3D structure of low-angle normal faulting and related tectono-sedimentary processes in the SE South China Sea

This contribution explores the formation and evolution of hyper-extended basins controlled by low-angle normal faults active at <30°. Such extensional structures are documented worldwide in different geodynamic settings (e.g., continental passive margins, collapsing orogens) but contradict classical fault mechanic models questioning how such extensional structures can form. Based on a recent industrial 3D seismic reflection survey along Sabah (southern margin of the SCS, Dangerous Ground), here we investigate the 3D structure of low angle normal faults and the related pre-, syn- and post-tectonic stratigraphic architecture of hyper-extended rift basins. We mapped and analyzed in 3D the surface of several normal fault systems active at low-angle associated with the interpretation of an array of seismic profiles across the basins.

The mapped faults show an average dip angle of 30° and appear planar, characterized by continuous reflections with no clear steepening at depth and sole-out at variable depths. They controlled the formation of two main depocenters (southern and northern basins) filled by up to 6 km of sediments including pre- to post-rift sequences. Intra-basement seismic reflectors dipping towards the north-west are observed, onto which extensional structures often seem to sole out. These reflectors are interpreted as interleaved thrust sheets from a dismantled accretionary wedge of the former Mesozoic active margin (Yanshan Arc).

Results show polyphased syn-rift infill during the development of the low-angle normal faults. The first syn-tectonic sequence appears as chaotic and discontinuous packages that has been dismembered during the activity of extensional structures. The second syn- tectonic sequence represent the main filling succession associated with numerous second order normal faults that become gradually younger towards the central depocenter. Antithetic to the main extensional structure, secondary normal fault soling out at the top of the pre-rift succession is observed. It controls the formation of growth strata showing a thickening opposite to the low-angle normal faults. The overall structure describes the geometry of an extensional fishtail.

Our results provide some key new elements on the 3D mechanisms of low-angle normal faulting and its control on sedimentary evolution as well as coeval crustal deformation.