Crustal Structure and Lithospheric Rupture Process of the Continent-Ocean Boundary of the South China Sea

Abstract:

Seismic reflection profiles acquired in the continent-ocean transition zone (COT) of South China Sea provide a detailed view of Moho and deep crustal reflectors and continental lithosphere extension and breakup styles. At the north margin, rift basins are often bounded by listric normal faults, most of which are terminated at the base of the upper crust. The upper-lower crust interface corresponds to the brittle-ductile transition zone, where listric faults tend to converge into a low angle detachment fault. According to calculated stretch factors in different depth scales (upper crust, lower crust) along several profiles, hyper-extended continental crust is widespread, with the upper crust being often preferentially more thinned than the lower crust. The ductile lower crust is more resistant to faulting and therefore stretching. A high-velocity lower crustal layer represents either magmatic underplating or pervasive lower crustal intrusions in the northern margin. The possible lower crust ductile flow and the high-velocity lower crustal layer may have contributed to extension discrepancy, leading to direct exposure of lower crust material landward of the continent-ocean boundary. Hyper-extended continental crust and thick syn-rift sequence developed during a long period of rifting prior to the inception of seafloor spreading are also observed in the southern continental margin, further suggesting depth-dependent continental extension in the South China Sea. Basement highs and discontinuity in Moho reflector are common features around the continent-ocean boundary of South China Sea. The basement ridges are located at the landward edge of the continent-ocean boundary and possibly composed by lower crust material. The COT is ~50 km wide, where the gravity anomaly is approximately zero and the Moho reflector is discontinuous. The COT here is narrower than those found in other magma-poor margins (e.g., Iberia-Newfoundland type), indicating that normal oceanic crust started to establish soon after crustal breakup.