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The Qiongdongnan Basin (QDNB) is a rift basin located in the extension direction of the oceanic ridge of the Northwest Subbasin of the South China Sea. This basin is surrounded by Late Cenozoic Ocean Island Basalts (OIBs) magmatism associated with the putative Hainan mantle plume. However, how the Hainan mantle plume has affected the QDNB has not been studied in detail. To reveal the crustal structure of the QDNB and the possible influence of the Hainan mantle plume, we conducted a wide-angle seismic exploration from Hainan Island across the QDNB to the Xisha Block, and obtained the crustal structure by travel-time tomography. The results show that the crustal stretching factors of the Northern and Southern Rises of the QDNB are estimated to be 1.3–2.0, indicating slight or moderate crustal thinning. Whereas, the crustal stretching factor in the Central Depression of the QDNB is estimated to be more than 3.0, which means that the crust has been hyperextended. Controlled by two detachment faults (F2 and F11), the upper and lower crust show brittle and ductile thinning, respectively. Three anomalous low-velocity conduits penetrating the crystalline crust are developed in the Songnan Uplift and the Southern Rise of the QDNB. Their P-wave velocities are 5.5–6.0 km s−1, which are significantly lower than those of the surrounding crust (6.0–6.8 km s−1). The Late Cenozoic magmatic intrusions (e.g., sills and dikes) and hydrothermal activities can be observed in the sedimentary strata above these low-velocity conduits. We interpret that the low-velocity conduits might be the crustal magmatic footprints of the Hainan mantle plume, due to the correlated distributions of OIB-type magmatism, the magmatic intrusion patterns, the deep structure of the Hainan mantle plume and the high heat flow of 95 mW m−2. Thus, we propose that the crustal structure of the QDNB is the product of crustal hyperextension in the rifting stage and subsequently affected by the Hainan mantle plume magmatism. This study provides a typical case for understanding the influence of a mantle plume on the continental crust with pre-existing rift structures.

The southernmost Mariana margin lacks a mature island arc and thus differs significantly from the central‐north Mariana and Izu‐Bonin margins. This paper presents a new Pwave velocity model of the crust and uppermost mantle structure based on a 349‐km‐long profile of wide‐angle reflection/refraction data. The active source seismic experiment was conducted from the subducting Pacific plate to the overriding Philippine plate, passing through the Challenger Deep. The subducting plate has an average crustal thickness of ~6.0 km with Vp of 7.0 ± 0.2 km/s at the base of the crust and low values of only 5.5–6.9 km/s near the trench axis. The uppermost mantle of the subducting plate is characterized by low velocities of 7.0–7.3 km/s. The overriding plate has a maximum crustal thickness of ~18 km beneath the forearc with Vp of ~7.4 km/s at the crustal bottom and 7.5–7.8 km/s in the uppermost mantle. A zone of slight velocity reduction is imaged beneath the Southwest Mariana Rift that is undergoing active rifting. The observed significant velocity reduction in a near‐trench crustal zone of ~20–30 km in the subducting plate is interpreted as a result of faulting‐induced porosity changes and fracture‐filling fluids. The velocity reduction in the uppermost mantle of both subducting and overriding plates is interpreted as mantle serpentinization with fluid sources from dehydration of the subducting plate and/or fluid penetration along faults. The southernmost Mariana margin is a convergence margin that lacks a mature island arc. The trench reaches the deepest point on Earth at the Challenger Deep, the origin of which is still little known. We acquired and analyzed new active source wide‐angle reflection/refraction seismic data along a profile across the Challenge Deep, obtaining a new model of crustal and uppermost mantle velocity structure of both the subducting Pacific and overriding Philippine Sea plates. Significant velocity reduction is observed in the crust of the subducting plate, especially in a 20‐ to 30‐km‐wide zone near the trench axis, which is interpreted as a result of faulting‐induced porosity changes and fracture‐filling fluids. A zone of slight velocity reduction is imaged beneath the tectonically active Southwest Mariana Rift. Significant velocity reduction is also observed in the uppermost mantle of both subducting and overriding plates and is interpreted as a result of mantle serpentinization. We conducted an active‐source seismic experiment across the Challenger Deep, revealing a new velocity model of the southernmost Mariana marginReduction in crustal velocity was observed in the subducting plate, especially in a near‐trench zone, possibly due to porosity changes and fluids; Relatively low upper mantle velocities were interpreted as mantle serpentinizationA low‐velocity zone of ~15‐ to 20‐km‐wide was imaged beneath the Southwest Mariana Rift, interpreted as a weak zone of initial rifting

We present a 2‐D seismic tomographic image of the crustal structure along the OBS2012 profile, which delineates the Moho morphology and magmatic features of the northeastern South China Sea margin. The image was created by forward modeling (RayInvr) and traveltime tomographic inversion (Tomo2D). Overall, the continental crust thins seaward from ~27 km to ~21 km within the continental shelf across the Zhu I Depression and Dongsha Rise, with slight local thickening beneath the Dongsha Rise accompanying the increase in the Moho depth. The Dongsha Rise is also characterized by ~4–7 km thick high‐velocity layer (HVL) (~7.0–7.6 km/s) in the lower crust and exhibits a relatively high velocity (~5.5–6.4 km/s) in the upper crust with a velocity gradient lower than those of the Zhu I Depression and Tainan Basin. Across the continental slope and continent‐ocean transition (COT), which contain the Tainan Basin, the crust sharply thins from 20 km to 10 km seaward and a ~2–3 km thick HVL is imaged in the lower crust. We observed that volcanoes are located only within the COT, but none exist in the continental shelf; the Dongsha Rise exhibits a high magnetic anomaly zone and different geochemical characteristics from the COT. Based on those observations, we conclude that the HVL underlying the COT is probably extension related resulting from the decompression melting in the Cenozoic, whereas the HVL beneath the Dongsha Rise is probably arc related and associated with the subduction of the paleo‐Pacific plate. These findings are inconsistent with those of some previous studies. Model of the northeastern South China Sea across the Dongsha Rise and the Tainan Basin is presentedA high‐velocity lower crustal layer is observed across the continental shelf/slope and the COTThe high‐velocity lower crustal layer across the COT is extension related, while it is arc related across the Dongsha Rise We show a seaward thinning trending of the crustal structure across the continental shelf and continent‐ocean transition (COT), whereas the crust was slightly and unconventionally thickened beneath the Dongsha Rise accompanying the increase in the Moho depth. The Dongsha Rise is also characterized by ~4–7 km thick high‐velocity layer (HVL) in the lower crust underlying the thick crust. In the COT, the crust sharply thinned seaward and a ~2–3 km thick HVL was imaged underlying the hype‐extended crust. We discuss the nature of the HVL in the northeastern South China Sea (SCS) by integrating the volcanic activity, magnetic anomaly, Moho morphology, and geochemistry of the region. We observed that the HVL underlying the COT is probably extension related resulting from the decompression melting in the Cenozoic, whereas that beneath the Dongsha Rise is probably arc related and associated with the subduction of the paleo‐Pacific plate. These findings are inconsistent with those of some previous studies. We believe that our viewpoint is first put forward through the detailed demonstration and provides new insights into the rifting processes of the northeastern SCS margin. It will be of great interest to the broad marine geosciences community, particularly those interested in the formation and evolution of the marginal seas.

As valuable land in the ocean, coral islands are not only important bases for making use of marine resources and protecting marine rights and interests, but also important for breakthrough research in many fields of earth science. Hence, the economical and efficient determination of the underground structure of coral islands has become significant in coral island engineering geology, but remains challenging for traditional marine geophysical prospecting and drilling methods. While ambient noise tomography with dense arrays has been widely used in continental regions, its applicability to coral islands remains undetermined. In this study, based on the data recorded by a dense array on an isolated coral island in the South China Sea, we analyzed the ambient noise characteristics and obtained a 3D subsurface structure of the coral island using ambient noise tomography. We made the following findings: ① The ambient noise frequencies can be roughly categorized into three levels: < 1, 1–5, and > 5 Hz. The spectral characteristics of the noise below 5 Hz were consistent at different stations, but there were significant differences in the characteristics of the noise above 5 Hz. ② For ambient noise frequencies below 5 Hz, cross-correlation functions with high quality could be obtained with only 24 h of waveform data. However, it was difficult to extract meaningful cross-correlation functions for ambient noise frequencies above 5 Hz. ③ The S-wave velocity in the coral island was higher toward the sea and lower toward the lagoon, which was consistent with the high degree of cementation of the outer reef flat stratum on the seaward side. ④ There were two low-velocity horizons at 25–75 and 200–300 m, which were in good agreement with the high-porosity horizons that were revealed by drilling core samples, reflecting the weathering history of the reef. Our research demonstrates that ambient noise tomography is a potentially economical, efficient, and environmentally friendly method for the geological prospecting of coral reefs.