High-resolution Multichannel Seismic Data Research Articles

The discovery of an active fault in the Qiongdongnan Basin of the northern South China Sea

Fault activities in sea areas always produce devastative marine geohazards. For marine geohazard assessment, it is important to know the location, geometric structure, and activity regularity of active faults. In this study, high-resolution multi-channel seismic and topographic data are used to investigate the exact location, geometric structure and latest activity of the Continental Slope Fault Zone (CSFZ) in the northern South China Sea. The results show that the Qiongdongnan (QDN) segment of the CSFZ is a 194 km long NEE-trending fault zone that developed near the transition of continental shelf and slope. In its geometric structure, the CSFZ (QDN segment) cuts through the Cenozoic basement and breaks upward to near the seafloor with high-angle dip. According to whether it ruptures to the seafloor, it can be divided into eastern and western segments. In the western segment, the fault scarps about 8∼9 m high can clearly be seen on the seafloor. Whereas in the eastern segment, the fault shows a blind characteristic below the seafloor. The analysis of fault throws and expansion indices show that CSFZ (QDN segment) have remained active since the late Pleistocene. Due to the CSFZ is an active fault and geographically adjacent steep slope, which may cause large interpolate landslides, and then leading to destructive tsunamis. As a result, the CSFZ has great seismogenic potential and may produce marine geohazard chain. Thus, it is of great significance to the assessment of marine geohazard chain posed by the CSFZ for improving hazard mitigation.

Fault structure, seismicity, and magmatism of the Littoral Fault zone, northern South China sea: Insights from high-resolution seismic reflection data

Four destructive M7+ earthquakes and 18 M6+ earthquakes demonstrate that the Littoral Fault Zone (LFZ) is one of the most seismically active regions in the offshore area of South China. However, the location, geometry, and structural characteristics of the LFZ, which spans over 1000 km, have not been properly examined. In this study, we investigate the precise position of the LFZ offshore East Guangdong (EGD) and map the fault structure using recently acquired high-resolution multi-channel seismic (MCS) reflection data. The LFZ consists of an array of normal faults that collectively form a terraced sidewall fault zone linked to the continental rifting of the South China Sea (SCS). This fault zone is approximately 35–75 km wide and runs parallel to the shoreline. The primary faults of the LFZ EGD segment correlate to a high-gravity gradient zone and serve as the northern boundary of the Zhu-I Depression. During the syn-rift phase, the LFZ experienced significant intra-plate deformation, resulting in fault throws of up to 1000 m. This deformation was locally influenced by pre-existing structures, giving rise to a listric geometry at depth. In the post-rift period, the LFZ exhibited inherited activity along its rift-related faults. The presence of growth fault structures suggests that the LFZ remained active throughout the Cenozoic, with numerous active faults extending practically to the seafloor. The negative flower structure confirms the strike-slip character of the LFZ, which is influenced by the modern tectonic stress field. Intense multi-phase Cenozoic magmatism, including igneous intrusion, volcanism, and lava flows, has been detected in the offshore area of Nan'ao Island. The frequent earthquakes in the study area are primarily attributed to far-field stress release resulting from the subduction of the Philippine Sea Plate beneath the Eurasian Plate. Additionally, the intense magmatism weakens crustal structures, further facilitating stress release and the frequent occurrence of earthquakes.

Undulating sediments of the Cape Fear submarine landslide system, offshore U.S. Atlantic margin: Sediment waves versus creep deformation

We interpret a region of undulatory sediments adjacent to a major headwall of the Cape Fear submarine landslide system offshore of North Carolina, USA, as sediment waves rather than creep or fault-related deformation. The wave package extends 19 km upslope from the S4 landslide headwall and thickens upslope from approximately 250 to 450 m. The field of undulating sediments displays the continuity of seismic horizons, upslope-migrating crests, downslope thinning, and wave heights and lengths of approximately 26 m and approximately 1 km, respectively, which are consistent with sediment wavefields. The Western Boundary Undercurrent formed contourites on the nearby Blake Ridge and it is possible that these sediment undulations were deposited via similar mechanisms. Ocean Drilling Program (ODP) cores near the sediment undulation field suggest that the turbidity currents also may have played a role in wave formation. Although most of the 19 km long field comprises unaltered sediment waves, we observe an approximately 5 km long zone adjacent to the landslide scarp that expresses evidence of faults that offset and deform the sediment wave strata. We interpret this deformation as the result of reduction in stress following the removal of the landslide mass. Given that the Cape Fear system has generated several episodes of potentially tsunamigenic slope failure, the future stability of the system is pertinent. Redefining these undulatory sediments as sediment waves eliminates a major slope instability mechanism of the system and is important for understanding the future slope stability hazards of Cape Fear. Our analysis highlights the importance of understanding sediment waves in hybrid submarine landslide-sediment wave systems. Geological feature: Cape Fear submarine landslide sediment waves Seismic appearance: Continuous undulating horizons Alternative interpretations: Downslope creep or faults Features with similar appearance: Extensional slope failure Formation: Alongslope and downslope currents Age: Quaternary Location: Cape Fear submarine landslide complex, offshore North Carolina Seismic data: High resolution multichannel seismic data Analysis tools: Multichannel seismic data, multibeam bathymetry, sub-bottom Chirp

Revisiting the 1899 earthquake series using integrative geophysical analysis in Yakutat Bay, Alaska, USA

Abstract A series of large earthquakes in 1899 affected southeastern Alaska near Yakutat and Disenchantment Bays. The largest of the series, a MW 8.2 event on 10 September 1899, generated an ~12-m-high tsunami and as much as 14.4 m of coseismic uplift in Yakutat Bay, the largest coseismic uplift ever measured. Several complex fault systems in the area are associated with the Yakutat terrane collision with North America and the termination of the Fairweather strike-slip system, but because faults local to Yakutat Bay have been incompletely or poorly mapped, it is unclear which fault system(s) ruptured during the 10 September 1899 event. Using marine geophysical data collected in August 2012, we provide an improved tectonic framework for the Yakutat area, which advances our understanding of earthquake hazards. We combined 153 line km of 2012 high-resolution multichannel seismic (MCS) reflection data with compressed high-intensity radar pulse (Chirp) profiles, basin-scale MCS data, 2018 seafloor bathymetry, published geodetic models and thermochronology data, and previous measurements of coseismic uplift to better constrain fault geometry and subsurface structure in the Yakutat Bay area. We did not observe any active or concealed faults crossing Yakutat Bay in our high-resolution data, requiring faults to be located entirely onshore or nearshore. We interpreted onshore faults east of Yakutat Bay to be associated with the transpressional termination of the Fairweather fault system, forming a series of splay faults that exhibit a horsetail geometry. Thrust and reverse faults on the west side of the bay are related to Yakutat terrane underthrusting and collision with North America. Our results include an updated fault map, structural model of Yakutat Bay, and quantitative assessment of uncertainties for legacy geologic coseismic uplift measurements. Additionally, our results indicate the 10 September 1899 rupture was possibly related to stress loading from the earlier Yakutat terrane underthrusting event of 4 September 1899, with the majority of 10 September coseismic slip occurring on the Esker Creek system on the northwest side of Yakutat Bay. Limited (~2 m) coseismic or postseismic slip associated with the 1899 events occurred on faults located east of Yakutat Bay.

Geophysical characteristics of a focused fluid flow system in the western South China Sea

High-resolution multichannel seismic data and multibeam terrain data are used to analyse and identify a focused fluid flow (FFF) system in the central and northern parts of the Zhongjiannan Basin in the western South China Sea. The authors classify and summarise the geophysical characteristics of an FFF system in the study area. It is divided into a fault-related FFF system and a column-related FFF system. In addition, the FFF system in the study area is mainly located within the tectonic activity area, thin slope break belt and land slope area. The rich FFF system in the study area is an important channel for fluid migration, which is conducive to gas hydrate accumulation. The FFF system in the study area is poorly studied, and the distribution research results of the FFF system in this area can be used more widely in hydrocarbon and gas hydrate exploration and must be taken into account when assessing seabed stability.

Oligocene magmatic accretion and transtensional tectonics in the Central Basin Fault rift of the West Philippine back-arc basin: New insights from high-resolution multichannel seismic data

High-resolution multichannel seismic transects that span from oceanic crust domains, and the Central Basin Fault rift to the Kyushu-Palau Ridge are utilized to investigate the dynamic mechanism of the Oligocene tectonic evolution in the West Philippine Basin based on an analysis of sismostratigraphic sequences and the geometries of acoustic substrates. In the oceanic domains, the physiography is characterized by ridges and troughs, and the pelagic stratigraphic succession with transparent facies covers the irregular top of the basement. Relatively homogeneous intracrustal reflections probably correspond to an area of abundant magma supply during the major spreading period. However, a widespread discrete, layered and high-amplitude intracrustal reflector is present in the Central Basin Fault rift that can be interpreted as an area of weaker and unstable magma supply after the major spreading period. The first group of reflectors shows typical low-angle fault shapes; the second and third groups of layered intracrustal reflectors are characterized by spatially limited high-amplitude seismic reflections. Inclined and mounded features of the intracrustal reflectors are interpreted as the result of tectonic processes, suggesting the presence of later magmatic accretion and transtensional tectonics within the acoustic substrate. Additionally, in the Kyushu-Palau Ridge, the anticline that appears within the stratigraphic infills of the basins serves as evidence of transtensional faulting. Overall, we propose that the Oligocene tectonic evolution of the West Philippine Basin occurred during a later transcurrent period due to Oligocene magmatic accretion followed by transtensional tectonics. The changes in the magmatic supply rate and tectonic processes of the Central Basin Fault rift may have originated from the compressive process within the Kyushu-Palau Ridge.

Sedimentary evolution and effects of structural controls on the development of the Zambezi mixed turbidite-contourite system (Mozambique Channel, Southwest Indian Ocean) since the Oligocene

High-resolution multichannel seismic reflection data that spans significant parts of the Mozambique margin offshore the Zambezi River permits the study of the Oligocene to present architectural evolution of the Zambezi turbidite system. In this time frame, five major depositional units are recognized that evidence a widespread spatial and temporal occurrence of both turbiditic and contouritic sedimentation. They indicate that the sedimentary regime within the turbidite system changed from dominantly aggradational during the Oligocene to mainly erosional during Miocene to an interplay of erosional and depositional processes during the Plio-Quaternary. Different episodes of incision, linked with the Serpa Pinto, Angoche and Zambezi valleys, are recognized in the upstream portion of the Zambezi Fan and highlight a westward (anticlockwise) shift of feeding axes. The central portion of the Zambezi Valley was affected by a progressive structural doming during the Miocene. The dominance of long-lasting erosional processes generated by the continuous rise of the seabed led to a deep entrenchment of the Zambezi Valley. This tectonically-controlled over-incision is believed to be the cause of the absence of Miocene levees, and has played an important role in the stabilization of the valley at its current position. Finally, our study revealed a quasi-constant development of contourite accumulations since the Late Miocene that occur most often synchronous with turbiditic sedimentation. The present study offers unique insight into the controls and stages of development of one of the largest turbidite systems in the world and demonstrates especially its susceptibility to structural activity.

Lateral coexistence of ductile and brittle deformation shapes magma-poor distal margins: An example from the West Iberia-Newfoundland margins

A long-standing problem in solid Earth science is to understand how low-angle normal faults form, their role in the development of tectonic asymmetry of conjugate margins, and how they relate to mantle hydration during continental breakup. The latter requires water to reach the mantle through active brittle faults, but low angle slip on faults is mechanically difficult. Here, we incorporate observations from high-resolution multichannel seismic data along the West Iberia-Newfoundland margins into a 2D forward thermo-mechanical model to understand the relationship between evolving rift asymmetry, detachment tectonics, and mantle hydration. We show that, during extreme extension, slip on active faults bifurcates at depth into brittle and ductile deformation branches, as a result of the cooling of the faults' footwall and heating of their hangingwall. The brittle deformation penetrates the Moho and leads to mantle hydration, while ductile deformation occurs in localized shear zones and leads to the formation of detachment-like structures in the distal margin sections. Such structures, as for example ‘S’ in the West Iberia-Newfoundland margins, are thus composed of several shear zones, active at low-angles, ∼25°-20°, and merging with the Moho at depth. The final sub-horizontal geometry of these structures is the result of subsequent back-rotation of these shear zones by new oceanward faults. Our results reproduce remarkably well the final sedimentary, fault, crustal architecture, and serpentinisation pattern observed at the West Iberia-Newfoundland margins. However, they challenge widely accepted ideas that such detachment-like structures formed by brittle processes, separate crust from mantle and caused conjugate margin asymmetry. Our model provides a quantitative framework to study hydrothermal systems related to serpentinization during extreme extension, their associated hydrogen, methane production, and the chemosynthetic life they sustain.

Seismic evidence of shallow gas in sediments on the southeastern continental shelf of Korea, East Sea (Japan Sea)

High resolution multi-channel seismic reflection data (~1000 km) and multibeam echosounder bathymetry from the southeastern Korean continental shelf of the East Sea (Japan Sea) reveal numerous shallow gas indicators and seepage-related features, such as bright spots, enhanced reflections, seismic chimneys, acoustic blanking, pockmarks, and bathymetric mounds. Bright spots, indicating gas-charged layers, appear as local negative-polarity reflection anomalies (up to 5 km wide) and occur at various stratigraphic levels within a subsurface depth of ~320 m. Bright spots covering an area of ~60 km2 are clustered at the tip of NE-SW–trending reverse faults in the northeastern and southeastern part of the investigated region, suggesting gas entrapment. Enhanced reflections (ca. 20-km-long) are developed along erosional unconformities and tilted sedimentary layers below them. This suggests that unconformities formed during sea-level low stands in the study area are potential reservoirs and may have acted as potential conduits for lateral migration of gas-rich fluids due to their permeable nature. Some enhanced reflections are formed along interfluves of channels where channel walls cut them, and thus they may potentially act as fluid reservoirs. Seismic chimneys, expressed as vertical disturbances in seismic data, are interpreted as the upward movement of fluids (i.e., either in liquid or gaseous form). Lack of faulting in some seismic chimneys suggests higher permeability in the sedimentary interval, which would allow the migration of deeper-sourced fluids. Pockmarks (up to 500 m in diameter) are typically associated with seismic chimneys in the sub-seabed, suggesting that they were formed by the explosive emission of gas or gas fluids. Some exhibit mound-like features near their crests that are interpreted as carbonate mounds. The locations of mounds above uplifted fault blocks in the central part suggest a structural control on the formation of these seabed features.

A buried submarine canyon in the northwestern South China Sea: architecture, development processes and implications for hydrocarbon exploration

High-resolution multichannel seismic data enables the discovery of a previous, undocumented submarine canyon (Huaguang Canyon) in the Qiongdongnan Basin, northwestern South China Sea. The Huaguang Canyon with a NW orientation is 140 km in length, and 2.5 km to 5 km in width in its upper reach and 4.6 km to 9.5 km in width in its lower reach. The head of the Huaguang Canyon is close to the Xisha carbonate platform and its tail is adjacent to the Central Canyon. This buried submarine canyon is formed by gravity flows from the Xisha carbonate platform when the sea level dropped in the early stage of the late Miocene (around 10.5 Ma). The internal architecture of the Huaguang Canyon is mainly characterized by high amplitude reflections, indicating that this ancient submarine canyon was filled with coarse-grained sediments. The sediment was principally scourced from the Xisha carbonate platform. In contrast to other buried large-scale submarine canyons (Central Canyon and Zhongjian Canyon) in the Qiongdongnan Basin, the Huaguang Canyon displays later formation time, smaller width and length, and single sediment supply. The coarse-grained deposits within the Huaguang Canyon provide a good environment for reserving oil and gas, and the muddy fillings in the Huaguang Canyon have been identified as regional caps. Therefore, the Huaguang Canyon is a potential area for future hydrocarbon exploration in the northwestern South China Sea. The result of this paper may contribute to a better understanding of the evolution of submarine canyons formed in carbonate environment.

Types, characteristics, distribution, and genesis of pockmarks in the South China Sea: insights from high-resolution multibeam bathymetric and multichannel seismic data

ABSTRACT Ubiquitous pockmarks in continental margins are important to understand the tectonic and sedimentary history and flow activity of their host margins. The study of an extensive field of pockmarks in the South China Sea (SCS) is important both scientifically and economically, for the exploration of oil, gas, and gas hydrate resources and the prevention and control of marine geological disasters. We report our new findings on the characteristics, distribution, and genesis of pockmarks in the SCS based on interpretation of high-resolution multibeam bathymetric data and multichannel seismic profiles collected in the past decade. Three pockmark fields (total area 572,824 km2), are recognized in the northern, western, and southern continental margins of the SCS. The pockmarks are highly variable in shape: circular, elliptical, crescent-shaped, elongated or forming a chain in plan view and U-, V- or W-shaped in cross-section. Pockmarks are normally 10–200 m in diameter, with maximum diameters/axial lengths of 7.3 km for circular, 5.5 km for elliptical, 8.6 km for crescent-shaped, 9.5 km for elongated, and 30 km for chain-type pockmarks. Pockmarks are normally less than 35 m deep, with a maximum depth of 341 m at the southwestern SCS margin. These pockmarks might have originated from methane seepages or from pore fluids from deeper sources, which migrated vertically along gas chimneys, mud diapirs, mud volcanoes, faults or buried channels. In addition to possible structural controls, the pockmark morphologies are affected by bottom currents.

Seismic stratigraphy and the sedimentation history in the Laxmi Basin of the eastern Arabian Sea: Constraints from IODP Expedition 355

Detailed interpretation of seismic stratigraphic sequences in the Laxmi Basin of the eastern Arabian Sea are presented in this study using closely spaced high resolution multi-channel seismic (MCS) data. Our stratigraphic interpretation is further corroborated using recent drilling results in the Laxmi Basin, derived from the long sediment cores collected during International Ocean Discovery Program (IODP) Expedition 355. Integrated core-log interpretation discussed in the present study, offer important insights about the lithostratigraphic variations in this region. Analyses of multi-channel seismic reflection data reveal five depositional sequences (ranging from Paleocene to Recent) that led to the development of this marginal basin since the Cenozoic period. Regional igneous basement is successfully imaged, which was also validated by deep sea coring during the IODP Expedition 355. In the present study, we primarily focus on the post-rift sedimentation in the Laxmi Basin and its possible mechanisms. Our detailed interpretation in the prevailing tectonic framework of the basin suggests that near-shelf oldest volcaniclastic sedimentation immediately overlying the acoustic basement is linked to the onset of India–Madagascar and India–Seychelles rifting activities during the Late Cretaceous period. Eventually, during the Early-Mid to Late Miocene, the basin received maximum sedimentation dominantly through an extensive mass transport mechanism implying possible large-scale deformation on the Indian shelf. Subsequent sediment input to the basin appears to have been fed variably via the Indus Fan as well as coastal discharge from the Indian mainland. The total sediment thickness in the Laxmi Basin ranges from 1.1 to 3.5 km. New stratigraphic information and sediment isopach maps presented here provide vital information about syn- and post-rift sedimentation pattern in the region and their long term tectonic implications.

Characterization of Gas Hydrates Reservoirs in Krishna-Godavari Basin, Eastern Indian Margin

Seismic inversion and attribute analysis techniques are frequently used in oil and gas industries to characterize a hydrocarbon reservoir. The gas hydrate reservoir in the Krishna-Godavari basin, eastern Indian margin, by analysis of highresolution multi-channel seismic (MCS) data has been characterised. The model-based seismic inversion was performed to derive acoustic impedance (3220 m/s*g/cm3) followed by computation of seismic attributes for delineating the bottom simulating reflector or BSR, main marker for gas hydrates, and characterizing the hydrate-bearing sediments. The study shows the characteristics of BSR at a depth of ∼125 m below sea floor (mbsf), and hence the presence of gas hydrates. The high velocity, low amplitude, high instantaneous frequency and low sweetness indicate presence of gas hydrates above the BSR, whereas low velocity, high reflection strength, low frequencies and high sweetness imply free gas underlying the BSR.

Gas occurrence and shallow conduit systems in the Western Sea of Marmara: a review and new acoustic evidence

Based on 3D and 2D high-resolution multichannel seismic reflection data in the Western High-Sea of Marmara, this study reviews shallow gas occurrence and related structures and classifies gas conduit systems within the upper, few hundred meter-thick sediment layers below the seafloor. Acoustic anomalies including high amplitude-reverse polarity reflections (bright spots), low amplitude transparent zones, chaotic or discontinuous reflections, pull-down effects, and plumes in the water column are interpreted in terms of natural gas occurrence and fluid flow structures (e.g., mud volcanoes, pockmarks). The gas occurrence is thought to be mostly of thermogenic origin. Mud volcanoes are one of the primary gas conduits forming craters on the seabed due to overpressure of fluidized gassy sediment flows. Following the reach of the Northern Branch of the North Anatolian Fault (NAF-N) to the Western High, the thermogenic fluids are believed to migrate vertically and horizontally to shallow depths mainly through the faults. Natural gas most probably originates from the Thrace Basin Eocene source rock or the Eocene-Oligocene reservoir rock, which extends below the Western High. Shallow gas is distributed by minor faults and gas pipes. Gas, to some extent, emanates from the seafloor via pockmarks and mud volcanoes or is trapped by the crests of the anticlines coinciding with erosional surfaces, impermeable sediments, and gas hydrate-bearing layers. Shallow traps below the tectonized “Western High” structure are likely located in thin layers of sands imbedded with impermeable silty clay layers. However, there is no shallow reservoir in the usual sense within the upper layers imaged by the 3D seismic data (< 300 ms two-way travel time). The existence of gas is an indicator of hydrocarbon-rich layers at depth and of active tectonics, and it also impacts the global climate and marine life conditions.

Subsurface controls on the development of the Cape Fear Slide Complex, central US Atlantic Margin

AbstractThe Cape Fear Slide is one of the largest (&gt;25 000 km3) submarine slope failure complexes on the US Atlantic margin. Here we use a combination of new high-resolution multichannel seismic data (MCS) from the National Science Foundation Geodynamic Processes at Rifting and Subducting Margins (NSF GeoPRISMS) Community Seismic Experiment and legacy industry MCS to derive detailed stratigraphy of this slide and constrain the conditions that lead to slope instability. Limited outer-shelf and upper-slope accommodation space during the Neogene, combined with lowstand fluvial inputs and northwards Gulf Stream sediment transport, appears to have contributed to thick Miocene and Pliocene deposits that onlapped the lower slope. This resulted in burial of an upper-slope bypass zone developed from earlier erosional truncation of Paleogene strata. These deposits created a broad ramp that allowed accumulation of thick Quaternary strata across a low-gradient (&lt;3.5°) upper slope. Upslope of one of the larger headwalls, undulating Quaternary strata appear to downlap onto a buried failure plane. Many of the nested headwalls of the upper-slope portion of slide complex are underlain by deformed strata, which may be the result of fluid migration associated with localized subsidence from salt migration. These new data and observations suggest that antecedent margin physiography, sediment loading and substrate fluid flow were key factors in preconditioning the Cape Fear slope for failure.

Late Pleistocene sequence architecture on the geostrophic current-dominated southwest margin of the Ulleung Basin, East Sea

High-resolution multichannel seismic data were collected to identify depositional sequences on the southwestern shelf of the Ulleung Basin, where a unidirectional ocean current is dominant at water depths exceeding 130 m. Four aggradational stratigraphic sequences with a 100,000-year cycle were recognized since marine isotope stage (MIS) 10. These sequences consist only of lowstand systems tracts (LSTs) and falling-stage systems tracts (FSSTs). Prograding wedge-shaped deposits are present in the LSTs near the shelf break. Oblique progradational clinoforms of forced regressive deposits are present in the FSSTs on the outer continental shelf. Each FSST has non-uniform forced regressional stratal geometries, reflecting that the origins of sediments in each depositional sequence changed when sea level was falling. Slump deposits are characteristically developed in the upper layer of the FSSTs, and this was used as evidence to distinguish the sequence boundaries. The subsidence rates around the shelf break reached as much as 0.6 mm/year since MIS 10, which contributed to the well-preserved depositional sequence. During the Quaternary sea-level change, the water depth in the Korea Strait declined and the intensity of the Tsushima Current flowing near the bottom of the inner continental shelf increased. This resulted in greater erosion of sediments that were delivered to the outer continental shelf, which was the main cause of sediment deposition on the deep, low-angled outer shelf. Therefore, a depositional sequence formation model that consists of only FSSTs and LSTs, excluding highstand systems tracts (HSTs) and transgressive systems tracts (TSTs), best explains the depositional sequence beneath this shelf margin dominated by a geostrophic current.

Sediment waves in the Caroline Basin suggest evidence for Miocene shifts in bottom water flow in the western equatorial Pacific

We describe abyssal sediment waves in seismic profiles from the Caroline Basin, western Pacific Ocean, which indicate bottom currents at this location have shaped the seabed beginning in the early to mid-Miocene. Our evidence is based on 2D high-resolution multi-channel seismic data we collected in 2013 during a site survey for IODP Expedition 363. Drilling within this grid (at IODP Site U1490) was completed in late 2016 and we expect results will provide the age and composition needed to evaluate the paleoceanographic implications of the geomorphologic history we describe. Our seismic grid reveals roughly 150ms (two-way travel time; roughly 135m) of sediment waves with 10–40ms wave heights (roughly 9–36m) and 1–2km wavelengths. These waves most likely accumulated in carbonate-rich sediments, based on DSDP Site 62 (450km to the south but lacking evidence of bottom currents in profiles or cores). The wave-like bedforms in our survey grid are independent of underlying structures and may have developed in response to an early Neogene intensification of bottom water flow documented at ODP Site 1124 in the SW Pacific (Carter et al., 2004). Steady growth and slow NW migration of the waves in the survey grid continued for much of the Miocene. Above ~250ms (roughly 212m) below the seafloor lateral migration halted and wave growth slowed. Today waves are present on the seafloor due to vertical aggradation along the flank of a gentle ridge at the northern edge of our survey grid. We propose this evolution of wave morphology is linked to global climatic events that drove thermohaline circulation throughout the world's oceans.

Carbonate mounds in the Santaren Channel, Bahamas: A current-dominated periplatform depositional regime

New hydroacoustic data, high resolution multichannel seismic reflection data, CTD casts and sampling document the dominance of bottom currents and the occurrence of carbonate mounds in the Santaren Channel off Great Bahama Bank and Cay Sal Bank. Based on these data sets, three types of carbonate mounds have been identified in water depths between 285 and 685m. Their distribution is strongly related to the present current regime. Bank-driven sediments are caught up by the currents and redeposited mainly in the channel center, resulting in the accumulation of giant confined drifts flanked by moats. The moats at the banks toe-of-slope are formed by a southward and northward flowing water mass, respectively, mainly Antarctic Intermediate Water and Subtropical Underwater. The observed carbonate mounds are concentrated in these moats. Mound type 1 has an elongated shape parallel to the prevailing bottom current direction, a gently inclined flank facing the water flow and a steep flank in its current shadow. Its upstream flank is covered by a thin sediment wedge whereas the lee side is marked by a parabolic scour. Type 1 occurs at both bank margins but mainly off Great Bahama Bank, consisting of cold-water coral rubble floating in a sediment matrix and is, thus, interpreted as cold-water coral mound. By contrast, type 2 mounds are restricted to the Cay Sal margin and have a predominately conical outline. Bottom samples including living sessile crinoids show that its surface must be lithified and colonized by macroepibenthos, except corals. The mounds are situated in pockmark-like depressions that are underlain by fluid escape structures like pipes and chimneys. These fluid pathways are generated by a network of fractures. Mound type 2 is probably formed by abiotic carbonate precipitation; however, a chemoherm nature cannot be ruled out. Mound type 3 has a conical to polygonal shape and occurs off Cay Sal Bank and Great Bahama Bank. It is surrounded by a scour which is elongated in downcurrent and crescent in upcurrent direction. A sediment bulge formed at its front. This type is consistent with blocks and boulders colonized by macroepibenthos and related to gravity-controlled mass movements at the steep bank slopes.Principally, two habitats can be distinguished: the Great Bahama Bank periplatform deep-water province dominated by mound type 1 and the Cay Sal Bank slope to basin province occupied by mound type 2 that is associated with pockmark-like structures. Mound type 3 occurs in both habitats. Physical water mass properties for the existence of cold-water corals are limited to a depth range of ca. 430 to 685m, with seawater densities (σ-θ) of 26.93 to 27.29kg/m and temperatures below 13°C. Their dominance off Great Bahama Bank might be addressed to a more effective food supply induced by local downwelling.

Cenozoic magmatism in the northern continental margin of the South China Sea: evidence from seismic profiles

Igneous rocks in the northern margin of the South China Sea (SCS) have been identified via high resolution multi-channel seismic data in addition to other geophysical and drilling well data. This study identified intrusive and extrusive structures including seamounts and buried volcanoes, and their seismic characteristics. Intrusive features consist of piercement and implicit-piercement type structures, indicating different energy input associated with diapir formation. Extrusive structures are divided into flat-topped and conical-topped seamounts. Three main criteria (the overlying strata, the contact relationship and sills) were used to distinguish between intrusive rocks and buried volcanos. Three criteria are also used to estimate the timing of igneous rock formation: the contact relationship, the overlying sedimentary thickness and seismic reflection characteristics. These criteria are applied to recognize and distinguish between three periods of Cenozoic magmatism in the northern margin of the SCS: before seafloor spreading (Paleocene and Eocene), during seafloor spreading (Early Oligocene–Mid Miocene) and after cessation of seafloor spreading (Mid Miocene–Recent). Among them, greater attention is given to the extensive magmatism since 5.5 Ma, which is present throughout nearly all of the study area, making it a significant event in the SCS. Almost all of the Cenozoic igneous rocks were located below the 1500 m bathymetric contour. In contrast with the wide distribution of igneous rocks in the volcanic rifted margin, igneous rocks in the syn-rift stage of the northern margin of the SCS are extremely sporadic, and they could only be found in the southern Pearl River Mouth basin and NW sub-sea basin. The ocean–continent transition of the northern SCS exhibits high-angle listric faults, concentrated on the seaward side of the magmatic zone, and a sharply decreased crust, with little influence from a mantle plume. These observations provide further evidence to suggest that the northern margin of the SCS is a magma-poor rifted margin.

Submarine explosive volcanism in the southeastern Terceira Rift/São Miguel region (Azores)

Morphologic studies with sonar data and in situ observations of modern eruptions have revealed some information suggesting how submarine volcanic cones develop, but the information only addresses the modern surfaces of these features. Here, we describe a study combining morphological data with high-resolution seismic reflection data collected over cones within the southeastern Terceira Rift — a succession of deep basins, volcanic bathymetric highs and islands (e.g. São Miguel) representing the westernmost part of the Eurasian–Nubian plate boundary.The cones (252) are distributed in depths down to 3200m and exhibit an average diameter of 743m, an average slope of 20° and heights mainly between 50 and 200m. The cones are here classified into three different categories by physiographic or tectonic setting (we find no particular morphometric differences in cone shapes between these areas). First, numerous cones located at the submarine flanks of São Miguel's Sete Cidades and Fogo Volcano are considered to be parasitic structures. Second, in the southeast of the island, they form a superstructure possibly reflecting an early submarine stadium of a posterior subaerial stratovolcano. Third, some cones are controlled by faults, mostly in a graben system southwest of the island. High-resolution multichannel seismic data indicates that the graben cones evolved synchronously with the graben formation. Bottom currents then probably removed the surficial fine grain-size fraction, leaving rough surface textures of the cones, which backscatter sonar signals strongly in the data recorded here. However, a young cone investigated in detail is characterized by a smooth surface, a marked increase of internal stratification with increasing distance from the summit and upwards concave flanks. Others exhibit central craters, suggesting an explosive than an effusive evolution of these structures. The morphological characteristics of these submarine cones show that they have similar sizes and shapes to cinder cones onshore São Miguel.