Evidence of low flexural rigidity and low viscosity lower continental crust during continental break-up in the South China Sea

Abstract

The South China Sea was formed by seafloor spreading in the Late Oligocene at ∼30 Ma following a series of extensional events within crust formed by Mesozoic continental arc. In this study, we interpreted faults along seismic reflection profiles from both the northern and southern conjugate margins of the South China Sea, and forward modeled these using a flexural cantilever model to predict modern basin geometries. When compared with the observed structure, the models based on upper crustal faulting consistently underpredicted the amount of subsidence, especially towards the continent–ocean transition (COT). We interpret this to indicate preferential extension of the continental lower crust along the COT on both margins, extending up to ∼80 km landward from COT. The regional slope of the South China continental shelf indicates lower crustal viscosities of 10 19–10 18 Pa s, representing an offshore continuation of the weak crust documented onshore on the eastern flanks of the Tibetan Plateau. Only in the region of Hainan Island in the western South China Shelf does lower crustal viscosity increase (10 21–10 22 Pa s) and the preferential loss of lower crust become less pronounced and limited to <40 km from COT. This western area represents a rigid block analogous to the Sichuan Basin onshore. Forward models based on upper crustal faulting support the idea of a very weak continental crust because models where the effective elastic thickness of the plate ( T e) exceeds 5 km fail to reproduce the geometry of the sub-basins within the Pearl River Mouth Basin (PRMB) of the South China Margin. The observed basins are too deep and narrow to be consistent with models invoking high flexural rigidity in the upper crust or mantle lithosphere. The fact that rifting and seafloor spreading seem to co-exist for ∼5 my. adjacent to the PRMB is consistent with very weak continental crust during break-up.