip (transtensional) tectonics. This interpretation supports the “rotational” model of opening of Amerasia Basin with a transform to transtensional margin at Lomonosov Ridge. As spreading continued, however, the tectonics became increasingly extensional perpendicular to Lomonosov Ridge. There is no evidence of major tectonic deformation in Makarov Basin beyond the late Paleocene.

Spatio-temporal evolution of sediment waves developed on the Gulf of Valencia margin (NW Mediterranean) during the Plio-Quaternary

Several fields of large-scale sediment waves have been observed along the Gulf of Valencia continental margin (NW Mediterranean). The largest sediment waves develop on the continental slope, extending from 250 to 850m water depth, with wavelengths ranging between 500m and 1000m and wave heights from ~2m to ~50m. On the lower part of the slope, sediment waves are quasi-stationary “vertically accreting”, becoming up-slope migrating towards the mid- and upper part of the slope. A second group of sediment waves have developed over the outer continental shelf, with wavelengths of 400 to 800m and heights of 2 to 4m, also displaying an up-slope migrating pattern. Multi-channel seismic lines crossing the continental margin show that the sediment waves over the continental slope region have been continuously developed on the foreset region of the prograding margin clinoform. Several units of sediment waves have been identified in the sedimentary record, evolving in accordance with the margin progradation. Detailed analysis of single-channel (sparker) seismic profiles revealed the presence of several sediment depositional subunits over the outer continental shelf, some of them with successive development of sediment waves being truncated by erosive surfaces, likely related to Quaternary eustatic sea-level oscillations. These erosional surfaces can be followed downslope into paraconformable strata of the sediment waves on the continental slope, where constant bedform growth is observed, without being affected by sea level changes. Based on geophysical data, the thickness of the sediment waves mapped units show that the largest sediment waves (in wave ratio, length and height) develop where sediment deposition rates are the highest, coinciding with the upper part of the continental slope (foreset clinoforms), confined by the presence of structural highs. The development of these sediment waves has been previously explained by the interaction of internal waves over the continental slope. Because sediment waves are preserved in the sedimentary record since the Lower/Pliocene, internal waves activity could have been present in this part of the margin shortly after the Zanclean reflooding of the Mediterranean Basin, following the Messinian desiccation event ~5.6 My ago. Deep water hydrodynamic conditions were re-established at that time, modulating sediment transport and deposition over the continental slope and outer continental shelf.

Seamount subduction at the North-Ecuadorian convergent margin: Effects on structures, inter-seismic coupling and seismogenesis

At the North-Ecuadorian convergent margin (1°S–1.5°N), the subduction of the rough Nazca oceanic plate leads to tectonic erosion of the upper plate and complex seismogenic behavior of the megathrust. We used three selected pre-stack depth migrated, multi-channel seismic reflection lines collected during the SISTEUR cruise to investigate the margin structure and decipher the impact of the subducted Atacames seamounts on tectonic erosion, interseismic coupling, and seismogenesis in the region of the 1942 Mw7.8 earthquake.This dataset highlights a subducted ∼30×40 km, double-peak seamount that belongs to the Atacames seamount chain and that is associated with a deep morphologic re-entrant containing mass transport deposits.The seamount subduction uplifted the margin basement by ∼1.6 km and pervasively broke the margin by deep and intense reverse faulting ahead of the seamount, a process that is likely to weaken considerably the margin. In the seamount wake, the basement reverse fault system rotated counter-clockwise. This faulted basement is overlain with slope sediment sliding along listric normal faults that sole out onto the BSR. This superposition of deep tectonic contraction within the basement and shallow gravitational extension deformation within the sediment highlights the key role of gas hydrate on outer slope erosion.In addition to long-term regional basal erosion, the margin basement has thinned locally by an extra 0.8–1 km in response to the subduction of the Atacames seamount chain and hydrofracturing by overpressured fluids at the margin toe. This pervasively and deeply fractured margin segment is associated with a seismically quiet and GPS-modeled low interseismic coupling corridor that terminates downdip near the 1942 epicenter and locked zone. We suggest that the deeply buried double-peak Atacames seamount triggered the 1942 earthquake ahead of its leading flank. This result supports previous studies proposing that subducted seamounts provide unfavorable conditions for locking the updip segment of the plate boundary limiting the updip extent of seismogenic zones, but may favor large subduction earthquakes at greater depths.

Shallow gas transport and reservoirs in the vicinity of deeply rooted mud volcanoes in the central Black Sea

The principal objective of this paper is to understand the gas migration process and the structures of mud volcanoes in the central Black Sea. Multichannel seismic lines were acquired across 6 mud volcanoes in the central Black Sea: MSU, Yuzhmorgeologiya, Malyshev, Kornev, Goncharov and Vassoevitch. Based on a high resolution seismic processing, we analyzed acoustic anomalies and studied near-surface sediment structures of these mud volcanoes. Four types of pathways for gas and fluid migration and three types of gas reservoirs were recognized. A regional “Bottom Simulating Reflection” (BSR) seems to be absent in most parts of the study area, however a clear BSR was observed in one of the seismic profiles. The free gas migrating upwards along these pathways were sealed by gas hydrates or fine-grained sediments in the gas hydrate stability zone. Four seismic units were separated according to a suggested age model and identified seismic facies. Combining with the possible sedimentary processes of the central Black Sea, we try to reveal the active mechanism of these mud volcanoes. These mud volcanoes reveal two to three major active stages. These active stages might be related to distinct sea level falls, which seem to be one of the main trigger factors of the mud volcano eruptions in the central Black Sea. A structural model for the MSU mud volcano was presented with respect to mechanisms of gas migration and the origin of the gas.