Tectonic Subsidence of the East Subbasin of the South China Sea

Abstract:

International Ocean Discovery Program (IODP) Expedition 349 provides downhole logging data for studying the subsidence history of the South China Sea. Time-depth relationships are estimated from P-wave velocity data at Sites U1431, U1432, U1433 and U1435. We find that this relationship in the East Sub-basin is similar to that in the Southwest Sub-basin, but quite different from that obtained at the northern continent-ocean boundary (COB). Regional and local tectonic subsidence is analyzed based on selected seismic profiles and coring/logging data from Site U1431.We first establish a function of thermal subsidence caused by lithosphere cooling by backstripping along a regional seismic profile extending from the COB to the relict spreading ridge in the East Sub-basin. By comparing the observed with theoretical thermal subsidence, we find an anomalous upwelling near the relict ridge but anomalous subsidence near the northern COB. Tectonic subsidence rate at Site U1431 peaked from 17 to 13 Ma, and then slowed down with time. By subtracting the thermal subsidence estimated from the regional seismic profile out of the tectonic subsidence at Site U1431, we estimate the local subsidence caused by local tectonic events. The largest rate of local subsidence occurred from 9 to 12.5 Ma, coeval with the seamount volcanism nearby that was active from 8 to 13 Ma at the relict spreading center and which may have triggered local tectonic subsidence.