Seismic Profiles Research Articles

Reflection seismic profiling along the Longitudinal Valley Basin, Eastern Taiwan: In search of the Central Range fault

A thorough understanding of the internal structures of the Longitudinal Valley is crucial for comprehending the architecture of the suture zone between the Eurasian and Philippine Sea plates. Here, we present the findings of eight new reflection seismic profiles spanning north to south of the Longitudinal Valley. Our primary objective is to investigate the internal sedimentary and tectonic features of the Longitudinal Valley Basin, the nature of the contact between it and the metamorphic basement rocks to the west, and the geometry of the Longitudinal Valley fault system along its eastern flank. A significant outcome of our research is that there is minimal internal deformation within the Longitudinal Valley Basin, with sedimentary features prevailing. The presence of inclined reflections suggests the presence of fan deltas and therefore a marine or lacustrine environment in the Longitudinal Valley. The sedimentary characteristics indicate that the source region for the sediment was predominately the Central Range, but that locally the Coastal Range also supplied sediment. Furthermore, at least in the shallow subsurface (1 to 2 km), a west-dipping Central Range fault does not exist along the western side of the Longitudinal Valley. The Longitudinal Valley fault system does not affect the middle of the basin, instead it is located along the eastern flank, adjoining the topography of the Coastal Range.

Quaternary stratigraphic architecture of the Pearl River Mouth continental shelf, northern South China Sea: Unraveling stratigraphic imprints of tectonism, paleoceanography, and climate change

The complexity of continental shelf stratigraphy is determined by the interactions among various global or local factors. However, their specific roles are still poorly understood. Using a newly-acquired grid of single-channel seismic reflection profiles, combined with piston cores, shallow boreholes, and industrial wells, this study investigates the sequence stratigraphic architecture and evolution of the Quaternary continental shelf offshore the Pearl River Mouth (PRM), northern South China Sea (SCS). Three types of bounding surfaces have been identified according to their stratal terminations: erosional surfaces (ESs), flat-lying surfaces (FSs), and locally dipping surfaces (LDSs). Given the limited extent of LDSs, we utilized ES/FS couplets to subdivide the upper Quaternary section into six high-frequency sequences (SQ6–SQ1) and their corresponding systems tracts. Within each sequence, regional progradational clinoforms (RPCs) prevail across the entire shelf and are accompanied by downstepping shoreline trajectories, which we interpreted as deposits of falling-stage systems tract. RPCs are interrupted by intercalated sheeted units (ISUs) characterized by high-amplitude and continuous seismic reflections, which may consist of transgressive and lowstand deposits. In younger sequences particularly SQ2 and SQ1, progradational-aggradational clinoforms (PACs) with ascending shoreline trajectories are well developed but confined to the shelf margin, indicating efficient sedimentation during lowstand.Our results revealed significant spatial and temporal variability in stratigraphic architectures of the PRM shelf. Along the SW–NE direction, the stratigraphic pattern changes from progradation to retrogradation and back to progradation, resulting in three sectors with distinct shoreline trajectories. Retreating shoreline trajectories and retrogradational stacking patterns are preferentially present in areas adjacent to the Dongsha Rise, which are jointly influenced by several local factors such as tectonism and intense marine processes. Temporally, a dramatic change in stratigraphic patterns can be observed across a key surface ES4, dated ca. 0.8 Ma. Below ES4, fluvial incisions are scarce and RPCs are overall low-angle and capped with relatively thick ISUs. Also, no lowstand PACs were observed. Above ES4, large-scale incised valley networks appear, together with increased scale and dominance of RPCs and PACs. This stratigraphic imprint is attributed to the Mid-Pleistocene Transition (MPT), which caused a noticeable increase in periodicity, amplitude, and asymmetry of glacio-eustatic cycles. The increased scale of RPCs and PACs after the MPT may reflect increased sediment input due to high denudation rate under intensified monsoon climate.

Evolution of the Manabí forearc basin in Ecuador: From the collision of Caribbean oceanic plateau to the build up of the Andes and Coastal Cordilleras

We investigate the evolution of the onshore Manabí forearc basin in Ecuador, which has developed on accreted oceanic terranes. We use 2D seismic profiles to identify 10 seismic units, separated by 9 unconformities. These units were dated by correlation with Ricaurte-1 well and regional stratigraphy. Unit S1 represents the first depocenter developed on the oceanic plateau before accretion. The emplacement of NE-dipping oceanic plateau slices evidencing the accretionary process dated to ∼75 Ma (Unconformity U1), was followed by folds and east-dipping thrusts at the Andes foothills that were eroded by unconformity U2 (56–41.3 Ma hiatus). The large distribution of unit S3 (late Eocene-Oligocene) over the topography inherited from the collision, points to the configuration of the post-collision forearc basin. The geometry of slope front fill and the thickening of unit S4 at the foot of the Andes reveals the building of Andean reliefs during the early Miocene. Unit S5 (17.8–15.9 Ma) is fan-shaped and abuts against the Jama fault, evidencing local extensional. After unconformity U5 (∼16 Ma), differential subsidence causes the installation of a westward-prograding sedimentary platform during the middle to late Miocene (S6 to S8). Since the Pliocene, the uplift of the Coastal Cordillera is related to localized shortening across the Jama and Canandé faults. Enhanced tectonic erosion at the tip of the margin,likely related to the Carnegie ridge subduction, could have led to underplating of offscraped material beneath the Coastal Cordillera, contributing to the uplift and enlargement of the Coastal Cordillera. The Pleistocene eastward tilt of the Santo Domingo fan (unit S10) relates to the Coastal Cordillera uplift. The fan is then deformed along the Andean thrust. This study provides a remarkable example of a forearc basin-fill developed on accreted oceanic terranes and controlled by erosional and underplating subduction processes, reactivation of inherited crustal structures, and the rise of peripheral reliefs.

Crustal structure of the Xisha block and implications for along-strike segmentation in the northern South China Sea continental margin

Continental blocks complicate the pre-rift lithospheric inheritance of passive continental margins, hence exert unique influences on rifting process. Along-strike rifting contrasts of the continental margins may reflect the pre-rift crustal heterogeneity and segmented rifting processes. A wide-angle seismic profile acquired on the Xisha Block, the northwestern margin of the South China Sea (SCS), provides the first constrains on the along-strike crustal structures of the continental block. Seismic imaging of the profile reveals a continental crust with relatively uniform thickness of about 22 km and the thickness ratio of the upper crust to lower crust at 0.8. Crustal stretching factor is approximately 1.5, and high-velocity anomalies are absent in the lower crust. Crustal velocities of the upper and lower crust are approximately 5.4–6.5 km/s and 6.6–6.9 km/s, respectively. The results indicate that the Xisha Block underwent slight and uniform extension during rifting, and the crust has not suffered from strong magmatic activities that formed extensive magmatic underplating. As a rigid continental block, the Xisha Block has retarded the rifting onset and shortened the process from lithospheric rifting to breakup in the northwestern SCS margin. Hypothetic models for the rheological heterogeneity of the inherited crust are proposed tentatively to interpret the contrasting distribution of the high velocities in the lower crust along the northern SCS margin.

Bedform evolution along a submarine canyon in the South China Sea: New insights from an autonomous underwater vehicle survey

ABSTRACTTraditional mapping of bedforms in submarine canyons relied on vessel‐mounted and towed sensors, but their fine‐scale geomorphology and shallow structure requires higher resolution datasets. This study utilizes a high‐resolution dataset obtained from an autonomous underwater vehicle, combined with seismic reflection profiles and sediment cores, to analyse bedform sets within a 25.6 km long submarine canyon (canyon C14) in the northern South China Sea. A train of crescent‐shaped axial steps, indicative of cyclic steps formed by supercritical turbidity currents, is imaged along the canyon. Axial steps in the upper course show erosional truncations and sub‐horizontal reflectors on the lee and stoss sides, respectively, pointing to erosional–depositional cyclic steps formed by confined flows with high erosional capacity. This is facilitated by canyon narrowness and steeper axial gradient. After a transition segment, the lower course widens, with a gentler axial gradient, resulting in increased asymmetry and wavelength of axial steps. Backset bed deposits on the stoss sides of these steps indicate depositional cyclic steps with higher aggradation. Sediment filling, almost padding each cyclic step associated scour suggests the reworking of previously formed bedforms by gravity flows fed by destabilization processes on the canyon sidewalls and upstream lee faces and, possibly, by shelf‐edge and uppermost slope spillover into the canyon. At the lowermost course, cyclic steps transition to a furrow field, likely associated with flow velocity reduction facilitated by canyon floor widening and a further decrease in slope gradient. Flow braiding and re‐convergence, related to the erosion of fine‐grained deposits within the canyon floor, should have played a role to produce furrows under supercritical conditions. This work enhances our understanding of the detailed morphology and shallow relief configuration of bedforms in deep‐water submarine canyons, providing insights into their causative processes and evolution.

Оценка геомеханического состояния массива пород в окрестности глубоких выработок рудника «Скалистый» комплексом натурных методов

In recent years, deep levels of exploited deposits have been a priority for the Polar Branch of MMC Norilsk Nickel. In the first half of 2023, KANEX SHAKHTOSTROY LLC has completed exploration workings EW-1 at the –1650 m level and EW-2 at the –1750 m level from the ventilation shaft VS-10 to the skip-cage shaft SCS-1 (2 km below the daytime surface). Comprehensive studies of mining-induced disturbance were carried out at 17 measuring stations in EW-1 and EW-2 sections driven in 2020–2022 (the length of about 1500 m). The rheometric, ultrasonic logging and seismic profiling methods were used as the principal in-situ methods to solve the task. It has been established that the size of the disturbed zone in the vicinity of EW-1 and EW-2 is characterized by longitudinal wave velocities, Vp, in the range of 2–4 km/s, averaging over the entire length of 1m from the contour of the working. The results of the seismic studies indicate moderate disturbance of the rock mass in the horizontal plane of the working's wall. The results of in-situ complex studies are qualitatively comparable with the results of visual inspection of the fixed surface of the working, thus indicating its satisfactory geomechanical condition.

Microbialites on the northern shelf of Lake Van, eastern Türkiye#: Morphology, texture, stable isotope geochemistry and age

ABSTRACTLake Van, the world's largest alkaline lake, hosts some of the largest microbialite towers worldwide, which are considered as modern analogues of ancient stromatolites. This study investigates the links between microbialite evolution, geology, climate and hydrology, and the role of biotic and abiotic processes in microbialite growth and morphology. For these objectives, the northern shelf of Lake Van was surveyed by sub‐bottom seismic profiling and diving, and two 9 m and 15 m high microbialite chimneys were sampled at 25 m water depth. Samples were analysed for stable oxygen and carbon isotopes, X‐ray diffractometry, scanning electron microscopy and U/Th age dating. Lake Van microbialites precipitate wherever focused Ca‐rich groundwater flows onto the lake floor to mix with alkaline lake water. Variable columnar, conical and branching morphologies of the microbialites indicate various processes of formation by groundwater channelling within the chimneys. Collectively, our data suggest that the microbialite chimneys have formed within the last millennium, most likely starting during the warm and humid Medieval Climate Anomaly (ca ad 800–1300), when lake level rose approximately to the present level due to enhanced Inputs of riverine Ca‐rich freshwater and groundwater. Our new scanning electron microscopy observations indicate that the internal structure of the microbialites below the outer cyanobacteria‐covered crust is constructed by calcified filaments, globular aggregates and nanocrystals of algal, cyanobacterial and heterobacterial origins and inorganically precipitated prismatic calcite crystals. These textural features, together with dive observations, clearly demonstrate the important role of inorganic carbonate precipitation at sites of groundwater discharge, followed by cyanobacteria and algal mucilage deposition and microbially meditated calcification in the photic zone in the rapid growth of the microbialite chimneys. Considering the close similarities of some textures with those of ancient stromatolites and meteorites, the results of this study provide new insights into the environmental conditions associated with stromatolite formation and extra‐terrestrial life evolution.

Structural geometry and geomorphic evolution of the Trans Indus Manzai Ranges, Western Himalayas, Pakistan

At the leading edge of fold-thrust belts, structural deformation can either be accommodated by thick-skinned faulting, and development of structural wedges near the mountain fronts, or by foreland propagation into the basins along the detachment, forming thin-skinned structures. Manzai Ranges at the western extremity of the Trans Indus Ranges were chosen as a case study to describe the structural deformation accommodated in the foreland basin and to study the folded mountain front. These ranges also provide an opportunity to compare along-strike structural variations of the Himalayan frontal thrust system. Both surface (field structural measurements and digital elevation model) and subsurface (2D seismic reflection and well) data were integrated to document different structural styles and their corresponding geomorphic expressions. Based on seismic reflection profiles, the results of structural interpretations constrain westward dipping basement (2.5o) at a depth of ∼10 km in the foreland, which dips deeper in the hinterland. In addition, our interpretation shows a basin-wide Eocene-Miocene unconformity that marks the Himalayan orogenic events. The structural geometry of the frontal Surqamar anticline is quantitatively characterized to be detachment fold, while Jandola, Kundi and Manzai anticlines demonstrate staircase geometry and characterized as fault-bend folds. The kinematics of development varies along-strike in the Manzai Ranges as displacement transfers along the detachment from typical fault-bend folding in the Manzai anticline to detachment folding in the Surqamar anticline, forming thin-skinned structures. The results of this study highlight the structural styles and their evolution in the context of active Himalayan tectonics.

Research and Application of Excitation Method Optimization in Areas with Low Signal-to-Noise Ratio of Seismic Data

In areas with a low signal-to-noise ratio of seismic data, the continuity of the seismic reflection waves in the exploration target layer is very poor, which will reduce the imaging accuracy and make it impossible to solve certain geological tasks. This article suggests an approach to address the issue of seismic acquisition by optimizing excitation parameters. It involves conducting a detailed investigation of the surface structure, enhancing the observation system, increasing the coverage appropriately, and transitioning from combined-well excitation to single-well excitation. Additionally, the use of technical tools like qualitative evaluation of the observation system and forward modeling are employed to determine the final optimized seismic acquisition plan. The effectiveness of this approach is evident from the seismic profile obtained in an exploration area in Inner Mongolia.

Sequence Stratigraphy of the Jurassic Strata and Occurrences of Potential Sandstone Reservoirs in the ST Gas Field, Northern West Siberia Basin

Jurassic strata in the ST gas field of the northern West Siberia Basin have been regarded as a potential exploration target with undiscovered hydrocarbon resources. However limited research has been performed on the sequence stratigraphy of the Jurassic strata, as well as its sandstone distribution controlled by variable sea level change and sediment input. In this paper, four third-order sequences (SQ1, SQ2, SQ3, and SQ4) and nine fourth-order sequences for the Jurassic strata are interpreted based on seismic facies analysis and the lithology stacking patterns of seven wells. SQ1 is characterized by the special Bazhenov Formation which is featured by regionally distributed deep marine shales. SQ2 (J1) is composed of a coarsening upward sequence, the base of which is an unconformable surface that can be recognizable in both seismic profiles and well logging data. SQ3 (J2-J8) is composed of a complete fining-upward and coarsening-upward sequence, showing a series of transgressive and regressive successions. A complete SQ4 has not been drilled through by all the seven wells, only showing a coarsening upward succession on its top (J9) which evolves into a fining upward succession at the base of SQ3. Combined with the seismic inversion result, which predicts sandstone distribution, a sequence evolution model was built for SQ3 showing a full unit of transgressive system tract and highstand system tract (TST-HST) which often occurs in shallow marine shelves. During sequence development, most reservoir sandstones are deposited in the shelf and tidal delta environment at the bottom of the TST and the top of HST, and mudstones are deposited as shelf mudstones, especially at maximum flooding surface. That is controlled by both accommodation and sediment input. Generally, under this sequence framework, the depositional architecture can be further analyzed with implications for source rock, reservoir sandstones, and sealing rock, which may guide future gas exploration and exploitation in this area.

Early Cretaceous sedimentary records of the early-stage continental rifting in the Songliao Basin, NE China

Well SK-2, a continental scientific drilling well in the Xujiaweizi fault depression of the Songliao Basin, has drilled through ca 2359 m of the Early Cretaceous Shahezi Formation. Herein, we focus on the sequence groups SSI and SSII in the lower part of the Shahezi Formation (interval 4357.38–5695.02 m) from well SK-2. There are fourteen different lithofacies and seven lithofacies associations. Syn-eruptive pyroclastic deposits and their proportions in successions indicate that SSI varies from dominantly pyroclastic fill to dominantly weathering-derived clastic fill, reflecting the development of volcanism in the early syn-rift stage. SSII exhibits another, similar evolutionary cycle. Whole-rock geochemistry of rocks from the Shahezi Formation reveals that the source rocks are felsic, andesite, and sedimentary rocks from the upper crust, and that the intermediate-acid rocks are originated from partial melting crustal materials and are hybridized with input from the mantle. In the seismic profile that intersects well SK-2, the thinning direction switches back and forth during the formation of the Shahezi Formation’s four sequence groups; that is, there is turnover superimposition in vertical stratigraphic subsidence, demonstrating strike-slipping during deposition of the Shahezi Formation.The major rift stage, especially during the deposition of the lower part of the Shahezi Formation, involved strike-slipping dominated extension. This conclusion is supported by the presence of abundant dark mudstone and coarse clastic progradation in the transgressive stage, as well as by the periodicity of volcanism combined with stratigraphic turnover superimposition.

Detecting gas upwelling hazards in coastal areas through integration of active and passive electrical and seismic methods (Fiumicino, Central Italy)

The accurate location of gas upwelling flows is still an open problem for non-invasive imaging techniques in populated areas. Gas blowouts of deep origin may represent a serious threat to human health in urban areas and should be correctly imaged with high-resolution for assessing the related hazards. In this work, we propose an integration of active (electrical resistivity tomography and high-resolution sub-bottom profiling complemented with the multibeam bathymetry) and passive (self-potential and ambient noise recordings) geophysical methods to image gas upwelling flows in the coastal area of Fiumicino (Central Italy), where the gas presence is well-documented by previous works. We demonstrate that merging seismic sub-bottom profiling and electrical resistivity tomography has enormous diagnostic potential for gas detection, since they combine the high resolution needed to correctly image the subsurface and the interfaces between different media with the high diagnostic capability of electrical methods to detect anomalies associated with the gas emissions. Passive seismic methods complement the analysis enabling an estimation of the shear-wave velocity through array measurements. Finally, the reconstruction of the natural electrical sources, inferred from the inversion of self-potential data, confirms the location of the near-surface gas upwelling flows assessed through the resistivity model. This work demonstrates that the integration of high-resolution active and passive seismic and electrical methods can be an effective choice for the accurate location of risk-prone areas by imaging the near surface gas pathways where borehole drilling is strongly limited if not forbidden.

Slow response of the gas hydrate system to ridge erosion and sea-level rise: Insights from double BSRs on the southern Hikurangi margin (New Zealand)

We analyse reflection seismic profiles across the outer accretionary wedge at the convergent New Zealand Hikurangi margin. We identify several, in some case stacked, bottom simulating reflections (BSRs). We interpret these multiple BSRs to record changes in gas hydrate stability. With the aid of gas hydrate systems modelling, we identify two geological drivers that affect gas hydrate stability: (1.) rapid sedimentation in trough basins and (2.) uplift and erosion of thrust ridges. Rapid sedimentation in trough basins buries gas hydrates that formed above the former base of gas hydrate stability (BGHS). Locally, we observe a remnant BSR from this process, likely due to residual gas and possibly gas hydrate. The combined effects of uplift and erosion, in contrast, result in the preservation of a remnant BSR within the gas hydrate stability zone, whilst a new BSR forms locally at the present-day BGHS. However, the limited occurrence of double BSRs in seismic data and the model both suggest that the formation of a deeper BSR is limited by gas supply. Formation of significant gas hydrate at this deeper level only occurs in areas of focused gas migration. This slow formation of gas hydrate also has implications for the response to glacio-eustatic sea-level rise: gas hydrates are more likely to accumulate above the BGHS corresponding to the last glacial maximum, whereas only small amounts formed above the deeper present-day BGHS. Hence, future bottom water warming will, at least initially, not lead to significant methane release from dissociating gas hydrates in deep water.

High-resolution geophysical and geochronological analysis of a relict shoreface deposit offshore central California: Implications for slip rate along the Hosgri fault

Abstract The Cross-Hosgri slope is a bathymetric lineament that crosses the main strand of the Hosgri fault offshore Point Estero, central California. Recently collected chirp seismic reflection profiles and sediment cores provide the basis for a reassessment of Cross-Hosgri slope origin and the lateral slip rate of the Hosgri fault based on offset of the lower slope break of the Cross-Hosgri slope. The Cross-Hosgri slope is comprised of two distinct stratigraphic units. The lower unit (unit 1) overlies the post–Last Glacial Maximum transgressive erosion surface and is interpreted as a Younger Dryas (ca. 12.85–11.65 ka) shoreface deposit based on radiocarbon and optically stimulated luminescence (OSL) ages, Bayesian age modeling, seismic facies, sediment texture, sediment infauna, and heavy mineral component. The shoreface was abandoned and partly eroded during rapid sea-level rise from ca. 11.5 to 7 ka. Unit 2 consists of fine sand and silt deposited in a midshelf environment when the rate of sea-level rise slowed between ca. 7 ka and the present. Although unit 2 provides a thin, relatively uniform cover over the lower slope break of the older shoreface, this feature still represents a valuable piercing point, providing a Hosgri fault slip rate of 2.6 ± 0.8 mm/yr. Full-waveform processing of chirp data resulted in significantly higher resolution in coarser-grained strata, which are typically difficult to interpret with more traditional envelope processing. Our novel combination of offshore radiocarbon and OSL dating is the first application to offshore paleoseismic studies, and our results indicate the utility of this approach for future marine neotectonic investigations.

A GIS Based Mapping of Seismic Parameters in Tectonically Active Mikir Massif Region of Assam, India

The Mikir massif area of Assam is considered one of the deformed zones of the north-eastern region (NER) as demonstrated by its seismicity. The present study aims to study the active tectonics in the Mikir massif region with the help of hypocentral parameters, frequency–magnitude relation, and ‘b’ and ‘a’ values of earthquake events. The major structures and existing lineaments of the area are correlated with the seismicity pattern and areas having low seismicity and less heterogeneity has been demarcated based on seismic parameters. The seismicity in the area shows a higher concentration of earthquake events towards NW compared to other parts. The depth section of earthquake events along and across the regional faults shows that all the major structures and lineaments with distinct topographic expressions can be identified from the seismic profile. The activeness of those faults and lineaments can be accessed by the cluster of seismic events that occurred spatially and also at depth. Classification of the area into 4 different seismogenic zones shows higher values of seismic parameters (Mc, a, and b) for Zone A, followed by Zone C, Zone B, and Zone D. The higher seismicity associated with these zones is also confirmed by high lineament density in those areas. The lower b and a- value associated with Zone D implies that the area is seismotectonically stable with fewer numbers of seismic events and hence suitable for any civil engineering construction.

Bayesian analysis of ground motion models using chimney fragility curves: 2021, 5.9-Mw Woods Point intraplate earthquake, Victoria, Australia

The 22 September 2021 (AEST) MW 5.9 Woods Point earthquake occurred in an intraplate setting (southeast Australia) approximately 130 km East Northeast of the central business district of Melbourne (pop. ∼5.15 million). A lack of seismic instrumentation and a low population density in the epicentral region resulted in a dearth of near-source instrumental and “felt” report intensity data, limiting evaluation of the near-source performance of ground motion models (GMMs). To address this challenge, we first surveyed unreinforced masonry chimneys following the earthquake to establish damage states and develop fragility curves. Using Bayesian inference, and including pre-earthquake GMM weightings as Bayesian priors, we evaluate the relative performance of GMMs in predicting chimney observations for different fragility functions and seismic velocity profiles. At the most likely VS30 (760 m/s), the best performing models are AB06, A12, and CY08SWISS. GMMs that were preferentially selected for utility in the Australian National Seismic Hazard Model (NSHA18) prior to the Woods Point earthquake outperform other GMMs. The recently developed NGA-East GMM performs relatively well in the more distal region (e.g. >50 km) but is among the poorest performing GMMs in the near-source region across the range of VS30. Our new method of combining analysis of engineered features (chimneys) with Bayesian inference to evaluate the near-source performance of GMMs may have applicability in diverse settings worldwide, particularly in areas of sparse seismic instrumentation.

Pre-Quaternary Evolution of the Eurasian Basin: Results of Interpretation of Seismic Profile ARC1407A

The work examined the tectonics and stratigraphy of the Eurasian basin in pre-Quaternary times based on the results of interpretation of the ARC1407A seismic profile and calculations of the theoretical position of linear magnetic anomalies. The sedimentary packages identified on the seismic profile and their stratigraphic adjustments are close to similar studies in the western parts of the Nansen and Amundsen basins. The age assignment of sedimentary strata corresponds to the results of drilling ACEX wells and the main stages of development of the Eurasian basin. The reference horizon of ~34 million years old, previously identified in some scientific works and associated with the cessation of spreading in the western part of the North Atlantic and the entry of the Greenland Plate into the North American Plate, has not been established, which is similar to studies in the western parts of the basins of the Eurasian Basin. For the western part of the Nansen Basin, a reference horizon with an age of ~38 Ma was identified for the first time, previously traced in the western part of the Amundsen Basin, the appearance of which is associated with one of the stages in the development of the Eurekan Orogeny. Also, for the western part of the Nansen Basin, within the ARC1407A section, a reference horizon with an age of ~26 Ma, previously traced in the western part of the Amundsen Basin, is established. The appearance of this boundary is associated with the beginning of unstable spreading in the westernmost segment of the Eurasian basin between the Yermak Plateau and the Morris Jesup Rise. The end of the long stratigraphic hiatus from 44.4 to 18.2 Ma in the ACEX well section clearly correlates with the appearance of sedimentary strata with an age of ~19.6‒18.3 Ma, which confirms the point of view about the beginning of the formation of the deep-sea connection between the North Atlantic and Eurasian basins. This event coincides with a fundamental stage in the restructuring of the movements of the Eurasian and North American plates, expressed in a change in the general direction of migration of the instantaneous opening poles from north-northwest to south-southeast. It is assumed that thick sedimentary deposits in the Nansen Basin and in the rift valley of the Gakkel ridge, observed on seismic section ARC1407A are glaciomarine Late Pliocene-Quaternary in age 2.7 Ma. Apparently these deposits are making up a significant volume of sediment in the eastern part of the Eurasian Basin and the Gakkel Ridge.

Deep Seismic Reflection Imaging of Mesozoic Kachchh Rift, NW India: Implications for Evolution

AbstractDeep crustal seismic reflection profiling is carried out for first time across various tectonic domains of seismically active Mesozoic Kachchh rift basin, formed during the breakup of Gondwanaland. The seismic data, processed using the common reflection surface stack approach, provided maiden images of the shallow and deep sub‐surface structures in the region. These images reveal a 15 km thick subhorizontal lower crustal reflection fabric and crustal‐scale domal‐type structure extending from the surface to the Moho. We interpret the earlier structure represent magmatic underplating and the latter as the Kachchh Mainland uplift. We find large variations in the thickness of sediments from 150 m to 6.5 km and crustal thickness from 45 to 35 km from north to south, with a Moho up warp of 4 km beneath the Kachchh Mainland fault. The Kachchh rift basin exhibits an unusually thick crust of 45 km, contrary to many rift basins. We interpret the syn‐rifting and the Reunion mantle plume activity, manifested as Deccan volcanics, are responsible for magmatic underplating and crustal thickening. Uplift in the region is multi‐genetic in origin. Present study illuminated new faults and nature of various other faults. Moderate to large earthquakes in the region are attributed to the regional and local stresses resulting from the plate boundary and heterogeneous crustal structure. Based on the mafic lower crust and distribution of aftershocks through the entire crust, we interpret the lower crust is brittle, contrary to most models of continental rheology.

Paleontology, Stratigraphy and Biogeography of the Late Precambrian and Phanerozoic Formations of Russia

Abstract —This special issue is dedicated to discussing the possibility of the stage division of the Vendian system, specifically, a composite section of the Siberian Platform is proposed as a stratigraphic standard. For the Ordovician system of the Siberian Platform and Northeastern Russia, certain changes in stratigraphic charts are proposed, nomenclative issues and different variants of correlations of regional horizons with the General Stratigraphic Scale (GSS) of Russia are considered. A geochemical analysis of Bajocian and Bathonian belemnite rostra of Arctic Siberia was carried out and for the first time a comprehensive C, O, and Sr isotope characterization of the sections was obtained, making it possible to propose a comparison of the studied sections with the European standard and to develop paleogeographic models. Comprehensive studies of the Upper Jurassic and Lower Cretaceous series of the Anabar Bay deposits (Laptev Sea) led to an improved biostratigraphic division for this reference section and to the identification of the levels for interregional and global correlation. The magnetostratigraphic division of the Jurassic–Cretaceous boundary interval and its comparison with the Tethyan regions are discussed. Taking into account biostratigraphic data, the δ13Corg variation curve can also serve as a correlation tool. The clinoform structure of Upper Jurassic strata was revealed in the northwest of the Anabar–Khatanga depression, based on the interpretation of seismic profiles calibrated against studied sections. The formation of this complex allows us to consider this stage as the initiation of the Verkhoyan orogeny in this folded area. Analysis of variations in the thickness of sediments allows us to deduce that intense tectonic-sedimentary processes took place during the Late Jurassic epoch, along with subsidence of the bed of the sedimentary basin. Analysis of a specific assemblage of lacustrine fossils from the reference section of the upper part of the Lower Cretaceous series of Transbaikalia and of published data showed that this fossil assemblage was widespread in East Asia. Biofacies analysis made it possible to reconstruct the paleoenvironments of this time.

Seismic reflection characteristics and controlling factors of shallow gas in the alongshore mud clinoform of the East China Sea

This study aims to investigate seismic reflection characteristics and controlling factors of gas within the East China Sea clinoform based on seismic profiles and borehole core data, including organic geochemistry and lithology. The seismic profiles reveal distinct gas-related seismic reflection characteristics along the coast, extending from the Yangtze Estuary southward to the elongated inner shelf. In the northern region, specifically the northern part off the Yangtze Estuary, widespread acoustic blanking is topped by a shallow gas front reflector parallel to seabed. In the middle region, spanning the southern part off the Yangtze Estuary and the adjacent northern inner shelf, widespread acoustic blanking is topped by a gas front reflector following the clinoform stratigraphy. In the southern region, off the Oujiang Estuary, there are widespread acoustic turbidities that are topped by a severely undulated gas front reflector. Based on the combined seismic-core data, it is suggested that the seismic reflection in the northern region, characterized by the gas front reflector parallel to the seabed, is controlled by the combination of anaerobic oxidation of methane and active gas diffusion triggered by seabed erosion. In the middle region, the obstructive effects of clinoform stratification on gas diffusion contribute to the seismic reflection characterized by the gas front reflector following the clinoform stratigraphy. In the southern region, the seismic reflection of low-concentration gas characterized by widespread acoustic turbidities is primarily influenced by the interplay between abundant gas source supplies due to the clinoform depocenter and elevated hydrostatic pressure in deeper water conditions. Furthermore, local gas seepage is observed in the northern region, likely due to seabed erosion and lower hydrostatic pressure in coastal zones. Additionally, the coastal zones and seaward slope break sites along the rest of the clinoform sections are also identified as having higher potentials for gas seepage.