CHARACTERISTICS OF DETACHMENT FAULT SYSTEM WITHIN HESHAN SAG IN SOUTH CHINA SEA AND IMPLICATIONS FOR MECHANISMS OF CRUSTAL THINNING

Heshan Sag is located in the ultra-deepwater area in the north continental passive rifted margin of the South China Sea, where developed strong thinning crust of the ocean-continental transition zone. Based on theinterpretation of 2D seismic profiles covering the study area and geophysical data, guided by the basin dynamic and tectonic-strata analysis theory,we tried to define the deeper basin structure style of Heshan sag and build its tectonic-stratigraphic framework as well. During the research, we described the geometric features of detachment faults and quantitatively analyzed their activities. In addition by using subsidence analysis technique, basin subsidence and filling evolution processes can be simulated. Combined with previous research on deep basin and the latest information of adjacent areas, we try to discuss the detachment fault system and its tectonicdevelopment mechanism. The research achievements are as followed: (1) a set of largeseaward detachment faultswith a very low angle of 10 degrees to 20 degrees has been found inHeshan Sag. The hangingwall block is tilted and hyper extended. Comparing with the northern shelf basins of South China Sea and Heshan sag, we discussthe relationship between detachment faulting and lithosphere stretching and thinning, and presume the distal margin is exposed in Heshan Sag. (2) Based on the seismic data, two important tectonichorizons was brought out, i.e. SD (Syn-detachment) and PD (Post-detachment), representing the beginning and the end of detachment faulting respectively, which is closely connected with stages of tectonic evolution (fault-depression stage and sag stage).Fault-depression stage can be subdivided into 2 periods. First period is Wenchang Formation sedimentary period controlled by high-angle faults and developed many half-grabens and wedges during faulting in the early time. The early Enping Formation sedimentary period is the beginning of detachment faulting in Heshan Sag, controlling the formation of large half-grabensand the rotating of hanging wall.PD represents detachment fault activity, also the transferring surface from fault-depression period to the sag period. (3)Tectonicdevelopment mechanism is related to thebreakup of lithosphere and the seafloor spreading of South China Sea.