Geometric and kinematic analysis of normal faults bordering continental shelves: A 3D seismic case study from the northwest South China Sea

Abstract

Normal faults border many a continental shelf, but have only been considered to play a subordinate role to basin-controlling faults in previous studies. Their detailed geometries and kinematic histories are still poorly known. In this study, high-quality three dimensional (3D) seismic data are used to investigate distinct families of normal faults bordering the continental shelf of the Qiongdongnan Basin, northwest South China Sea. Sixty-six (66) normal faults are interpreted and found to mostly tip out in the Upper Miocene. Three large-scale faults offsetting Horizon T30 (the base of Pliocene strata), and three other faults terminating beneath this same horizon, were selected and studied in detail. We discriminate between growth and blind faults by analysing upper tip folding geometry and fault displacement distribution using throw-depth (T-z) plots. The dips of the upper parts of faults are almost twice that of their lower parts. A new three-stage model for fault reactivation in Qiongdongnan Basin is therefore proposed based on our results. In the northwest South China Sea, the propagation of normal faults on the continental shelf can be attributed to: (1) the rotation of the South China block and a reversal in the movement of the Ailao Shan-Red River Fault Zone from left-to right-lateral slip, and (2) large-scale slope instability in the Qiongdongnan Basin at ~5.5 Ma. The results presented here outline the structural evolution of the continental shelf of the northwest South China Sea after 5.5 Ma, and how regional-scale structures such as the Ailao Shan-Red River Fault Zone controlled the evolution of normal faults on continental shelves.