Seismic stratigraphy and tectonic structure from a composite multi-channel seismic profile across the entire Dangerous Grounds, South China Sea

摘要

We interpret a more than 500km long composite multi-channel seismic reflection profile across the entire Dangerous Grounds, the South China Sea. Five tectonostratigraphic units are determined, together with seven sequence boundaries. Detailed analysis of extensional features, based on measurements of fault heaves, revealed two major phases of extension, separated by a distinct unconformity, which likely correspond with the beginning of sea-floor spreading in the South China Sea. Early extension occurred during continental rifting (Late Cretaceous-Early Oligocene), and resulted in formation of half-grabens and rotated blocks, controlled by a deeply rooted detachment system. Extension continued in our study area during the drifting phase of the East Subbasin of the South China Sea (Late Oligocene-Early Miocene) until the subsequent opening of the Southwest Subbasin at about 25Ma, but its intensity decreased markedly thereafter. Deeply rooted detachment systems evolved possibly during this second phase of extension at the continent-ocean transition area. From a coincidence of the lowermost reflections and Moho depth as derived from gravity inversion we tentatively interpret the flattened part of this detachment system as the crust-mantle boundary. It is suggested that the Dangerous Grounds reflect a magma-poor rift system at the initial stage of mantle unroofing. We suggest that a widespread carbonate platform developed across the Dangerous Grounds, concurrent with the period of seafloor spreading in the Southwest Subbasin of the South China Sea. Fault-related stretching factors (βf) along the line show discrepancies with the whole crustal stretching factors (βc). The continental crust of the Dangerous Grounds may have experienced nonuniform thinning.