Continent-Ocean Transition at the Northern Margin of the South China Sea

Abstract

The northern margin of the South China Sea (SCS) has particular structural and stratigraphic characteristics that are somewhat different than those described in typical passive margin models. The differences are due to poly-phase tectonic movements and magmatic activity resulting from interaction of Eurasian plate, Philippine Sea plate and the Indian-Australia plate. Based on several crustal-scale multi-channel seismic reflection profiles and satellite gravity data across the northern SCS margin, this paper analyses the structures, volcanoes and deep crust of the continent-ocean transition zone (COT) at the northern margin of the SCS to study the patterns and model of extension there. Results show that the COT is limited landward by basin-bounding faults near shelf-slope break zone, and bounded by seaward-dipping normal faults near the oceanic basin in our seismic lines. The shallow anatomy of the COT is characterized by rift depression, structural highs with igneous rock, and may include a volcanic zone or a zone of tilted faults blocks at the distal edge. Gravity modeling revealed that a High Velocity Layer (HVL) with a thickness of 0.8–6 km is frequently present in the slope below the lower crust in the mid-northern margin of the SCS. Our study shows that the HVL is only located in the eastern portion of the northern SCS margin based on the available geophysical data. We infer from this that the presence of an HVL is not required in the COT at the northern SCS margin. The magmatic intrusions and HVL may be related to partial melting caused by decompression of passive, upwelling asthenosphere which resulted primarily in post-rifting underplating and magmatic emplacement or modification of the crust. Based on this study we proposed that an intermediate mode of rifting was active in the mid-northern margin of the SCS with characteristics that are closer to those of the magma-poor margins than those of volcanic margins.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015