Evolution of Low-Angle Normal Faults in the Enping Sag, the Northern South China Sea: Lateral Growth and Vertical Rotation

Abstract

Low-angle normal faults (dip <30°, LANFs) are widespread in the northern margin of the South China Sea where the maximum crust thickness is approximately 30.0 km. Based on 3D seismic survey data and drilling wells in the Enping sag, evidences for LANFs that initially formed at high-angles are discussed. After a detailed investigation of extensional fault system and description of 3D fault geometry, the initial fault dips under the model of distributed vertical simple shear are also calculated. The results indicate that the present-day dip angles of the LANFs are in the range of 12° to 29°, and the initial fault dip angles are in the range of 39° to 49°. Deep seismic imaging suggests that the upper crust in the footwall block of the LANFs was tilted at an angle of ~14° to 22° due to the isostatic rebound during rifting. Moreover, the temporal and spatial sequences of the lateral growth of the LANFs have been investigated by the seismic interpretation of four isochronous stratigraphic interfaces, which demonstrates that two individual fault segments propagated towards each other and subsequently, were hard-linked during the Early Eocene.