Preferential mantle lithospheric extension under the South China margin

Abstract

Continental rifting in the South China Sea culminated in seafloor spreading at ∼30 Ma (Late Oligocene). The basin and associated margins form a classic example of break-up in a relatively juvenile arc crust environment. In this study, we documented the timing, distribution and amount of extension in the crust and mantle lithosphere on the South China Margin during this process. Applying a one-dimensional backstripping modeling technique to drilling data from the Pearl River Mouth Basin (PRMB) and Beibu Gulf Basin, we calculated subsidence rates of the wells and examined the timing and amount of extension. Our results show that extension of the crust exceeded that in the mantle lithosphere under the South China Shelf, but that the two varied in phase, suggesting depth-dependent extension rather than a lithospheric-scale detachment. Estimates of total crustal extension derived in this way are similar to those measured by seismic refraction, indicating that isostatic compensation is close to being local. Extension in the Beibu Gulf appears to be more uniform with depth, a difference that we attribute to the different style of strain accommodation during continental break-up compared to intra-continental rifting. Extension in PRMB and South China slope continues for ∼5 m. y. after the onset of seafloor spreading due to the weakness of the continental lithosphere. The timing of major extension is broadly mid-late Eocene to late Oligocene (∼45–25 Ma), but is impossible to correlate in detail with poorly dated strike–slip deformation in the Red River Fault Zone.