Final breakup of continental block and opening of oceanic lithosphere: insights from deep crustal structure and tectonic evolution of the ocean–continent transition zone in the northern South China Sea

Abstract

As an important transition domain between the thinnest continental crust and the initial oceanic crust, the ocean–continent transition zone (OCT) preserves a wealth of information on the processes of lithospheric extension and breakup. The northern margin of the South China Sea (SCS) is a non‐typical, magma‐poor passive continental margin with a particular structural pattern and sedimentary formation. The OCT in the northern margin of the SCS was developed by a series of continental rifting and seafloor spreading. Based on several multi‐channel seismic reflection profiles, crustal‐scale structural profiles extracted from gravity data and ocean‐bottom seismometers (OBS) data, this paper studies the deep crustal structures, deformation and geodynamics of the OCT to further understand the structural patterns and tectonic process from the lithospheric extension to its breakup. The OCT of the northern margin of the SCS is characterized by a crustal thinning and high‐velocity layer (HVL) in the lower crust. The OCT has three structural units: rift depression, volcanic zone and tilted fault blocks. The HVL is mainly limited in the eastern portion of the northern SCS. The HVL is distributed essentially within the OCT but does not clearly develop everywhere in the OCT. We propose a hybrid model of rifting to fit characteristics of the northern margin of the SCS. A crustal‐scale largely symmetric process predominated at the initial rifting stage. The basin‐controlling faults subsequently penetrated the entire crust, resulting in asymmetric tectonic evolution. Then, the OCT occupied this asymmetric location. Copyright © 2016 John Wiley & Sons, Ltd.