Margins of the Southwest sub-basin of the South China Sea—A frontier exploration target?

The deep South China Sea Basin is about 500 km narrower (when measured along fracture zone directions) in the southwest sub-basin than in the eastern sub-basin. Reconstruction of the South China Sea Basin to predrift configurations thus creates an unacceptable 500-km overlap of continental or transitional crust in the western half of the proto-South China Basin. In order to maintain continuity through the plate-tectonic evolution of this basin, about 500 km of differential crustal extension must have occurred in the west in relation to that in the east. Virtually no deep-penetration seismic data exist across the northwest and southwest foundered and extended continent-transition crust. However, plate tectonics allows us to place constraints on the effective β even in the absence of direct information on crustal thickness, heat flow, or subsidence history. The problem here is essentially the inverse of that normally presented. Assuming various endmember models and knowing the effective β (i.e., extension), we can predict the cumulative subsidence (and hence maximum sediment thicknesses), the regional thermal history, and crustal thicknesses. The total continental plus transitional margin crust in the western sub-basin represents a differential stretching factor about twice that inferred for rifted margins of the eastern sub-basin. The actual mean stretching factor for the western sub-basin margins may be as high as β = 5 or 6. The anomalous crustal extension inferred for the western half of the basin leads us to speculate on the likelihood that numerous rift basins and their associated sediments may have been subjected to unusual thermal conditions that would be favorable for the early maturation of hydrocarbons. Although this region remains virtually unexplored, the foregoing analysis suggests it should perhaps be one focus for additional frontier exploration. This portion of the South China Sea margin (young, thin sediments) offers the ideal natural laboratory for the further testing of continental margin thermal subsidence models.