STRUCTURE OF THE BEIBUWAN BASIN, CHINA: AN ANALOG TO CORDILLERAN EXTENSIONAL BASINS ON YOUNG CRUST

The Beibuwan Basin (BB) in south China and Vietnam is a component basin of the South China Sea (SCS) extensional system, of Paleogene age, which along with the Pearl River Mouth (PRMB) Basin form the extensional ramp from mainland China down into the South China Sea oceanic basin. It sits at a sloppy triple junction in the NW corner of the SCS behind an intact block, Hainan Island, within the extensional collage. Built on thermally young continental crust of Neoproterozoic age tectonically reworked during the Paleozoic and Mesozoic, it represents a good analog to Cordilleran extensional basins that is controlled by seismic imaging and drilling. The basin is characterized by a breakaway system that includes a conventional down-to-the basin normal fault, the Weixinan Fault on the west side of the BB that displays no footwall uplift, and cuts through pre-rift sediments and syn-rift (growth) section. Half way along the basin rim, the Weixian Fault transfers to the SE to the Luzhou Fault along a blind tipline of the basal detachment. The tipline stretches seaward under landward-facing half graben immediately in front of the edge of the BB. Beyond those, the basal detachment extends offshore under a half-dozen depocenters or ‘sags’ across the BB that are themselves underlain by shallower detachments. Hainan Island itself forms a relatively intact block displaced seaward from the Mainland, riding on a separate shallower detachment. The internal depocenters are bounded by growth faults, forming separate rift elements of the overall BB that collectively gather into multiple detachment surfaces. The summed extension related to separate beta factors of each BB 'sag' provides a much larger extension than any one depocenter displays. This synthesis derives from details from secondary sources as scant primary information resides in the public domain. Our synthesis, built on the SEATIGER non-exclusive study, is supported by reinterpretation from this literature using techniques of restoration and depth-to-detachment calculations to build a structural framework for the BB.