Post-rift Magmatism And Submarine Landslides On The Northern South China Sea Continental Margin

Three-dimensional(3D) seismic, borehole and geochemical data reveal a prolonged phase of post-rift magmatism on highly extended crust of the Baiyun Sag, South China Sea. Two volcanic complexes are identified and described in the context of continental rifting and diachronous continental breakup of the South China Sea. The volcanic complexes include volcanic mounds, numerous igneous sills and lava flows. Petrological analyses from borehole sidewall cores suggest these volcanic mounds, or edifices, are composed of basalt lavas and tuffs intercalated with thin limestone and clastics beds. Biostratigraphic data from exploration wells BY7-1 and BY2, complemented by K-Ar datings from core samples, confirm that magmatic activity in the Baiyun Sag occurred in two main stages:(1) a first episode at the base of the Miocene(23.8 Ma); and(2) a second episode occurring at the end of the Early Miocene(17.6 Ma). The observation that some volcanic mounds overlie and are aligned along fault traces suggest that pre-existing normal faults also influenced the localisation of volcanic activity, likely by providing preferential, high-permeability, magma flow pathways. Importantly, the identification of interbedded extrusive and sedimentary rocks implies that eruptive episodes occurred intermittently and were separated by volcanically-quiescent periods when sediment deposition occurred. The relative location of volcanic complexes in the Baiyun Sag, and their stratigraphic position, reveal prolonged magmatism inboard of the ocean-continent transition zone during continental breakup. We suggest that magmatism in the Baiyun Sag reflects progressive continental breakup in the South China Sea, with the last volcanic episode marking the end of a breakup sequence representing the early post-rift tectonic events associated with the continental breakup process. Seismic and borehole data from this breakup sequence records diachronous magma emplacement and complex changes in depositional environments during continental breakup. The onset of deepwater circulation recorded after the breakup sequence marks the establishment of oceanic circulation in the South China Sea. The discovery of the Early Miocene volcanic complexes has important implications for the understanding of the evolution of the Pearl River Mouth Basin and for regional hydrocarbon prospectivity.High-quality 3-D seismic data are used to assess the significance of mass-transport deposits(MTDs) to the evolution of the Pearl River Mouth Basin(South China Sea). Basal shear surfaces and lateral margins of seven recurrent MTDs are mapped to reveal a general NE-SW transport direction throughout the Late Miocene Quaternary. A key result of our analysis is the perceived relationship between the recurrence of slope instability in the study area and the Dongsha Tectonic Event. Using borehole data to constrain the ages of interpreted MTDs, we show that tectonic uplift in the northern South China Sea led to slope oversteepening in the Late Miocene(between 10.5 Ma and 5.5 Ma), preconditioning it to fail recurrently for more than10 Ma. Interpreted MTDs are shown to enhance burial depths of source and reservoir units, and improve seal competence in lower-slope areas. Conversely, upper slope regions record important erosion and reduced sealing capacity in Late Cenozoic strata. As a result, we postulate that the thickness variations imposed by MTDs on Late Miocene-Quaternary strata have important implications to petroleum plays in the South China Sea.