Morphotectonics and sedimentation in convergent margin basins: An example from juxtaposed marginal sea basin and foreland basin, Northern South China Sea

Abstract

Using reflection seismic profiles and bathymetric mapping this paper reveals the tectonic–sedimentary characteristics of the convergent margins in the northern South China Sea, where it is strongly related to flexure of Chinese rifted margin and overthrust of Taiwan orogen. Convergent margin tectonics of the South China Sea near southern Taiwan is characterized by a progressively northward transition from oceanic subduction along the Manila Trench to the incipient collision zone offshore southern Taiwan where the continental crust of the Eurasian plate subducts beneath the Philippine Sea plate. North of 21°N, dip angles of the Benioff zone increase up to 80° in the incipient collision zone where the Manila Trench becomes shallower, gradually loses its morphological identity and finally merges into the nearly N–S trending Penghu Canyon. Convergent margin tectonics in the initial collision zone in SW Taiwan is manifested by the beginning of flexure of the Chinese margin under the westward migrating overthrust belt of Taiwan, forming two distinct basins. On the passive Chinese margin, the marginal sea basin becomes smaller and is underlain by the South China Sea Slope, while on the active Taiwan margin, a wedge-top basin has formed above the frontal thrust sheets of the Taiwan orogenic wedge. Sediments derived from the Taiwan orogen progressively overlie the strata of the passive Chinese margin, resulting in sediment thickening and basin shallowing from south to north. Sedimentary facies shows that offshore deep-water mud is gradationally overlain by shallow marine sediments. Sediments of the wedge-top basin are being actively deformed into mud diapiric intrusions and a series of west-vergent thrusts and folds with their associated piggy-back basins, resulting in irregular topography of the sea floor with alternating sea ridges and troughs. Pliocene–Quaternary strata of the passive Chinese margin are a little deformed under the westward compression induced by the arc–continent collision in the southern Taiwan. The slope profile of the passive Chinese margin is characterized by a sigmoidal curvature, representing a typical primary depositional setting of a passive margin. Slope canyons occur mainly on the upper slope and cut the sea floor, resulting in irregular topography and representing effects of erosion. Being under the initial arc–continent condition, the offshore SW Taiwan has experienced the transition from a rifted margin to an overthrust belt and becomes a foreland basin, which is acting as a catch basin for orogenic debris derived from SW Taiwan. Arguments from tectonics and sedimentation suggest that the Koping shelf-slope region is considered to be an immature foreland basin rather than an accretionary wedge. More specifically, the Kaoping shelf-slope can be regarded as a depositional wedge top where exists an overlap area between rapid sedimentation and active deformation of the Kaoping shelf-slope sediments in the external orogenic wedge of southern Taiwan.