Transition from subduction to hyper-extension: the example of the Liwan sag (North South China Sea)

Regional tectonic transition from subduction to rifting in the late Mesozoic has occurred along the northern continental margin of the South China Sea. However, the processes of this transition and the role of inheritance in forming hyper-extended rift systems still remain poorly investigated. In this study, we present new constraints about this transition by studying the hyper-extended Liwan sag. We establish a new tectono-stratigraphic framework by defining tectono-stratigraphic sequences based on the interpretation of new long-cable seismic profiles, high-resolution 3-D seismic data together with deepwater drill hole data.

In terms of geometry of boundaries, structures and seismic architecture, we define three seismic-sedimentary units below HARs (a décollement layer made of shales), which include from top to bottom M0, M1 and M2. By comparing the Mesozoic sedimentary reflectors calibrated by drill wells in the northern continental margin, we propose that the sediments in the sub-HARs are Mesozoic (pre-hyperextension), the sediments of M0 are deposited in a Cretaceous terrestrial or littoral environment. Stratigraphic correlations demonstrate at least three major tectonic events in the area, including two events related to Mesozoic subduction and a subsequent rifting related to rollback of the paleo-pacific plate. Based on the new data, we propose a new model to explain the evolution of the hyper-extended Liwan sag involving complex transition from subduction to episodic rifting. The major implication of our study is that: 1) the establishment of a tectono-stratigraphic framework helps to constrain the evolution and present deep structure of the hyper-exteded basin and 2) deciphering the pre-rift inheritance is fundamental to explain the rift process.