Abstract
The South China Sea (SCS) is a marginal basin characterized by propagating breakup and subsequent seafloor spreading during the Cenozoic. Our study investigates the crustal architecture and tectonic evolution of the Xisha Islands - Northwest Borneo conjugate margins using a 1000 km long seismic profile and two shorter seismic lines. This system is characterized by wide margins showing a succession of rift basins with thinned continental crust, associated with large extensional detachment faults soling on top of the lower crust. In relation with crustal extension, several syn-rift sequences have been identified. The lower syn-rift unit (syn-rift I) shows well expressed wedge-shape growth strata. The upper one (syn-rift II) presents a more uniform thickness, and is slightly offset by normal faults dipping toward the depocenters. To illustrate the evolution of the southwestern SCS margins during continental breakup, we restore two conjugate sections near the oceanic magnetic anomaly C5En (18.8 Ma). The conjugate COT presents an asymmetrical hyper-extension architecture in cross sections, with the southern margin being the lower plate margin. In map view, the spatial variation of crustal structures illustrates a bayonet shape characterized by an initial N–S extension coeval with the first phase of seafloor spreading in the east sub-basin in the SCS. The extension direction changed shortly later (circa 23Ma) to NW-SE opening in the southwest sub-basin in relation to a ridge jump. We suggest that variable extension direction is linked to changes in the compressional tectonic regime along Borneo and Palawan.
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