3D seismic anatomy of a low angle normal fault in the southern South China Sea rifted margin

This study investigates the fault zones of a low-angle extensional structure along the southern South China Sea (SCS) rifted margin. The SCS in Southeast Asia is the best-known marginal basin. It is characterized by a V-shaped oceanic domain formed by seafloor spreading propagating from northwest to southeast between the earliest Oligocene and middle Miocene. Rifting produced broad margins (>600 km) with hyper-extended basins exhibiting varying degrees of crustal thinning. Our study focuses on the southern edge of Dangerous Grounds, in the proximal domain of the southern SCS margin, at its transition to the Sabah Trough.

High-resolution 3D seismic data reveal a well-defined top-basement low-angle normal fault zone. The continental basement preserves evidence of a former orogenic wedge characterized by succession of shallow dipping reflectors similar to thrust sheets. Although not drilled, this geometrical relationship is consistent with imbricated thrust sheets of metasediments likely associated with the Yanshan orogen.

The fault surface exhibits pronounced corrugations with wavelengths of 500m-1 km and a fault zone thickness of up to several 500-700m meters based on seismic resolution. Seismic reflections immediately beneath the fault surface show shear zones of variable thickness with phacoidal blocks, analogous to structures observed in oceanic core complexes.

Overlying the fault surface, we identify dismembered blocks ranging from tens of meters to kilometers in size. They are interpreted as basement material scrapped from the underlying basement made of imbricated thrust sheets. These “rider blocks” are associated with seismic facies consistent with breccias, forming a discontinuous cover over the fault plane. Breccias are interpreted in two categories: mechanical breccias dragged as a tail downdip of the allochthons and classical sedimentary breccias associated with the fault scarps.

These observations provide new insights into the geometry, kinematics, and lateral variability of low-angle normal fault systems, with implications for the evolution of hyper-extended rifted margins.