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The southern Mariana subduction zone, home to the Challenger Deep—the deepest known point on Earth—poses significant challenges for studying the hydration of the subducting plate due to its extreme depth. This study uses S‐wave seismic tomography and Vp/Vs ratios to investigate hydration and serpentinization at the Challenger Deep. We observe a low Vp and Vs layer in the upper mantle with Vp/Vs ratios exceeding 1.8, reaching up to 1.95 at the Moho. These high ratios indicate a strong serpentinized layer (>15 vol%) with significant changes in the mechanical properties of the serpentinized peridotite. Additionally, Vp/Vs ratios in the crust and uppermost mantle increase from the outer rise to the trench axis, demonstrating that bending‐related faulting and hydration intensify as the plate approaches the trench. Our results suggest extensive faulting, hydration, and mantle serpentinization at the Challenger Deep, making this region an extreme example of water cycling in subduction zones. Plain Language Summary The southern Mariana Trench, containing the deepest point on Earth's surface, is where the old Pacific Plate (∼125 Ma) is subducting beneath the Philippine plate. Understanding the processes of bending‐related faulting and hydration of the incoming subducting plate has been challenging due to the limitations of using only P‐wave velocity (Vp), which does not provide detailed lithological information. In this study, we identified valuable converted S‐wave arrivals from the incoming plate, allowing us to determine the S‐wave velocity (Vs) structure and calculate the Vp/Vs ratios. Our results reveal that the low Vp layer in the upper mantle is a strongly serpentinized layer. Compared to other subduction zones, the combination of lower Vp and Vs values with higher Vp/Vs ratios suggests more intense serpentinization within the incoming plate at the southern Mariana subduction zone. This study provides a clearer understanding of mantle hydration processes in extreme subduction environments and highlights how plate characteristics influence serpentinization intensity. Key Points S‐wave tomography and Vp/Vs ratios reveal extensive serpentinization and hydration in the subducting plate of the southern Mariana Trench Vp/Vs ratios in the crust and uppermost mantle increase toward the trench axis, indicating intensified hydration as the plate approaching Challenger Deep is an extreme example of water cycling in subduction zones

The Trans‐Atlantic Geotraverse (TAG) field on the Mid‐Atlantic Ridge is one of the largest currently active seafloor hydrothermal fields known. An underlying detachment is inferred to maintain TAG's long‐lived hydrothermal discharge, but the detachment lacks a widespread corrugated surface. We used dense wide‐angle seismic data to define TAG's detachment structure at a finer scale than has previously been possible. We generated two P‐wave velocity profiles of the shallow section of the detachment using first‐arrival travel‐time tomography, preconditioned by downward continuation. Our results reveal a low‐angle detachment, dipping at ∼15° (±5°) at 5 km east of the ridge axis, and evidence for uplifted lower‐crustal gabbro in the footwall. Increasing footwall velocities southward suggest a more intense exhumation of deep‐seated rocks, showing the detachment's geometry changes along the ridge axis. We conclude the detachment is a complex 3‐D structure, and a young system without a dome‐shaped footwall can exhumes deep‐seated crustal rocks. Plain Language Summary Hydrothermal vents, where seawater is superheated by magma and discharges mineral‐rich fluids through the seafloor, help regulate global ocean chemistry and are a potential resource for base metals. The Trans‐Atlantic Geotraverse (TAG) field on the Mid‐Atlantic Ridge is one of the largest known deep‐sea hydrothermal systems. A long‐lived extensional fault, called an oceanic detachment, is believed to localize hydrothermal circulation and venting off the ridge axis. However, current knowledge is still limited on the 3‐D geometry of the underlying detachment and how it influences high‐temperature hydrothermal activity. We present active‐source seismic data to characterize the detachment's subseafloor structure and properties at TAG, where no widespread seafloor outcrop of the fault surface is observed. Our results reveal lower crustal rocks are uplifted by the detachment, with possibly more intense fault deformation to the south of the TAG field. We suggest that the TAG detachment is a young system and expresses variable geometry along strike, with a series of concurrent faults to the north merging into one main fault dominating the south. This observation indicates detachments are complex 3‐D structures and even a young detachment lacking a clear exposed fault surface, can accommodate significant tectonic displacement and uplift of deep‐seated lithologies. Key Points P‐wave velocity model from dense seismic refraction data reveal the detailed structure of the detachment at the TAG hydrothermal field The TAG hydrothermal field is underlain by a detachment fault with a complex 3‐D geometry Young detachments, such as under TAG, could exhume lower‐crustal gabbro despite limited corrugations and a lack of a dome‐shaped footwall

The 26 December 2004 Sumatra earthquake (Mw = 9.1) initiated around 30 km depth and ruptured 1300 km of the Indo‐Australian–Sunda plate boundary. During the Sumatra‐OBS (ocean bottom seismometer) survey, a wide‐angle seismic profile was acquired across the epicentral region. A seismic velocity model was obtained from combined travel time tomography and forward modeling. Together with reflection seismic data from the SeaCause II cruise, the deep structure of the source region of the great earthquake is revealed. Four to five kilometers of sediments overlie the oceanic crust at the trench, and the subducting slab can be imaged down to a depth of 35 km. We find a crystalline backstop 120 km from the trench axis, below the fore‐arc basin. A high‐velocity zone at the lower landward limit of the ray‐covered domain, at 22 km depth, marks a shallow continental Moho, 170 km from the trench. The deep structure obtained from the seismic data was used to construct a thermal model of the fore arc in order to predict the limits of the seismogenic zone along the plate boundary fault. Assuming 100°–150°C as its updip limit, the seismogenic zone is predicted to begin 5–30 km from the trench. The downdip limit of the 2004 rupture as inferred from aftershocks is within the 350°–450°C temperature range, but this limit is 210–250 km from the trench axis and is much deeper than the fore‐arc Moho. The deeper part of the rupture occurred along the contact between the mantle wedge and the downgoing plate.

We have analyzed the MZ6 onshore‐offshore wide‐angle seismic profile of the MOZ3‐5 survey to investigate the crustal structure of the South Mozambique passive margin. The NNW‐SSE, 625 km‐long profile runs across the Mozambique coastal plain (MCP), the Continental Shelf and Slope, the Almirante Leite Ridge (ALR), the North Natal Valley (NNV), the Ariel Graben and the Dana Plateau of the Mozambique Ridge. Forward modeling through combined interpretation of the multichannel seismic, the main reflected and refracted phases of the wide‐angle, drill hole data and bathymetric data reveal: (a) a sedimentary cover poorly compacted up to 3 km‐thick, intruded by magmatic dykes that reach the seafloor at the ALR forming 0.5 to 5 km‐wide corrugated mounts, (b) between 2 and 7 km, thick magmatic or volcano‐clastic deposits are observed both at the MCP and NNV, forming a 40 km‐wide terrace at the center of MZ6 and southward‐dipping reflectors in the southern part interpreted as the Pre‐Neocomian Formation contemporary of the Karroo and/or Bombeni‐Movene magmatic events reached in several wells, (c) onshore, the 3‐layered crust reaches 39 km thickness, gradually thinning to ∼27 km at the southern end of MZ6. In the deepest layer, velocities exceed 7.15 km/s, reaching at its base 7.55 km/s at the vertical of the ALR. (d) the seismic Moho is marked by a strong reflection on the wide‐angle data. These results suggests that the basement is composed of slightly thinned and altered continental crust, most likely intruded by several phases of intense magmatism. Plain Language Summary About 200 Ma ago, the mega‐continent Pangaea broke up. The dispersion of the pieces, linked to the closure and disappearance of the Thetys paleo‐ocean, gave the birth of the Atlantic and Indian Oceans. In detail, the initial position of each piece of this jigsaw is of great importance as it has an impact on our understanding of the genesis of the continental passive margins, the role of tectonic inheritance, the pre‐rift and post‐rift evolution of the topography dynamic (vertical movement) and of the geodynamic of the plates (horizontal movement). Nevertheless, in the Western Indian Ocean, the initial pre‐beak‐up position of Antarctica plate respect to Africa plate is still under debate, mainly due to the lack of deep geophysical data. In 2016, an academic‐industrial collaboration succeeded in acquiring deep information along seven seismic profiles crossing the North Natal Valley (NNV) off the coast of Mozambique. The results falsify the presence of an oceanic crust in that area and thus most of the plate reconstruction models. The NNV presents a thick continental crust intrudedded by several phases of intense magmatism, with at its top, a volcano‐clastic pre‐Neocomian Fm contemporary of the crustal thinning and subsequent spreading between Africa and Patagonia plates. Key Points The North Natal Valley (NNV) presents a ∼30 km‐thick continental crust, in contrary to what is proposed in most geodynamic models Velocity variations in the middle‐lower part of this crust probably represent mantle intrusions during several phases of intense magmatism On the top of the basement, there is a volcano‐clastic pre‐Neocomian Fm, probably contemporary of the movement of the Patagonia plate

Magnetotelluric (MT) and wide-angle seismic reflection/refraction surveys play a fundamental role in understanding the crustal rheology and lithospheric structure of the Earth. In recent years, the integration of the two methods in order to improve the robustness of the inversion has started to gain attention. We present a new approach for joint 3-D inversion of MT and wide-angle seismic reflection/refraction data to accurately determine crustal structures and Moho depth. Based on H-κ stacking of teleseismic receiver functions (RFs), we estimate an initial reference Moho. This is used as input for the subsequent MT/seismic joint inversion, where the Moho interface is updated and crustal structures are added to the model. During the joint inversion process, structural similarity is facilitated through the cross-gradient constraint. Synthetic model tests show an improvement of the inversion results over separate inversions. In particular, the tests based on two geologically realistic models demonstrate that the crustal structure and even the trade-off between velocity and Moho interface can be sufficiently resolved by combined MT and seismic datasets when using the estimates from analysis of RFs. These results show that the new method can provide useful constraints on crustal structures including their geophysical properties and discontinuities.

We present the results of new seismic reflection and wide‐angle data across the SE Caribbean plate boundary. The 550 km long N–S profile crosses the structures involved in the active 55 Ma long continent‐arc oblique collision between the Caribbean (CAR) and the South American (SA) plate. From the north to the south these structures include the accretionary prism, the extinct volcanic arc (Leeward Antilles arc), the Tertiary Bonaire basin, the continental‐size dextral strike‐slip fault system (San Sebastián–El Pilar fault), the allochthonous exhumed terranes, and the authocthonous fold and thrust belt (Caribbean Mountain system) and foreland basin. The wide‐angle data show that these elements are characterized by different velocity structures and that they are separated by sharp lateral velocity variations. The Leeward Antilles arc exhibits a velocity structure similar to that of the Lesser Antilles active volcanic arc, indicating that the extinct arc has not been modified by the collision with the SA plate. The data show a ∼20 km change in crustal thickness across the San Sebastián fault, suggesting that the dextral strike‐slip fault is a crustal feature that likely continues in the mantle as a primary strand of the plate boundary between the South American and the Caribbean plates. South of the strike‐slip fault and beneath the exhumed eclogitic terranes, the data image a north dipping, high‐velocity (>6.5 km/s) anomaly in the upper crust (3–11 km), indicating that high‐pressure/low‐temperature rocks are the likely lithologies responsible for the high seismic velocities and suggesting that exhumation of these assemblages is enabled by the strike‐slip fault.

Post-critical seismic waves are widely used in crustal exploration of the seismic velocity structure, and are gaining interest in the oil gas seismic community to image the deeper structure beneath the high velocity basalt layer. They are featured with their phase shifts and strength changes, which should be taken into account in seismic data processing, such as velocity analysis and true amplitude migration, etc. In order to simplify the exact but complicated formula of reflection and transmission coefficients, numerous approximate expressions for reflection and transmission coefficients for pre-critical incidence are obtained. In the post-critical case, there is Downton's approximation with acceptable accuracy approximation when the velocity changes smoothly. However if the velocity model changes rapidly, the error will be relatively very large, limiting the use of the approach. In order to improve the post-critical approximation, we utilize Taylor expansion of ray parameters with angle increment (compared to critical angle) in wide-angle seismic reflection and transmission coefficients. The explicit expressions for amplitude and phase shift (time shift) for the post-critical incident angle are obtained. Our results confirm that the wide-angle seismic reflection transmission phase shifts are strongly frequency dependent; phase shifts of low frequency wide-angle seismic waves are more predominant and their correction should be considered in seismic processing and imaging. Numerical examples demonstrate that (1) the accuracies of these approximations are high compared to the classic Aki's formula and Downton's approximation, and (2) the wide-angle effect can be effectively reduced with phase-shift correction by utilizing our time-shift approximation to the seismic traveltimes.

To investigate the geodynamic processes of Mesozoic large-scale mineralization in South China, we deployed a 350-km-long, wide-angle seismic reflection/refraction sounding profile between Yingshan in Hubei and Changshan in Zhejiang. This profile traverses the Cu-Au metallogenic belt in the middle and lower reaches of the Yangtze River (YMB), the Jiangnan W-polymetal metallogenic belt (JNMB), and the Qinhang Cu-polymetal metallogenic belt (QHMB). Our imaging results reveal various interesting velocity features along the profile. (1) The velocity structure is characterized by vertical layering and horizontal blocking; (2) the YMB is marked by high velocity and high V p / V s ratios in general with a significantly uplifted Moho interface and a thin crust of ∼31 km, and the lower crust contains high-velocity anomalies and has the characteristics of a crust-mantle transition zone; (3) the JNMB is bounded by the Jiangnan fault and Jingdezhen-Huangshan fault and has low-velocity anomalies and low V p / V s ratios; and (4) the QHMB is characterized by high-velocity anomalies and high V p / V s ratios. The high-velocity anomalies in the YMB and QHMB represent relatively Cu-Au-rich mafic juvenile lower crust. The formation of this kind of crust is considered to be related to mantle-derived magma underplating or residues of Neoproterozoic oceanic crustal materials, and it also provided sources for large-scale Cu-Au mineralization in the Mesozoic. The JNMB has features similar to those of ancient crusts enriched in W-Sn, the partial melting of which played a leading role in the formation of the superlarge W deposits in this belt. Considering these results and other regional geological data, we propose that a large-scale oblique upwelling of the asthenosphere along the collisional belt of the Yangtze and Cathaysia blocks during the Mesozoic was the deep driving mechanism for the explosive mineralization of Cu, Au, and W in northeastern South China. The boundaries of the blocks or terrains and discontinuities of the lithosphere were the main channels for deep heat and magmas and therefore controlled the spatial distribution of the metallogenic belt.

We applied reverse time migration (RTM) to offshore wide-angle seismic data acquired with airgun shots and sparsely deployed ocean bottom seismographs (OBSs) for reflection imaging of the Moho discontinuity in the eastern margin of the Sea of Japan. While seismic tomography is generally applied to wide-angle seismic data for estimating regional velocity, reflection imaging is uncommon due to the low folds from wide-spacing OBS deployment. The long offset reflection data obtained by airgun-OBS surveys are promising for profiling deep crustal structures, which may be able to add constraints on the velocity structures estimated by tomographic inversion. Furthermore, reflection imaging from wide-angle seismic data is useful when only airgun-OBS data are acquired without any MCS data due to weather conditions or restrictions of using streamer cables. In this study, we validated the feasibility of RTM, which is an effective reflection imaging method based on wavefield modelling with the two-way wave equation, using offshore wide-angle seismic data acquired along two crossing survey lines off Niigata–Yamagata. Airgun shot intervals were 200 m in both surveys, and the OBS spacings were 5 km along a 297-km-long line and 8 km or 16 km along a 366-km-long line, except for OBSs near the coast. By applying RTM with velocity models estimated by traveltime tomography of the same OBS data, we successfully imaged clear reflections around depths of 20–30 km. We confirmed that reflections observed in the long offset range were effective in imaging the deep structures that were not imaged by the MCS survey in this region. The depths of reflectors were traced from approximately 20 km in the offshore area to approximately 30 km near the coast, which corresponds to the Moho discontinuity. The depth variation is consistent with the crustal classification that was inferred based on tomography analyses: thick oceanic crust in the Yamato Basin and rifted continental or island arc crust beneath the areas from the Sado Ridge to the coast. Our results from two surveys with different OBS spacings suggested the high potential of the application to a wide variety of wide-angle seismic data for crustal-scale seismic exploration.Graphic Abstract

The Emeishan large igneous province（ELIP） in SW China is interpreted to be associated with an ancient mantle plume. Most of the constraints on the role of mantle plume in the generation of the Emeishan flood basalts were provided by geological and geochemical methods, but the geophysical investigation is very limited. In order to better understand the deep structure and features of ELIP, we have studied the crustal velocity structure using the data acquired from the Lijiang-Panzhihua-Qingzhen wide-angle seismic profile. This profile crosses the three sub-zones of the ELIP（the inner, intermediate, and outer zones）, divided based on the differential erosion and uplift of the Maokou limestone. The results provided by the active source seismic experiment demonstrate：（1） The average depth of the crystalline basement along the profile is about 2 km.（2） The middle crust in the Inner Zone is characterized by high-velocity anomalies, with the average velocity of 6.2-6.6 km/s, which is about 0.1- 0.2 km/s higher than the normal one. The velocity of the lower crust in the inner zone is 6.9-7.2 km/s, higher than those observed in the intermediate and outer zones（6.7-7.0 km/s）. Relatively low velocity anomalies appear in the upper, middle and lower crusts near the junction of the inner zone and intermediate zone, probably due to the effect of the Xiaojiang fault（XJF）.（3） The average velocity of the crust is comparatively low on both sides of XJF, especially on the east side, and the average velocity of the consolidated continental crust is also low there. This may suggest that the XJF extends at least down to 40 km deep, even beyond through the crust.（4） The depth to the Moho discontinuity decrease gradually from 47-53 km in the inner zone, via 42-50 km in the intermediate zone to 38-42 km in the outer zone. In the inner zone, the Moho uplifts locally and the（consolidated） crust is characterized by high-velocity anomalies, which are likely related to intensive magma intrusion and underplating associated with melting of plume head. Overall the crustal velocity structure in the study area recorded the imprint left by the Permian Emeishan mantle plume.