Evolution of the continental margins of the South China Sea (SCS) is one of the open questions when discussing continental breakup and seafloor spreading. We processed data from a wide-angle seismic profile (OBS2011–1), which passes through the northwestern margin of the SCS, and performed travel time modelling to obtain the seismic velocity structures. The modelling results show a stepwise variation of the crustal thicknesses from continental margin to oceanic basin. Stretching factor of the upper crust is nearly double the estimate of the lower crust along the Zhongsha Trough. The lower crust shows asymmetrical upwelling towards the trough center, accompanied by ∼0.3 km s–1 of the velocity increase due to magmatic addition. The upper and lower crusts have almost the same stretching factor beneath continental blocks, indicating a uniform extension. Crustal structures of the conjugate margins of the Southwest Subbasin show similar velocity range and different thickness distribution, supporting the common origin and asymmetric extension of these two margins. The Ocean–Continent Transition zones (OCT) are much wider in the southern part (∼50 km) than the northern part (∼25 km) crossing the margins. We propose a tectonic model for the asymmetry of both the conjugate margins and the OCTs, favoring the highly stretched upper crust and accompanied by rising of the ductile middle-lower crust controlled by major low-angle faults. The rigid blocks may also act as a kind of hindrance for further evolution of the failed rifts and affect the shape of the OCT.
Abstract Upper mantle discontinuities are influenced by convection‐related thermal heterogeneities arising in complex geodynamic settings. Slab rollback of the Pacific plate and mantle upwelling in the Meso‐Cenozoic caused the extension and spreading of continental segments in the South China Block leading to profound variations of the local temperature conditions. We processed 201 teleseismic events beneath 87 stations in the Hainan, Guangdong, and Fujian provinces in the South China Block, and extracted 4172 high‐quality receiver functions. We imaged the topography of the local mantle discontinuities by using phase‐weighted common conversion point stacking of the receiver functions, which effectively improves the P‐to‐S‐converted phases. We found that the average depths of the discontinuities at 410 and 660 km depth are 414 and 657 km, respectively, while no clearly defined discontinuity at 520 km depth was detected. We mapped the thickness of the mantle transition zone (MTZ), which can reflect temperature and/or compositional heterogeneities as well as the presence of water, and discussed possible geodynamic implications. In particular, we found that the MTZ beneath the Leizhou Peninsula in the Hainan province is 42 km thinner than average. This scenario suggests that the Hainan plume is responsible for positive temperature anomalies between ∼270 and 380 K and between ∼200 and 240 K at the 660 and 410 km discontinuities, respectively. We also observed a prominent uplifting of the 660 km boundary beneath the coast regions that may be indicative of lateral flow of the Hainan plume.
The land-sea transition zone in the northern South China Sea (SCS) records important information from the continental rifting to the seafloor spreading. The crustal structure is the key to explore the deep tectonic environment and the evolution of the SCS. In 2015, the onshore-offshore 3D deep seismic experiment was carried out on the Pearl River Estuary (PRE). Explosions and air guns were used as sources on land and at sea respectively in this experiment.Onshore seismic stations and Ocean Bottom Seismographs (OBSs) synchronously recorded the seismic signals. We focus on an onshore-offshore seismic profile (L2, SE-trending) along the eastern side of the PRE. By modelling the seismic travel times, we constructed a P-wave velocity model along the profile. The model shows that the sediment on land is thin and has seismic velocities of 4.5–5.5 km/s. In contrast, thickness of the offshore sediment gradually increases to more than 4.0 km, and the velocities vary between 2.0 km/s and 4.5 km/s. The onshore and offshore crustal velocities are 5.8–6.8 km/s and 5.5–6.8 km/s, respectively. At depth between 15 km and 20 km, a low-velocity layer (LVL; only 5.9 km/s) is detected, pinching out under the Littoral Fault Zone (LFZ). The LVL has probably accommodated the crustal extension beneath the land area, resulting in low extent of the crustal thinning. A slightly uplifted Moho exists beneath the Dongguan fault depression zone, representing a place where hot mantle materials ascend. Localized thickening of the sediments and rapid thinning of the crust characterize the LFZ, and it can be regarded as a tectonic boundary between the South China (SC) with normal continental crust and the northern SCS margin with extended continental crust.
Abstract We present a high-resolution shear-wave velocity model of the upper-middle crust beneath the Pearl River Delta. The sedimentary basins are characterized by low-velocity anomalies at depths of 6–8 km and reflect the lateral migration of the deposition centers. High-velocity structures are located predominantly in Hong Kong, and outline the supervolcano conduit and magma reservoir in the crust. We observe contrasting velocity structures across the Lianhuashan fault zone (LFZ) where low-velocity anomaly zones (LVZs) are developed in different strikes. Microseismic swarms at depths of 8–16 km mainly occurred at the intersection of the LVZs. Metamorphic fluids and migration in the crust are proposed to interpret the seismic activity. The intersecting LVZs would increase the lithologic permeability to allow deep-rooted fluids to flow up, whereas the overlying high-velocity structures representing the overflowing magma act as a permeable barrier that confines the seismicity in the middle-lower crust. We anticipate that the detailed subsurface structures will help correlate magmatic remnants with the distribution of intraplate earthquakes and assess the earthquake risks in the Greater Bay Area of China.
Summary We evaluate quantitatively the degree of serpentinization in the mantle wedge of Kyushu subduction zone by using the seismic velocity. The results show that the degree of serpentinization in the forearc mantle wedge of Kyushu is strongly heterogeneous and varies from 0 to 12%, containing about 0-1.8% water contents. In general, the degree of serpentinization gradually decreases with depth from 40 -80 km and the largest degree usually occur in about 40-50 km depth. Localized high anomalies of serpentinization are revealed in the northern and southern portions of Kyushu, respectively. We suggest that in the northern portion of the forearc mantle wedge, the water contents are relatively large, which might result from the abundant fractures and cracks with more fluids in the subducting slab because of the subduction of Kyushu-Palau ridge and the sudden change in its subduction angle of Philippine Sea lithosphere. But the high degree of serpentinization in the southern portion is closely associated with the active left-lateral shear zone revealed by global positioning system site velocities and earthquake focal mechanisms. The present results also display that the low degree of serpentinization in the central domain of the forearc mantle wedge is consistent with the location of anomalous arc volcano.
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