Revisiting South China Sea Oceanic Spreading in Light of IODP Expedition 349 Results

Abstract:

One of the main objectives of International Ocean Discovery Program (IODP) Expedition 349 was to constrain the age of the initiation and termination of seafloor spreading in the South China Sea (SCS) basin. Expedition 349 drilled three sites (U1431, U1433, and U1434) into the basaltic crustal basement near the fossil spreading center in the East and Southwest Subbasins. Shipboard biostratigraphic analysis of microfossils from the sediment immediately above or between flows in the basaltic basement indicates early Miocene ages: 16.7–17.6 Ma for Site U1431 in the East Subbasin and ~18–21 Ma for Site U1433 in the Southwest Subbasin. The similarity in preliminary crustal age between sites suggests a similar age for the cessation of spreading in both the East and Southwest Subbasins. This observation rules out some of the tectonic models for the opening of the SCS in which the Southwest Subbasin was older than the East Subbasin. Expedition 349 also drilled Site U1435 on a bathymetric high along the northwestern continent-ocean boundary. Onboard core description, biostratigraphy and magnetostratigraphy revealed that sediment at this site shows a sharp discontinuity at about 33 Ma, with shallow-water sandstones and mudstones of unknown age below the disconformity and deep-water marine deposits above. We infer that this represents the transition from breakup to the beginning of oceanic spreading. The oceanic seafloor spreading in the SCS, from 33 to ~16-18 Ma, is thus coeval with a large part of the left-lateral motion along the Ailao Shan-Red River Fault Zone (dated 34 to 17 Ma). Through the calibration of magnetic anomaly models, the expedition results will therefore permit a better reconstruction of the spreading history of the basin. We use the age constraints provided by the biostratigraphic, magnetostratigraphic and radiometric dating analyses to build new synthetic profiles and identify magnetic anomalies in the SCS. A compilation of this new data with existing magnetic anomaly data allows us to trace isochrons through most of the East and the Southwest Subbasins.