Multichannel Seismic Data Research Articles

Delayed Subsidence After Rifting and a Record of Breakup for Northwestern Zealandia

AbstractContinental rifting and breakup of eastern Gondwana during the Cretaceous separated northern Zealandia from eastern Australia, but the processes leading to this highly extended and largely submerged block of continental crust are unknown. We acquired and processed multichannel seismic reflection data across northern Zealandia and examine the stratigraphy of the Middleton Basin. We identified a two‐phase formation process for the basin, as evidenced by an unconformity separating two postrift units. After initial basin formation and slow deposition of the lower postrift unit, deposition rates within the Middleton Basin rapidly increased in response to the latest stage of subsidence and to create the modern basin. We propose a tectonic model wherein the Middleton Basin initiated through oceanic spreading and the subsequent postrift subsidence of the newly created oceanic lithosphere was delayed due to thermal buoyancy associated with nearby oceanic spreading in the Tasman Basin. Our results provide new constraints on rifting and breakup processes of wide, magma‐poor, and asymmetric margins and indicate that multiple regions of weak lithosphere may have influenced the breakup.

Three-dimensional multichannel seismic imaging of water columns in the Gulf of Mexico

Seismic reflection profiling technique, previously used to image the sediments beneath the seafloor, is herein used to image the ocean’s water columns, namely, seismic oceanography. The imaging has a much higher lateral resolution (~10 m) than traditional oceanographic measurements such as CTD (usually >100 m). Prior work on seismic oceanography was limited on imaging in two dimensional vertical transects. This work develops the three-dimensional (3D) seismic oceanography technique to image the 3D dynamic processes of water columns. 3D multichannel seismic survey data in a seismic volume of 625 km3 in the Gulf of Mexico are processed and produce images containing detailed 3D water-column structures near the continental slope. Some mesoscale and sub-mesoscale structures are visualized from different viewing angles. Spectral analyses of the seismic images reveal 3D spatial features of the structures, suggesting the potential of 3D seismic oceanography. [Work supported by NOAA.]

Seaward growth of Antarctic continental shelves since establishment of a continent-wide ice sheet: Patterns and mechanisms

The location and setting of the continental shelf break along the Antarctic continental shelves influences the paleoceanographic circulation patterns as well as sedimentation and ice sheet dynamics. By evaluating the available multichannel seismic reflection data and drill core information, we mapped the base of progradational features associated with the first advances of grounded ice sheet to the outer shelf. The distance between the modern shelf edge and the pre-glacial shelf edge varies from >200 km (e.g. Ross Sea) to <6 km (e.g. Thurston Island, West Antarctica). We identified multiple processes and mechanisms which influence the amount of shelf progradation during the establishment of a grounded ice sheet on the outer Antarctic shelves. The tectonic structure of the shelves proves to be a key factor by establishing the width of the shelf as well as local tectonics such as horst/graben and fault systems, which guide the ice stream flow and therefore the resulting sedimentation. Unlike expected, the erosional potential of the hinterland seems to play a minor role in building progradational sequences, whereas fewer long-term grounding events on the shelf edge result in more continental shelf progradation than presumed from a more dynamic ice sheet. By using these observed relationships to extrapolate the former shelf edge to unmapped regions around the Antarctic continent, we estimate an overall seaward growth of the continental shelves by 7% in area and 1.28 ∗ 106 km3 of approximated sedimentary volume deposited along the shelf edge and upper slope.

The influence of bottom currents on the Zambezi Valley morphology (Mozambique Channel, SW Indian Ocean): In situ current observations and hydrodynamic modelling

Mixed turbidite-contourite systems can be found in oceans where bottom currents and turbidity currents interact. The Zambezi turbidite system, located in the Mozambique Channel (SW Indian Ocean), is one of the largest sedimentary systems in the world in length and area of the related catchments. The oceanic circulation in the Mozambique Channel is intense and complex, dominated by eddies flowing southwards and deep currents flowing northwards along the Mozambican margin. Current measurements obtained from moorings at 3400–4050 m water depth in the Zambezi and Tsiribihina valleys show periods of intense currents at the seafloor with peaks of 40–50 cm s−1 that last up to one month and are not related to turbidity currents. These strong bottom-current events are correlated with a change in current direction and an increase in temperature. The periods of current intensification may be related to eddies, since they present similar frequencies (around 7 per year). Moreover, modelling results show that during periods of intense deep circulation an anticyclonic eddy is present between the Mozambican slope and the centre of the Mozambique Channel, which may block the northward transport of the deep water mass and thus enhance the southward transport along the western slope of Madagascar. According to our hydrodynamic modelling of the circulation near the seafloor, intense currents are often present along the Zambezi Valley, especially along the valley flanks. Multi-channel seismic reflection data show that the Zambezi turbidite system does not show the typical characteristics of turbidite systems, being dominated by erosional processes, which mainly affect the valley flanks. Levees associated with the valley are absent in the main axis of the system. The effect of bottom currents on sedimentation in the basin is evidenced by the low sedimentation rates that witness winnowing in the basin, the presence of contouritic sand in the Zambezi Valley flanks and the abundance of current-related bedforms observed in multibeam bathymetry and seismic data. The intense oceanic processes observed in the Mozambique Channel may transport a large part of the fine sediment out of the basin and erode the seafloor even at great depths. Therefore, the Zambezi turbidite system could at present be considered as a mixed turbidite-contourite system, with important implications for source-to-sink studies.

Extension Structures in the Central Arctic Submarine Elevations Complex

Abstract—The available body of geological and geophysical data indicates that the morphologic structures of the Central Arctic submarine elevations complex (CAE) form a single complex block of continental crust that broke away from the Barents–Kara continental margin in the late Paleocene. Seismostratigraphic interpretation of the multichannel seismic reflection data acquired within the CAE, based on seismostratigraphic benchmarks confirmed by drilling and continuous tracing of pre-Cenozoic unconformities from the offshore North Chukchi Trough to its deep-water extension (Vilkitsky Trough), makes it possible to draw the following conclusions: The sedimentary-basin depocenters of the Vilkitsky Trough and Chukchi basin include pre-Upper Jurassic sediments in addition to Cretaceous complexes. However, the former are not common in the rest area of the CAE.Synrift extension of the continental crust is the key factor that affected the tectonic evolution of morphologic structures of the Central Arctic basin. Multichannel seismic reflection data show the clearest signs of the synrift extension in the Lomonosov Ridge, Mendeleev Rise, Chukchi plateau, and their flanks sloping to the sedimentary basins of the Vilkitsky Trough and Chukchi basin. At the same time, the depocenters of these sedimentary basins formed by pre-Upper Jurassic deposits are characterized by an almost undisturbed bedding of all sedimentary complexes.Pre-Upper Jurassic deposits might be interpreted as a relic of the Ellesmerian structural stage preserved in the deep-water extension of the North Chukchi Trough since the preoceanic evolution stage. Pre-Upper Jurassic complexes seem to be affected by deep rift activity only within the elevations of the Central Arctic area and near-flank zones of the depressions separating them. Pre-Upper Jurassic deposits in the sedimentary basin depocenters of the Vilkitsky Trough and Chukchi basin structurally linked to the shallow-water shelf were barely affected by the rifting processes. The tectonic evolution of the depocenters and their submergence relative to the flank zones might have been affected not only by crustal extension processes but also by compensation mechanisms.

Depositional Evolution of the Western Amundsen Basin, Arctic Ocean: Paleoceanographic and Tectonic Implications

AbstractA new stratigraphic model and estimated sedimentation rates of the western Amundsen Basin, Arctic Ocean, are presented based on multichannel seismic reflection data, seismic refraction data, magnetic data, and integrated with the sedimentary sequence from the central Arctic Ocean, obtained during the Arctic Coring Expedition. This places new constraints on the postbreakup Cenozoic depositional history of the basin, the adjacent Lomonosov Ridge, and improves the understanding of the tectonic, climatic, and oceanographic conditions in the central Arctic region. Four distinct phases of basin development are proposed. During the Paleocene‐mid‐Oligocene, high sedimentation rates are linked to terrestrial input and increased pelagic deposition in a restricted basin. Deposition of sedimentary wedges and mass transport into marginal depocenters reflect a period of tectonic instability linked to compression associated with the Eurekan Orogeny in the Arctic. During the late Oligocene‐early Miocene, widespread passive infill associated with hemipelagic deposition reflects a phase of limited tectonism, most likely in a freshwater estuarine setting. During the middle Miocene, mounded sedimentary buildups along the Lomonosov Ridge suggest the onset of geostrophic bottom currents that likely formed in response to a deepening and widening of the Fram Strait beginning around 18 Ma. In contrast, the Plio‐Pleistocene stage is characterized by erosional features such as scarps and channels adjacent to levee accumulations, indicative of a change to a higher‐energy environment. These deposits are suggested to be partly associated with dense shelf water‐mass plumes driven by supercooling and brine formation over the northern Greenland continental shelf.

Neogene Modifications of Circulation in the Northeastern African‐Southern Ocean Gateway

AbstractIn the African‐Southern Ocean gateway, several water masses originating in the Atlantic, Indian, and Southern oceans meet and mix. As a consequence, the gateway is crucial for the maintenance of the global thermohaline circulation. Newly acquired multichannel seismic reflection data collected across the southern Mozambique Ridge are used to reconstruct the impact of paleoceanographic modifications on the Neogene and Quaternary circulation in the northeastern African‐Southern Ocean gateway. The data show the occurrence of mid‐Miocene (~15 Ma) to early Pliocene (~5 Ma) contourite drifts and erosional features interpreted as evidence for the onset of current‐controlled sedimentation in the late Neogene resulting from mid‐Miocene cooling and closure of the Indonesian gateway. The Quaternary is characterized by a relocation of Antarctic Bottom Water inflow and the inception of two branches of the North Atlantic Deep Water circulation subsequent to the final closure of the Central American Seaway and the Northern Hemisphere Glaciation. Therefore, the two events triggered the onset of unhindered deep and bottom water circulation from the Atlantic into the Indian Ocean, whereas the Antarctic Intermediate Water circulation decreased due to the final closure of the Indonesian gateway. Our results show that tectonic and climatic events, which themselves may be linked, continuously modified the Cenozoic paleoceanic circulation in the African‐Southern Ocean gateway, and indicate that ocean gateways governing the global water mass exchange act as an excellent location to reconstruct such modifications based on the interpretation of contourite drifts and erosional features.

A novel measurement of the surface wave dispersion with high and adjustable resolution: Multi-channel nonlinear signal comparison

Dispersion measurement of surface-waves is critical for utilizing surface-waves. However, the traditional dispersion measurements of multi-channel seismic data are usually based on linear signal comparison and achieve a low resolution at low frequencies. To improve the accuracy of surface-wave dispersion measurement for multi-channel seismic data, we propose a novel measurement that can produce a high and adjustable resolution among a wide detected frequency range. This measurement is based on a nonlinear signal comparison using multi-channel seismic data. We evaluated the performance of this proposed multi-channel nonlinear signal comparison (MNLSC) on several datasets: one synthetic shotgather containing only surface waves, one synthetic shotgather with full elastic waves, and one field shotgather. These examples show that the MNLSC is promising to be a reliable and effective tool for measuring the surface dispersion from the multi-channel seismic data.

New interpretation for the main sediment source of the rapidly deposited sediment drifts on the northern slope of the South China Sea

The slope of the northern South China Sea (SCS) is one of the world’s most active areas of modern marine sedimentary processes. A rapidly deposited sediment drift (RDSD) was identified on the slope near Dongsha Island. The sources of this body have been studied and discussed by several authors, but no consensus has been achieved. Based on multi-channel seismic profiles interpretation and data analysis of long-term observation of bottom currents, we identified that although the amounts of sediments input from the Pearl and Hanjing Rivers, and from Kao-ping and Tseng-Wen Rivers are large, considering the current situation of northern SCS, none of the Pearl River, the Hanjiang River, the Kao-ping River and the Tseng-Wen River can maintain the deposition pattern of the northern SCS margin as the single source, especially to maintain the supper high sedimentation rate at the RDSD area. The Miocene drape formation (MDF) calmly draped over an unconformity after rifting stage resulting in a relatively constant thickness of about 2000 m throughout the overall northern SCS continental margin, and followed by a standard configuration of a clinoform, the Pliocene formation and another clinoform of about another 700 m in total thickness. Due to Dongsha uplift since 3 Ma, roughly 1/2 of MDF and the N2 formation in the Dongsha area measures 150 km by 200 km were denudated resulting in an average annual denudation rate of 60 × 106 t/y. If these sediments were deposited on a slope region within an area measuring 100 km by 100 km, the sedimentation rate could reach 250 cm/ky. This is clearly an impressive new sediment source that has never been discussed before. Moreover, the RDSD is near the Dongsha Uplift and on the adjacent slope. We believe that this new sediment source should be the main contributor for the RDSD development.

Impact of seismicity on Nice slope stability—Ligurian Basin, SE France: a geotechnical revisit

The shallow Nice submarine slope is notorious for the 1979 tsunamigenic landslide that caused eight casualties and severe infrastructural damage. Many previous studies have tackled the question whether earthquake shaking would lead to slope failure and a repetition of the deadly scenario in the region. The answers are controversial. In this study, we assess for the first time the factor of safety using peak ground accelerations (PGAs) from synthetic accelerograms from a simulated offshore Mw 6.3 earthquake at a distance of 25 km from the slope. Based on cone penetration tests (CPTu) and multichannel seismic reflection data, a coarser grained sediment layer was identified. In an innovative geotechnical approach based on uniform cyclic and arbitrary triaxial loading tests, we show that the sandy silt on the Nice submarine slope will fail under certain ground motion conditions. The uniform cyclic triaxial tests indicate that liquefaction failure is likely to occur in Nice slope sediments in the case of a Mw 6.3 earthquake 25 km away. A potential future submarine landslide could have a slide volume (7.7 × 106 m3) similar to the 1979 event. Arbitrary loading tests reveal post-loading pore water pressure rise, which might explain post-earthquake slope failures observed in the field. This study shows that some of the earlier studies offshore Nice may have overestimated the slope stability because they underestimated potential PGAs on the shallow marine slope deposits.

Practical approaches to maximizing the resolution of sparker seismic reflection data

Sparkers are a type of sound source widely used by the marine seismic community to provide high-resolution imagery of the shallow sub-bottom (i.e., < 1000 m). Although sparkers are relatively simple, inexpensive, and high-frequency (100–2500 Hz) sources, they have several potential pitfalls due to their complicated and unpredictable signature. In this study we quantify the source characteristics of several sparker systems and develop a suite of simple processing approaches for both single channel and multi-channel sparker data. In all cases, the results show improved vertical resolution and reflection coherency. Correcting for small static variations in multi-channel seismic (MCS) data is a critical first step to preserve the broad frequency content during stacking, and to reduce the shot-to-shot variability of outgoing and incoming signals. Application of predictive deconvolution to static-corrected, post-stack traces suppresses short-path multiples and restores the latent high-resolution reflection patterns. However, if shot-to-shot source signatures are recorded directly, pre-stack deterministic deconvolution followed by post-stack predictive deconvolution produces the most robust results. Processing sparker data without broadband techniques results in less confident or completely missed interpretations when compared to the broadband equivalent. If processed correctly, marine sparker data can provide exceptional sub-bottom imagery that rivals other more repeatable marine seismic sources (e.g., high-frequency air-guns).

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.

Post-spreading volcanism in the central South China Sea: insights from zircon U–Pb dating on volcaniclastic breccia and seismic features

Post-spreading volcanism in the central South China Sea (SCS) is of broad interest and has been well-studied using mainly geophysical data. The zircon U–Pb dating of volcaniclastic breccia cores recovered by the International Ocean Discovery Program (IODP) Expedition 349 provides important insights into the characteristics and origin of the volcanism in conjuction with the evolution of the SCS and related regional tectonic events. The results show that two volcanic eruptions were dated as 11.29 ± 0.53 Ma at Site U1431 in the East sub-basin and 7.56 ± 0.28 Ma at Site U1434 in the Southwest sub-basin; both ages are in good agreement with the shipboard ages derived from biostratigraphic and paleomagnetic datums and are a little older than the published K–Ar ages of dredged samples from nearby seamounts. Furthermore, by integrating the IODP borehole data and regional multichannel seismic data, the distribution of the post-spreading volcanism in the central SCS is mapped through geological time. The volcanism was mainly concentrated in the Late Miocene and did not occur immediately after the cessation of seafloor spreading at ~ 15.5 Ma. The origin of the post-spreading volcanism in the central SCS is discussed and we propose that it resulted from the post-spreading cooling and subsidence of the SCS, the changed subduction direction of the Philippine Sea plate, the rapid uplift of the southeastern Tibetan Plateau in the Late Miocene and potential Hainan mantle plume activities.

Multichannel seismic reflection data from the southern part of the Japan Sea

Multichannel seismic reflection data from the southern part of the Japan Sea

High-density surveys conducted to reveal active deformations of the upper forearc slope along the Ryukyu Trench, western Pacific, Japan

In this study, which was conducted to investigate present-day geological deformations occurring off the central Ryukyu Arc along the Ryukyu Trench (Okinawa-jima) in the vicinity of Japan, we obtained and examined dense, high-resolution, 2D grid (high-density) multichannel seismic reflection data. The forearc slope of the central Ryukyu Arc lacks a large forearc basin but has a steep narrow slope that dips to the southeast in the direction opposite to the plate convergence. However, while the seismic profiles obtained in our investigation do not reveal any active structures indicative of compressional stress in this convergent subduction zone, recent tectonic deformations are characterized by normal faults that strike perpendicular to the Ryukyu Trench axis. Examinations of high-density geological data show that the recent relative motion of the overriding plate has resulted in an active, arc-parallel extensional field. Furthermore, the results of our observations indicate two stages of extensional stress beneath the forearc slope of the central Ryukyu Arc that resulted from back-arc rifting in the Okinawa Trough.

Defining megathrust tsunami source scenarios for northernmost Cascadia

For assessing tsunami hazard in northernmost Cascadia, there is an urgent need to define tsunami sources due to megathrust rupture. Even though the knowledge of Cascadia fault structure and rupture behaviour is limited at present, geologically and mechanically plausible scenarios can still be designed. In this work, we use three-dimensional dislocation modelling to construct three types of rupture scenarios and illustrate their effects on tsunami generation and propagation. The first type, buried rupture, is a classical model based on the assumption of coseismic strengthening of the shallowest part of the fault. In the second type, splay-faulting rupture, fault slip is diverted to a main splay fault, enhancing seafloor uplift. Although the presence or absence of such a main splay fault is not yet confirmed by structural observations, this scenario cannot be excluded from hazard assessment. In the third type, trench-breaching rupture, slip extends to the deformation front and breaks the seafloor by activating a frontal thrust. The model frontal thrust, based on information extracted from multichannel seismic data, is hypothetically continuous along strike. Our low-resolution tsunami simulation indicates that, compared to the buried rupture, coastal wave surface elevation generated by the splay-faulting rupture is generally 50–100% higher, but that by trench-breaching rupture is slightly lower, especially if slip of the frontal thrust is large (e.g. 100% of peak slip). Wave elevation in the trench-breaching scenario depends on a trade-off between enhanced short-wavelength seafloor uplift over the frontal thrust and reduced uplift over a broader area farther landward.

The architecture of the lower parts of submarine canyons on the western Nigerian continental margin

Multi-beam, sub-bottom and multichannel seismic data acquired from the western Nigerian continental margin are analysed and interpreted to examine the architectural characteristics of the lower parts of the submarine canyons on the margin. The presence of four canyons: Avon, Mahin, Benin, and Escravos, are confirmed from the multi-beam data map and identified as cutting across the shelf and slope areas, with morphological features ranging from axial channels, moderate to high sinuosity indices, scarps, terraces and nickpoints which are interpreted as resulting from erosional and depositional activities within and around the canyons. The Avon Canyon, in particular, is characterised by various branches and sub-branches with complex morphologies. The canyons are mostly U-shaped in these lower parts with occasional V-shapes down their courses. Their typical orientation is NE–SW. Sedimentary processes are proposed as being a major controlling factor in these canyons. Sediments appear to have been discharged directly into the canyons by rivers during the late Quaternary low sea level which allows river mouths to extend as far as the shelf edge. The current sediment supply is still primarily sourced from these rivers in the case of the Benin and Escravos Canyons, but indirectly in the case of the Avon and Mahin Canyons where the rivers discharge sediments into the lagoons and the lagoons bring the sediments on to the continental shelf before they are dispersed into the canyon heads. Ancient canyons that have long been buried underneath the Avon Canyon are identified in the multichannel seismic profile across the head of the Avon Canyon, while a number of normal faults around the walls of the Avon and Mahin Canyons are observed in the selected sub-bottom profiles. The occurrence of these faults, especially in the irregular portions of the canyon walls, suggests that they also have some effect on the canyon architecture. The formation of the canyons is attributed to the exposure of the upper marginal area to incisions from erosion during the sea level lowstand of the glacial period. The incisions are widened and lengthened by contouric currents, turbidity currents and slope failures resulting in the canyons.

Active Seafloor Seepage Along Hydraulic Fractures Connected to Lateral Stress From Salt‐Related Rafting: Regab Pockmark, Congo Fan

AbstractSeafloor seepage is a widespread phenomenon within salt‐influenced basins as the deformation provides pathways for hydrocarbons to reach the seafloor. However, only minor attention has been given to the distal parts of such systems where the impact of salt tectonic deformation is relatively unpronounced. The stress put on the sedimentary column by moving salt on a continental margin may influence fluid flow systems even outside of the salt province. This stress may lead to overpressure formation within reservoirs and determine the orientation of overpressure‐induced fractures. Seepage in the Congo Fan has been discovered in such a distal position at the Regab pockmark, about 35 km west of the salt front, and its geology and biology have been studied extensively in recent years. We present high‐resolution multichannel seismic data from the Regab pockmark that reveal the underlying migration pathways from a buried channel flank 300 m below seafloor to the seafloor via hydraulic fractures in the sealing overburden. Local doming of the reservoir and the remobilization and uplift of sedimentary strata along the migration pathways are interpreted as the result of overpressure within the reservoir. The orientation of the hydraulic fractures is WSW‐ENE, and the fracture outline corresponds to the area of most intense seepage activity within the seafloor pockmark. Along with a similar orientation of other fractures in the vicinity, we propose that this alignment is due to the stress imposed on the sedimentary column in the fan by the seaward moving salt and rafting sedimentary packages of the salt province further east.

Rift–drift transition in the Dangerous Grounds, South China Sea

The South China Sea (SCS) has a long record of rifting before and after subsequent seafloor spreading, affecting the wide continent of the Dangerous Grounds, and its scissor-shape opening manner results in the rifting structures that vary along this margin. Some 2000 km of regional multichannel seismic data combined with borehole and dredge data are interpreted to analyze the multistage rifting process, structural architecture and dynamic evolution across the entire Dangerous Grounds. Key sequence boundaries above the Cenozoic basement are identified and classified into the breakup unconformity and the rift end unconformity, which consist of the rift-related unconformities. Reflector T70 in the east of the Dangerous Grounds represents the breakup unconformity, which is likely corresponding to the spreading of the East Subbasin. T60 formed on the top of carbonate platform is time equivalent to the spreading of the Southwest Subbasin, marking the breakup unconformity of the central Dangerous Grounds. The termination of the spreading of the SCS is manifested by the rift end unconformity of T50 in the southwest and the final rift occurring in the northwest of the Dangerous Grounds is postponed to the rift end unconformity of T40. On the basis of the stratigraphic and structural analysis, distinct segments in the structural architecture of the syn-rift units and the ages of rift–drift transition show obvious change from the proximal zone to the distal zone. Three domains, which are the Reed Bank-Palawan Rift domain, the Dangerous Grounds Central Detachment domain and Nam Con Son Exhumation domain, reflect the propagation of the margin rifting developed initially by grabens formed by high angle faults, then large half-grabens controlled by listric faults and detachments and finally rotated fault blocks in the hyper-extended upper crust associated with missing lower crust or exhumed mantle revealing a migration and stepwise rifting process in the south margin of the SCS.

Seismic evidence for arc segmentation, active magmatic intrusions and syn-rift fault system in the northern Ryukyu volcanic arc

Tectonic and volcanic structures of the northern Ryukyu arc are investigated on the basis of multichannel seismic (MCS) reflection data. The study area forms an active volcanic front in parallel to the non-volcanic island chain in the eastern margin of the Eurasian plate and has been undergoing regional extension on its back-arc side. We carried out a MCS reflection experiment along two across-arc lines, and one of the profiles was laid out across the Tokara Channel, a linear bathymetric depression which demarcates the northern and central Ryukyu arcs. The reflection image reveals that beneath this topographic valley there exists a ~ 3-km-deep sedimentary basin atop the arc crust, suggesting that the arc segment boundary was formed by rapid and focused subsidence of the arc crust driven by the arc-parallel extension. Around the volcanic front, magmatic conduits represented by tubular transparent bodies in the reflection images are well developed within the shallow sediments and some of them are accompanied by small fragments of dipping seismic reflectors indicating intruded sills at their bottoms. The spatial distribution of the conduits may suggest that the arc volcanism has multiple active outlets on the seafloor which bifurcate at crustal depths and/or that the location of the volcanic front has been migrating trenchward over time. Further distant from the volcanic front toward the back-arc (> 30 km away), these volcanic features vanish, and alternatively wide rift basins become predominant where rapid transitions from normal-fault-dominant regions to strike-slip-fault-dominant regions occur. This spatial variation in faulting patterns indicates complex stress regimes associated with arc/back-arc rifting in the northern Okinawa Trough.