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We utilized receiver functions to study the crustal structure beneath the Pearl River Delta in the southwestern part of the Cathaysia Block, which was characterized by widespread tectono-magmatism during the late Mesozoic. Combination of waveform modeling and H - k stacking was used to derive crustal thickness, V p / V s ratio and average P-wave velocity. The results show that the crustal thickness ranges from 26 km in the center of the Pearl River Delta to 32 km in the periphery; the V p / V s ratio is between ~1.70 and 1.74. The average P-wave velocity ranges from 6.4 to 6.8 km/s and shows higher value beneath the islands along the Lianhuashan Fault zone. A weakly positive correlation between the crustal thickness and V p / V s ratio is observed in southwestern Pearl River Delta, indicating depth-dependent extension of the continental crust, that is, ductile lower crust was stretched to a much higher degree in comparison with the brittle felsic upper crust. The crust in the northeastern Pearl River Delta may be more basaltic owing to underplating caused by the subduction of the Paleo-Pacific Plate, and thus a relatively high V p / V s ratio is observed there. The overall low crustal V p / V s ratio and the relatively higher crustal P-wave velocities beneath the Lianhuashan Fault zone may imply a felsic-intermediate composition and high metamorphic grade of the crust along the fault. The Lianhuashan Fault zone may provide a pathway for rapid magmatic transportation from the underplating beneath the crust to the surface. Unlabelled Image • Combination of receiver function modeling and H-k stacking can reveal more reliable estimates of the Vp/Vs ratio. • Average V p / V s ratio and P-wave velocity imply a felsic-intermediate and strongly deformed crust beneath the PRD. • Low-volume magma was transported rapidly from the mantle-derived underplating to surface through the Lianhuashan Fault zone. [ABSTRACT FROM AUTHOR]

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. • The along-strike crustal structures are presented of the Xisha block in the NW South China Sea (SCS) continental margin. • The Xisha block underwent slight and uniform deformation under the lithospheric extension during rifting. • Rigid continental blocks contributed to the along-strike segmentation of rifting in the northern SCS continental margin. [ABSTRACT FROM AUTHOR]

By processing the vertical component data of OBS2019–2, a wide angle seismic line situated adjacent to the Liyuexi Trough in the Nansha Block of the South China Sea (SCS), we obtained a crustal model of P-wave velocity structure beneath the line and acquired information about the deep crustal structure in the Liyuexi Trough region. However, the lithology remains to be fully constrained. This paper presents the results of processing the horizontal component data of OBS2019–2 to obtain the S-wave velocity structure model and Vp/Vs ratios. The results show that the Vp/Vs ratios of sedimentary layer range from 1.90 to 4.60, while the corresponding S-wave velocities range from 0.4 to 1.8 km/s. As for the crust, the Vp/Vs ratios fall between 1.70 and 1.89, while the S-wave velocities range between 2.9 and 4.2 km/s. The lithological compositions of oceanic crust from top to bottom may consist of basalt, diabase, and gabbro. In contrast, the lithologies of continental crust transition from felsic to mafic downwards, including andesite, granodiorite, granite, greenschist, and anorthosite. The rock compositions of the Continent-Ocean Transition (COT) region are intermediate between the continental and oceanic crusts, exhibiting transitional features. The rock composition of the high-velocity layer (HVL) in the lower crust may consist of amphibolite, and the presence of magma intrusions and underplating in this region could be associated with the Zhongnan-Liyue Fault. Compared to the Dongsha Block, the lithologies of the Liyue Block and Zhongsha Block are more similar, indicating a higher possibility of conjugation between the Liyue Block and the Zhongsha Block. • S-wave and Vp/Vs profiles were developed near the Liyuexi Trough in the southern continental margin of the South China Sea. • The formation of the thin high-velocity layer composed primarily of amphibolite was related to the Zhongnan-Liyue Fault. • The Liyue Block is inferred to be conjugated more likely with the Zhongsha Block. [ABSTRACT FROM AUTHOR]

The Zhenbei and Huangyan seamounts located close to the extinct spreading ridge (ESR) in the South China Sea (SCS) have provided fundamental insight into postspreading magmatism. Long debates on the source and evolutional process of the postspreading seamounts are due to the lack of detailed structural crustal constraints. Here, we present three-dimensional (3D) crustal thickness and the Moho geometry of the Zhenbei and Huangyan seamounts and their surrounding area by forwarding and interpolating P-wave forward velocity structures along 14 seismic refraction and wide-angle profiles. We analyzed the variation in crustal velocity and thickness from the waning stage of seafloor spreading to the postspreading magmatism stage. Results show that the original oceanic crust, with an average thickness of 4.5 ± 0.7 km, is characterized by thin lower crust, thick upper crust, and low crustal velocity, compared with Atlantic oceanic crust, reflecting reduced magma supply and intense tectonic fracturing. Both the Zhenbei and Huangyan seamounts show high extrusion to intrusion ratio (2.6 and 2.7) and low P-wave velocities in the upper crust, indicating that extrusive processes are dominant during the postspreading volcanism in the SCS. The total magma volumes for building Zhenbei and Huangyan seamounts are estimated to be 4995 km3 and 3674 km3, respectively, with a related volume flux of ∼0.032 m3/s and ∼ 0.023 m3/s. These values are larger than those of Longnan seamount (2884 km3, 0.018 m3/s) but smaller than those of plume-derived seamounts. The relationship between average lower crustal velocity and crustal thickness also indicates that the postspreading magma may not be caused by the higher degree of mantle melting driven by elevated temperature, but by multiple lower degree of melting caused partly by the enriched mantle. • Seismic structures of the Zhenbei and Huangyan seamounts based on refraction data. • Original oceanic crust is thin with low velocity. • Extrusion to intrusion ratios of post-spreading seamounts are close to 3. • Magma flux of post-spreading seamounts is smaller than plume-derived seamounts. [ABSTRACT FROM AUTHOR]