Depositional architecture and structural evolution of a region immediately inboard of the locus of continental breakup (Liwan Sub-basin, South China Sea)

Abstract

Abstract New 3-D seismic data and regional 2-D seismic profiles from the northern South China Sea, the most extensive data set imaging a distal rifted margin in the world, are used to characterize a region located immediately inboard of the locus of Cenozoic continental breakup. The interpreted data set images a ∼6-km-thick continental crust in which the Moho and the base of syn-rift sediment are observed as clear, well-resolved seismic reflections. This extremely thinned continental crust was offset at its base by a complex detachment fault system from which oceanward-dipping listric faults propagated vertically to bound six separate tilted blocks, in a style akin to tectonic rafts. The seismic reflection data allowed us to investigate the thickness of syn- and post-rift strata above tilt blocks, revealing that the early-middle Eocene syn-rift topography was gradually blanketed in the late Eocene (ca. 38 Ma). After 33 Ma (earliest Oligocene), the main depocenter on the margin migrated to the south of the Liwan Sub-basin, i.e., oceanwards, as recorded by the thickening of strata within a breakup sequence. This work is important as it demonstrates how closely structures and sedimentation within the Liwan Sub-basin were controlled by a basal, rift-related detachment system, which is imaged in detail by 3-D seismic data for the first time on a rifted continental margin. Continental breakup was marked by a shift in the locus of subsidence (and crustal stretching) toward ocean crust, within a time period spanning ∼16 m.y. We extrapolate our findings from the South China Sea to the development of asymmetric passive margins across the world.