Crustal structure across the xisha trough, northwestern South China Sea

Located at the northwestern part of the South China Sea (SCS) between the H ainan and Xisha (Paracel) Islands, the Xisha Trough represents a failed rif t in conjunction with the opening of the SCS between 32 and 17 Ma. From wes t towards east within a scale of several hundred kilometers, it presents al l major stages of the rifting process, and thus, provides an ideal place to study the rifting process in great details. In the autumn of 1996, a joint team of Sino-German scientists carried out a wide-angle seismic experiment across the Xisha Trough with 10 ocean bottom hydrophones (OBH) along a 237 -km NNW-SSE-oriented profile, which was surveyed in 1987 with multi-channel seismic (MCS) method by BGR of Germany and SOA of China. Favorable weather conditions and the powerful 4 x 12-1 air gun array rendered very good qual ity data with seismic signals observed at the offset of up to 110 km, A det ailed velocity-depth model was obtained by using an interactive trial-and-e rror 2D ray-tracing method. Interpretation of the NICS data published by BG R provides very good geometrical constraints of the complex upper crustal s tructure, which is characterized by fault blocks, half-horsts and half-grab ens filled with syn- and post-rift Cenozoic sediments. The velocity model i n turn confirms the major structure outlined by the interpretation of the M CS data, showing a varying sedimentary layer between I and 4 km of thicknes s and velocities between 1.7 and 4.5 km/s. The P-wave velocity of 5.5 km/s on the top of the crystalline basement is relatively low, suggesting strong weathering. Within the crystalline crust, the velocity increases downward continuously to 6.8 km/s at the bottom of the crust without a clear differe ntiation in the middle crust, showing clearly its continental nature even b eneath the Xisha Trough. The Moho is marked by a sharp first-order interfac e with a velocity of 8.0-8.1 km/s at the uppermost mantle. The Moho depth i s 15 km beneath the center of the trough and increases gradually to more th an 25 kin towards north and south, corresponding to an extreme thinning of the pre-rift continental crust from more than 25 kin under the coast line t o only 8 kin beneath the Xisha Trough. The similar velocity structure of th e continental nature on both sides of the Xisha Trough suggests a homogeneo us pre-rift continental setting. The sharp Moho and the lack of high veloci ty body (HVB) in the lower crust imply no magmatic underplating, which is v ery different to interpretations across the eastern part of the continental margin in northern South China Sea. The intense faulting in the upper crus t, the strong but rather symmetrical crustal thinning centered at the Xisha Trough and the close neighborhood to the open SCS NW subbasin suggest a pu re shear rifting, which failed most likely at the phase shortly before the continental breakup. (C) 2001 Elsevier Science B.V. All rights reserved.