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Sediment Loading and Lower Crustal Flow on the Rifted Margins of the South China Sea


Rifting of the South China Sea involved extension of flexurally weak, arc-type continental lithosphere. Subsidence on this rifted margin is generally greater than might be predicted given the degree of brittle upper crustal extension that can be observed in seismic profiles. This anomalous additional subsidence cannot only be explained by poor resolution in the seismic data. I interpret this to reflect preferential extension in the ductile lower crust, much of which occurs during the separation of the Dangerous Grounds from southern China around 30 Ma. Evidence from the Nam Con Son basin and adjoining deep-water areas show that this flow occurs prior to the breakup. However, subsidence anomalies continue to be observed starting around 20 Ma in the Baiyun Sag and 5 Ma in the Song Hong-Yinggehai Basin, at times when there is no evidence for upper crustal extension in the form of normal faulting, within the continental margin and also after the end of seafloor spreading. I propose that lower crustal flow in the post-rift period may be driven by a combination of loading in the offshore by sedimentation, coupled by uplift onshore driven by intensified erosion triggered by a strengthening of the South Asian monsoon at this time. Low viscosities in the lower crust, even lower than that seen in the mantle asthenosphere, mean that much of the isostatic compensation to changing loads is accomplished within the lower crust by flow of material back under the rifted edge of the continent. New numerical models now show that post-rift flow of lower crust is possible thanks to the thermal blanketing effects of the thick sedimentary deposits seen in both Baiyun Sag and the Song Hong-Yinggehai Basin. The process can be envisaged as a re-adjustment of the isostatic equilibrium following continental breakup. Surface processes onshore, driven by climate change, therefore influence the structure of the continental crust in the offshore region, at least in areas where crustal strength is low, such as in active margin settings like the South China Sea, but should be applicable to other regions, such as in the Southwest Pacific and the Gulf of California.