Seismic Reflection Profiles Research Articles

Subduction retreat recorded in the southern Beishan orogenic collage of the Central Asian Orogenic Belt: New evidence from deep seismic reflection profiling

The crustal and upper mantle structure of the Beishan orogenic collage, which serves as the southern part of the middle Central Asian Orogenic Belt, provides crucial insights into the history of the multiple openings and closings of the Paleo-Asian Ocean during the Paleozoic. There is considerable dispute over the eventual closure position, timing, and subduction polarity of the Paleo-Asian Ocean, particularly in the southern Beishan orogenic collage. The main cause of these controversies is the lack of a high-resolution lithospheric structure in this area. In this study, we first present a 140-km-long, high-resolution seismic reflection profile taken across the northern Dunhuang Block and the southern Beishan orogenic collage. The seismic imaging provides new constraints on the structure of the lower crust, the Moho, and the upper mantle beneath the southernmost Central Asian Orogenic Belt. A subhorizontal reflector in the middle crust, two sets of north-dipping reflectors from the lower crust to the upper mantle, and several south-dipping reflectors in the upper crust in the northern part of the profile were imaged. Based on our study and other geological, chronological, and geophysical data, we propose that the two sequences of north-dipping reflectors from the lower crust to the upper mantle represent two stages of north-dipping subduction of the southern branch of the Paleo-Asian Ocean. The first stage in the southern Beishan orogenic collage is Late Silurian−Early Devonian, and the second stage is late Carboniferous−Early Permian. The two-stage subduction process gave rise to the Huaniushan arc and the subsequent Shibanshan arc, respectively. These findings provide new constraints on the controversial subduction polarity and the multistage amalgamation of the microcontinental blocks and arcs in the southern Central Asian Orogenic Belt.

‘’Geological characterization of a potential CO2 storage play in the Gela offshore (southern Sicily) and the role of a gravitational slide’’

In a context where European policies mandating a 55% reduction in CO2 emissions by 2030, Carbon Capture Storage (CCS) presents a viable solution for achieving substantial emissions cuts. Identifying existing and new potential sites for CCS in offshore southern Sicily is crucial to meeting this goal.The Gela offshore region is predominantly characterized by the Gela Thrust System (GTS) and its associated Gela Foredeep (GF), a narrow (less than 20 km) and elongated (approximately 100–120 km) depozone that comprises the thicker Plio-Pleistocene sandy sediments of the offshore Sicilian sector. This area is covered by the Gela Slide, a 1500 km2 gravitational slide.The potential of Plio-Pleistocene clastic deposits in this regionfor CCS implementation has not been previously explored. In this study, we investigated a potential caprock-reservoir system covering around 840 km2 in the Gela offshore area. Our analysis involved interpreting key horizons from a dense grid of 2D seismic reflection profiles, conducting well-to-seismic tie analysis and developing a refined velocity model.The study reveals a promising storage play within the foredeep basin-filling deposits. This potential storage play consists of Lower Pleistocene foredeep turbidite sands of Sabbie di Irene Fm. (reservoir) and of the Middle-Upper Pleistocene pelitic deposits of the Argo Fm. (primary seal). Additionally, the basal shear level of the Gela Slide has been investigated as a potential secondary seal. Analyzing its extent, age, and interaction with the GTS offers valuable insights into the relationships between tectonics and sedimentation, which could lead to the identification of suitable sealing layers for CCS purposes.This study thus provided new data on the thickness, depth, facies and rock volumes of the main reservoir and seals, highlighting their potential suitability for CCS applications.

A 3D Seismotectonic Model and the Spatiotemporal Relationship of Two Historical Large Earthquakes in the Linfen Basin, North China

The Shanxi Graben is a transitional zone between the Ordos Block and North China Plain with complex structures and frequent earthquakes. Six earthquakes with M ≥ 7.0 have been recorded in the area, including the 1303 Hongtong M 8 and 1695 Linfen M 7.8 earthquakes in the Linfen Basin. Research on these two large earthquakes, closely related in time and space, is lacking. Our objective was to use deep seismic reflection profiles and 3D velocity structure data from previous research, along with seismological observation results, to interpret the geological structure near the source of the two earthquakes. A 3D geometric model of the seismogenic fault was constructed, and the relationships among the deep and shallow structures, deep seismogenic environment, and two large earthquakes were explored. Differences in seismogenic environment between the southern and northern Linfen Basin were identified. The distribution of small earthquakes in the southern Linfen Basin was scattered, and the overall distribution was at depths <25 km. The small earthquakes in the northern part of the basin were dense and concentrated at depths of 25–35 km. Low-velocity layers at an approximate depth of 15–20 km in the southern basin led to differences in seismogenesis between the two regions. Based on the area of the 3D geometric model of the Huoshan Fault, the maximum magnitude of an earthquake caused by fault rupture is Mw 7.7, so the magnitude of the 1303 Hongtong earthquake might be overestimated. Numerical simulation results of Coulomb stress showed that the 1303 Hongtong earthquake had a stress-loading effect on the 1695 Linfen earthquake. The change in Coulomb rupture stress was 1.008–2.543 bar, which is higher than the generally considered earthquake trigger threshold (0.1 bar). We created a new 3D source model of large earthquakes in the Linfen Basin, Shanxi Province, providing a reference and typical cases for risk assessment of large earthquakes in different regions of the Shanxi Graben.

REWARE: a seismic processing algorithm to retrieve geological information from the water column

SUMMARY When interpreting marine very high-resolution (VHR) single-channel seismic reflection data, the signal in the water column is generally considered as noise and is often eliminated by a water-mute application to focus on geological information under the seafloor. Alternatively, the signal in the water column can be used to study ocean currents or gas/fluid emissions. To provide images of the sedimentary formations and tectonic structures beneath the seafloor in shallow water regions, such as continental shelves and lakes, marine seismic reflection profiles are often acquired using a single-channel streamer and sparker-type source, providing VHR data, with limited penetration depth. To exploit the full potential of these single-channel data, we propose a simple algorithm, called REWARE (Recovery of Water-column Acoustic Reflectors). This algorithm allows to extract further geological information from the water-column data using open-source codes (Seismic Un*x), adding the coherent signal from the previous shots, recorded in the water column, to the previous traces. The record length becomes longer while maintaining a very high trace-to-trace consistency. To demonstrate its efficiency, we present two examples of the REWARE processing in two different geological contexts: the East Sardinia shelf (Italy) and the North Evia Gulf (Greece). This method provides deeper images than with original data for seismic data acquired across steep slopes, such as canyons or continental shelf breaks. Thus, depending on the seafloor geometry and subseafloor structures, it is possible to image or map sediment layers and tectonic structures at depth, keeping a very high structural resolution.

Integrated analysis of the Neogene–Quaternary Valdera‐Volterra Basin (Northern Apennines). Evidence for composite development of hinterland basins

AbstractThe Neogene and Quaternary hinterland basins of the Northern Apennine have been the subject of different tectonic interpretations. Several studies considered these basins as the result of polyphase normal faulting framed in a continuous crustal extensional regime since the middle Miocene. On the contrary, geophysical and geological studies provided evidence of the important role played by out‐of‐sequence thrusts and backthrusts in the evolution of these basins during a prolongated and intense period of shortening. Here we present an integrated analysis of 2D stacked seismic reflection profiles, stratigraphic and geophysical data from deep exploration wells, gravity data, and published geological and biostratigraphic data for the Valdera‐Volterra basin (central Tuscany, Italy). The results support a polyphase and composite evolution of the basin, subdivided into three main phases. During the late Tortonian–Zanclean, the growth of major thrust‐related anticlines controlled the evolution of the sedimentary basin. The growth of a syncline determined the creation of accommodation space for the sediments. This main compressional deformation occurred during the Messinian and ended during the Late Zanclean. NE migration of the depocentre during the Early Zanclean was identified, likely possibly due to a differential activity growth between the bordering anticlines. During the Piacenzian, an extensional phase has been recognised, superposed to the previous compressive phase. During the Latest Piacenzian–Early Pleistocene (?), a final compressional phase took place resulting in the positive inversion of the Piacenzian WSW dipping main border fault.

21Ne and 10Be dating of folded fluvial terraces: Constraining active deformation of the Guman fold along the foothill of the western Kunlun mountains (Xinjiang, China)

Characterizing the active deformation of the foreland is particularly valuable for understanding the dynamics of regional structural evolution within active contractional orogens. We focus on the Guman fold, one of the most prominent fold-and-thrust belts along the foothill of the Western Kunlun Mountains (Xinjiang, China). The anticline growing above the blind thrust faults progressively deforms river terraces. However, contemporaneous terrace surfaces are buried under young deposits north of the fold. We propose a novel method that extends terrace surfaces above the forelimb of the fold within the extent of deformation determined on the seismic reflection profile, to estimate the sediment thickness after abandonment of the terraces. Furthermore, cosmogenic nuclide (10Be and 21Ne) depth profile dating was used to determine the exposure ages of the two terrace surfaces: 413–673 kyr for T1b and 3.7–5.2 Myr for T3b. Combining the age and deformation amount, the slip rate on the frontal fault ramp within the fold is estimated to be 0.4+0.2/-0.1 mm/yr and 0.17+0.07/-0.03 mm/yr since the abandonment of the T1b and the T3b terrace respectively. These rates represent the recent crustal shortening rate across the Guman fold and likely account for most of the total crustal shortening rate across the Western Kunlun Mountains. However, these rates appear to be notably lower by an order of magnitude compared to the long-term (∼23 Ma) average crustal shortening rate (∼1.1–2.6 mm/yr) in this region. This may indicate a rapid slowing down of the deformation at the scale of the whole Western Kunlun Mountains, possibly related to a regional reorganization.

Seismic Reflection Imaging Reveals Relict Subduction Zone of the Paleo-Pacific Plate in the Northeastern South China Sea

The South China Sea (SCS) is widely accepted as an active margin that is associated with the subduction of the Paleo-Pacific plate during the Mesozoic. However, the exact location of the subduction or suture zone remains unclear. Understanding the location of the subduction zone is crucial for comprehending the Mesozoic tectonic evolution of the South China Block and the Cenozoic rifting process of the SCS. To clarify the position of the subduction zone and the influences of preexisting structures, we used a multichannel seismic reflection profile to investigate the crustal architecture. The seismic profile reveals a crustal “crocodile” structure that is interpreted as relict subduction in the Chaoshan Depression and a set of south-dipping crust-mantle reflectors related to the initial rifting in the continent–ocean transition (COT) zone. The results indicate that a Mesozoic subduction zone is located at the northeastern margin of the SCS and that preexisting structures (subduction-related structures) facilitated the rifting process. Combined with previous studies on the oceanic plateau collision-accretionary zone of the northern SCS and the Mesozoic accretionary zone in Palawan of the southern SCS, we infer that a section of the suture zone of the Paleo-Pacific plate subduction is preserved at the northeastern SCS margin and that the rifting of the SCS may have initiated at the suture zone.

Post-glacial stratigraphy and late Holocene record of great Cascadia earthquakes in Ozette Lake, Washington, USA

Ozette Lake is an ~100-m-deep coastal lake located along the outer coast of the Olympic Peninsula (Washington, USA); it is situated above the locked portion of the northern Cascadia megathrust but also relatively isolated from active crustal faults and intraslab earthquakes. Here we present a suite of geophysical and geological evidence for earthquake-triggered mass transport deposits (MTDs) and related turbidite deposition in Ozette Lake since ca. 14 ka. Comprehensive high-resolution bathymetry data, seismic reflection profiles, and sediment cores are used to characterize the post-glacial stratigraphic framework and examine paleoseismic evidence in the lacustrine sediments. Stacked sequences of MTDs along the steep eastern flanks of the lake appear to grade basin-ward from thick, chaotic, blocky masses to thin, parallel-bedded turbidite beds. The discrete turbidite event layers are separated by fine-grained (silt and clay) lake sedimentation. The event layers are observed throughout the lake, but the physical characteristics of the deposits vary considerably depending on proximity to primary depocenters, steep slopes, and subaqueous deltas. A total of 30–34 event deposits are observed in the post-glacial record. Radiometric dating was used to reconstruct a detailed sedimentation history over the last ~5.5 k.y., develop an age model, and estimate the recurrence (365–405 yr) for the most recent 12 event layers. Based on sedimentological characteristics, temporal overlap with other regional paleoseismic chronologies, and recurrence estimates, at least 10 of the dated event layers appear to be sourced from slope failures triggered by intense shaking during megathrust ruptures; the recurrence interval for these 10 events is 440–560 yr. Thus, Ozette Lake contains one of the longest and most robust geological records of repeated shaking along the northern Cascadia subduction zone.

Spatial and Temporal Distribution of Igneous Sills in the Central Tarim Basin and Their Geological Implications

Interpretation of the seismic reflection profiles associated with borehole data from the petroleum industry offers a novel way to study sill emplacement in sedimentary basins. This study uses this approach to reveal the intrusive part of the Tarim Large Igneous Province (LIP) within the basin, which has not been systematically reported. A large number of igneous intrusions (sills) are identified in the sedimentary layers of the Central Tarim Basin. The burial depth of the sills is 6–8 km, and they are mainly located within the upper Ordovician strata. According to their seismic facies and drilling data, it is inferred that they are dolerite sills. Based on the uplift of the overlying strata above the intrusions, it is concluded that the sills were mainly formed during the depositional period of the middle Permian Kupukuziman Formation and Kaipailezike Formation (early stage), with a few formed during the depositional period of the upper Permian strata (late stage). It is likely that these two stages of sill intrusion correspond to the main basaltic eruptions within the basin and the mafic dike emplacement in the Bachu area of the Tarim LIP, respectively. The study suggests that that the dolerite sills reported in this study are also an important component of the Permian Tarim LIP.

Seismic Reflection Profiling Reveals New Accretionary Structure in the Solonker Zone of the Central Asian Orogenic Belt

AbstractThe Central Asian Orogenic Belt (CAOB) was formed by the aggregation and collage of numerous Paleozoic subduction‐accretion assemblages and Precambrian microcontinental blocks. However, the tectonic nature of the southeastern CAOB remains controversial, which complicates the reconstruction of the Paleo‐Asian Ocean. To address this issue, a deep seismic reflection survey was initiated across the southeastern CAOB and reveals broad gentle sub‐horizontal reflectors in the middle‐lower crust and a relatively transparent zone in the upper crust. Combining with the Precambrian geological outcrops and other geophysical features, we support a microcontinental block, the Xilinhot Block, existed in the Paleo‐Asian domain. Thus, the Paleo‐Asian Ocean was separated into two branches that underwent north‐dipping and double‐dipping oceanic plate subduction, respectively, to form the Hegenshan‐Heihe and Solonker sutures. Multiple relics beneath Hegenshan‐Heihe Suture indicate that multiple sets of unidirectional oceanic subduction‐accretion and magmatism were important mechanisms of continental growth.

Geodynamic modeling on continental collision in Qilian orogenic belt

The Qilian orogenic belt (QOB) located in the northeast margin of the Tibetan Plateau is featured by remarkable crustal thrusting and shortening, providing a key natural example to understand the lithospheric deformation of the Tibetan Plateau. Two types of continental collision are observed in the QOB: lithosphere subduction beneath Southern Qilian and crust underthrusting of Alxa terrain along the North Border Thrust (NBT). Deep seismic reflection profiles reveal complex stress field evolution, including compressional deformation in the lower crust, extensional deformation in the upper crust, and detachment deformation in the middle crust. In this study, we use 2D numerical modeling to investigate the dynamics of these two different collision types and the evolution of Qilian uplift. Model results suggest three patterns of continental collision, i.e., crust underthrusting follows lithosphere subduction, lithosphere subduction and the failed underthrusting/subduction. The key factors that may influence model evolution, including crustal rheology, convergence direction and rate, are systematically investigated. Our model results are further compared to observations, suggesting that lower convergence rate and crust underthrusting along NBT likely control the uplift and crust stress stratification of the QOB.

Misidentified subglacial lake beneath the Devon Ice Cap, Canadian Arctic: a new interpretation from seismic and electromagnetic data

Abstract. In 2018 the first subglacial lake in the Canadian Arctic was proposed to exist beneath the Devon Ice Cap, based on the analysis of airborne radar data. Here, we report a new interpretation of the subglacial material beneath the Devon Ice Cap, supported by data acquired from multiple surface-based geophysical methods in 2022. The geophysical data recorded included 9 km of active-source seismic-reflection profiles, seven transient electromagnetic (TEM) soundings, and 17 magnetotellurics (MT) stations. These surface-based geophysical datasets were collected above the inferred locations of the subglacial lakes and show no evidence for the presence of subglacial water. The acoustic impedance of the subglacial material, estimated from the seismic data, is 9.49 ± 1.92 × 106 kg m−2 s−1, comparable to consolidated or frozen sediment. The resistivity models obtained by inversion of both the TEM and MT measurements show the presence of highly resistive rock layers (1000–100 000 Ω m) directly beneath the ice. Re-evaluation of the airborne reflectivity data shows that the radar attenuation rates were likely overestimated, leading to an overestimation of the basal reflectivity in the original radar studies. Here, we derive new radar attenuation rates using the temperature- and chemistry-dependent Arrhenius equation, and when applied to correct the returned bed power, the bed power does not meet the basal reflectivity threshold expected over subglacial water. Thus, the radar interpretation is now consistent with the seismic and electromagnetic observations of dry or frozen, non-conductive basal material.

The Mesozoic Subduction Zone over the Dongsha Waters of the South China Sea and Its Significance in Gas Hydrate Accumulation

The Mesozoic subduction zone over the Dongsha Waters (DSWs) of the South China Sea (SCS) is a part of the westward subduction of the ancient Pacific plate. Based on the comprehensive interpretation of deep reflection seismic profile data and polar magnetic anomaly data, and the zircon dating results of igneous rocks drilled from well LF35-1-1, the Mesozoic subduction zone in the northeast SCS is accurately identified, and a Mesozoic subduction model is proposed. The accretion wedges, trenches, and igneous rock zones together form the Mesozoic subduction zone. The evolution of the Mesozoic subduction zone can be divided into two stages: continental subduction during the Late Jurassic and continental collision during the late Cretaceous. The Mesozoic subduction zone controlled the structural pattern and evolution of the Chaoshan depression (CSD) during the Mesozoic and Neogene eras. The gas source of the hydrate comes from thermogenic gas, which is accompanied by mud diapir activity and migrates along the fault. The gas accumulates to form gas hydrates at the bottom of the stable domain; BSR can be seen above the mud diapir structure; that is, hydrate deposits are formed under the influence of mud diapir structures, belonging to a typical leakage type genesis model.

Geomorphological features and formation mechanism of the Taixinan canyon‐channel system in the north‐eastern South China Sea

During the formation and evolution of the South China Sea, a series of multiscale submarine geomorphologies have been produced in the continental margin. The obvious submarine canyons and channels are widely distributed in the continental shelf and slope of the South China Sea. In the Taixinan Basin, several submarine canyons and channels termed as the Taixinan canyon‐channel system (TCCS) are distributed between the active and passive continental margins. Based on acquired ship‐borne multi‐beam bathymetry data in this study and the global GEBCO 2023 bathymetric dataset, we identify and define nine submarine canyons and seven submarine channels in the Taixinan Basin. The TCCS consists of the Dongsha, Taiwan, Jiulong, West Penghu, Penghu, Kaoping, Shoushan, Kaohsiung and the Fangliao canyons and submarine channels. The detailed geomorphological features of different submarine canyons and channels within the TCCS are analysed and summarized using multi‐beam bathymetry data and seismic reflection profiles across canyons. Based on the slope variations of the continental margin and the effects of turbidity currents and bottom currents on canyon, we propose a three‐stage evolutionary model of the TCCS. In the initial formation stage of canyon, the initial erosional grooves were created by tectonic activity on the continental slope and it represents the foundation of submarine canyons. During the growth and development stage, the submarine canyons are further evolved and the canyons began to deepen and widen from the continental slope to the deep‐water areas. It shows the weak erosion and sediment infilling within the canyons in this stage. On the northern continental slope of the South China Sea, continuous transportation and erosion of sediments led to the initial formation of grooves and it becomes the embryonic stage of submarine channels. The present stage of the TCCS was formed when the initial grooves on the continental slope have further developed and rebuilt under the erosion by the turbidity current and the scouring by the bottom current. In the last stage, the intense erosion by the turbidity current is supported by sediment waves around the submarine canyons and the migration of canyons is suggested by the cyclic steps formed within some canyons.

Deep-seated gravity instability of the southern apron of the Ischia volcanic island (Tyrrhenian Sea, Italy)

Ischia Island is an active volcano representing the emerged sector of an E-W trending volcanic ridge largely extending undersea. Its collapsing behaviour, mainly in the form of fast-moving, terrestrial and submarine debris avalanches, has been recognized during the Holocene, but much less is known about previous gravity-driven processes. Using high-resolution multibeam bathymetric data and seismic reflection profiles, we provide evidence that the Island's southwestern flank has been involved in a slow-moving, deep-seated slope deformation that has displaced large volumes of its apron since the Late Pleistocene and until very recent or contemporary times. A long tongue of deformed seafloor, spreading up to 45 km from the Island over an area of 330 km2, between 500 and 1300 m water depths, has been detected along its southwestern slope. Different types of mass movements, genetically associated with each other, characterize this landslide: 1) a basal slump anticline, corresponding to a bulge on the bathymetry detaching at about 400 m sub-bottom depth; 2) an intermediate-mass movement chiefly consisting of debris avalanches and debris/turbiditic flows; 3) an upper mass movement consisting of hundred-metre size slumps detaching at relatively shallow depths. Conservative estimates indicate that at least 50 km3 of volcano-clastic and hemipelagic deposits have been mobilized, most of which comprise the basal slump anticline. This submarine landslide can be explained as a gravity failure of the continental slope unrelated to volcanism or rather as a process related to the dynamics of the volcanic edifice, which would imply volcano-spreading.

Continuous distribution pattern and the origin of the high‐velocity layer in the Pearl River Mouth Basin in the northern South China Sea: Constraints by gravity modelling

The high‐velocity layer in the lower crust is widely distributed in the northern continental margin of the South China Sea. A detailed anatomy of the high‐velocity layer is crucial for understanding the continental rifting and crustal thinning. Based on three seismic reflection profiles across the Pearl River Mouth Basin (PRMB) and the global free‐air gravity anomaly data in this study, by the gravity modelling we construct the crustal structure along three seismic reflection profiles across the depression and uplift zones. The free‐air gravity anomaly data within the uplift and depression zones indicates the distinct zonation, and the high and low values of free‐air gravity anomalies in the basin show the northeastward trend. Based on the gravity modelling along the three seismic profiles, the crustal thickness is of 12–23 km beneath the basin and thickness of the crust is gradually thinning from the continental shelf to the continental slope. The high‐velocity layer extends eastward to the Dongsha Uplift and terminates westward beneath the Baiyun Sag. The average thickness of the high‐velocity layer is 4–6 km and the maximum thickness is about 8 km at the Dongsha Uplift. The thickness variation of the high‐velocity layer suggests the continuous distribution of mantle underplating and intense magma activity. According to the isolated distribution and the continuous distribution of the high‐velocity layer found by previous studies in the northern South China Sea, we use gravity modelling to test which models of the high‐velocity layer within the PRMB are reasonable. According to testing models and preferred models of the crustal structure by gravity modelling, we propose that the continuous distribution pattern of the high‐velocity layer is the best model to interpret the crustal thinning characteristics and the gravity anomaly responses of the high‐velocity layer in the basin. We propose that the high‐velocity layer in the Pearl River Mouth Basin was formed by the long‐term underplating of the high‐temperature melted mantle.

Integrated geological and geophysical workflow for structural modelling: a case study from the contractional foothills zone of the Colombian Eastern Cordillera

In fold-and-thrust belts where there is a high degree of structural complexity, artificial geometrical distortions are often present on seismic reflection profiles. These need to be minimized during modelling. We document a workflow in which depth mapping, velocity model building, well calibration and cross-section balancing are integrated into the seismic interpretation process to generate trustworthy structural models in complex zones. The proposed methodology is exemplified by a case study from the foothills zone of the Colombian Eastern Cordillera. In addition, sequential kinematic restoration of the modelled structure allowed an evaluation to be made of hydrocarbon migration routes during the period between the Oligocene and the middle Miocene. Following the previously mentioned workflow, we document a failed exploratory case study where all elements of the petroleum system are present except the trap. In this context, the documented case is a typical velocity pull-up. From this and published case studies, we conclude that in Andean settings and probably most on land contractional-foothill settings, the use of seismic images only does not provide enough evidence for the presence of traps and additional surface geological signatures must be documented. The proposed workflow therefore appears to be a useful tool for evaluating the exploration risk in structurally complex fold-and-thrust belt settings.

Structural Variation Along the Southern Hikurangi Subduction Zone, Aotearoa New Zealand, From Seismic Reflection and Retro‐Deformation Analysis

AbstractThe southern Hikurangi subduction zone exhibits significant along‐strike variation in convergence rate and obliquity, sediment thickness and, uniquely, the increasing proximity of southern Hikurangi to, and impingement on, the incoming continental Chatham Rise, an ancient Gondwana accretionary complex. There are corresponding changes in the morphology and structure of the Hikurangi accretionary prism. We combine widely spaced multichannel seismic reflection profiles with high resolution bathymetry and previous interpretations to characterize the structure and the history of the accretionary prism since 2 Ma. The southern Hikurangi margin can be divided into three segments. A northeastern segment (A) characterized by a moderately wide (∼70 km), low taper (∼5°) prism recording uninhibited outward growth in the last ∼1 Myr. Deformation resolvable in seismic reflection data accounts for ∼20 % of plate convergence, comparable with the central Hikurangi margin further North. A central segment (B) characterized by a narrow (∼30 km), moderate taper (∼8°) prism, with earlier (∼2‐∼1 Ma) shortening than segment A. Outward prism growth ceased coincidentally with development of major strike‐slip faults in the prism interior, reduced margin‐normal convergence rate, and the onset of impingement on the incoming Chatham Rise to the south. A southwestern segment (C) marks the approximate southern termination of subduction but widens to ∼50 km due to rapid outward migration of the deformation front via fault reactivation within the now‐underthrusting corner of the Chatham Rise. Segment C exhibits minimal shortening as margin‐normal subduction velocity decreases and plate motion is increasingly taken up by interior thrusts and strike‐slip faults.

Seismic and Potential Field Constraints on Upper Crustal Architecture of Inner Bering Shelf, Offshore Southwestern Alaska

Southwestern Alaska encompasses a group of fault-bounded tectonostratigraphic terranes that were accreted to North America during the Mesozoic and Paleogene. To characterize the offshore extension of these terranes and several significant faults identified onshore, we reprocessed three intersecting multichannel deep seismic reflection profiles totaling ∼750 line-km that were shot by the R/V Ewing across part of the inner Bering continental shelf in 1994. Since the uppermost seismic section is often contaminated by high amplitude water layer multiples from the hard and shallow seafloor, the migrated reflection images are supplemented with high-resolution P wave velocity models derived by traveltime tomography of the recorded first-arrivals to depths of up to 2000 m. Additionally, other geophysical datasets such as seismicity, well logs, high resolution satellite-altimetry gravity, air-borne magnetics, ship-board gravity and magnetics, are also incorporated into an integrated regional interpretation. We delineate the offshore extension of the major mapped geological elements, including the Togiak fault (TGF), East Kulukak fault (EKF), Chilchitna fault (CF), Lake Clark fault (LCF), Togiak terrane (TT), Goodnews terrane (GT), Peninsular terrane (PT), Northern Kahiltna flysch (NKF) and Southern Kahiltna flysch (SKF) deposits, and the regional suture zone. The geophysical evidence from this study suggest that the major faults and terrane boundaries of southwestern Alaska, excluding the LCF, not only extend beneath the Bering shelf offshore but also appear to rotate, forming a trend parallel to the inactive Beringian margin. The LCF extends offshore but likely terminates in the northeastern part of the Bristol Bay basin. Additionally, the constraints from seismicity data indicates that while the major faults in southwestern Alaska exihibit some activity onshore, they remain dormant in the offshore region. These new findings will contribute to a better understanding of terrane accretion processes and existing fault models of southwestern Alaska.

High-resolution geophysical data unravel the post-Variscan structural history of the NW Cotentin inner shelf (Central English Channel)

The Cotentin Peninsula (CP) is one of the only area in Europe which contains records of a > 2.5 Ga-lasted geological history including three orogenic events (Archaean, Cadomian and Variscan) followed by a polyphase basin/inversion evolution during Meso-Cenozoic times. The CP area sensu lato is thus a suitable place for discussing how the structural configuration of the basement might have influenced the development of part of the southern shelf margin of the Central English Channel, even if sediments and post-Variscan tectonic records are limited at this place. This issue is addressed through an onshore/offshore structural approach combining newly-acquired high resolution bathymetric data and reflection seismic profiles, further constrained by lateral correlations onshore. The resulting Land-Sea Digital Elevation Model and corresponding geological map reveal a number of fault-bounded blocks involving a relatively thin package of Jurassic to Plio-Quaternary sequences, locally involved in slightly compressional deformations. These specific sedimentary and tectonic features typically characterize the southern shelf margin of the Central English Channel. They are discussed in terms of basin development and inversion processes in relation with basement structures and then integrated in the English Channel basin framework. Special attention is paid to three major structural features, i.e. the La Hague Offshore Fault, the La Hague Deep Fault network and the La Hague Deep, which emphasize, respectively, the role of structural inheritance and erosion/incision/deposition events during the post-Variscan tectono-sedimentary history of the southern elevated shoulder of the Central English Channel.