Seismically Imaged Crustal Breakup in the Southwest Taiwan Basin of the Northeastern South China Sea Margin

The Southern Depression of the Southwest Taiwan Basin is an ENE-trending wide rift in the northeastern South China Sea margin. Though known as highly thinned, the maximum degree of crustal extension remains unclear. Here, we present three NW-trending multi-channel seismic profiles crossing the depression askew, and another profile traversing it, all of which image the basement and Moho clearly. The basement deepens toward the rift axis while the Moho shoals toward the axis, indicating significant crustal thinning. Faults appear with only small throws and are usually untraceable to depth. As the integral fault offset seems insufficient to match the horizontal crustal extension, the high crustal thinning is mainly accommodated by detachment faults. Only small-scale post-rift magmatic bodies and sporadic sills are observed, and syn-rift magmatism is poor. The depression shows a recumbent V-shaped valley opening to the northeast with the width increasing from 50 km on the southwestern profile to 96 km on the northeastern profile. Along the valley axis, the crust thins from 8.1 to 3.6 km and more thinner to the northeast, demonstrating enhanced extensional degree toward the northeast. A unique conch-shaped package (up to & SIM;2 s two-way travel time thick) with moderately coherent reflections is imaged atop the axial mantle on the northeastern profile. It was possibly formed by mantle exhumation, syn-rift sedimentary uplift, or volcanic eruption. All these possibilities indicate that the crust probably broke up there, which represents a special style of crustal thinning different from other magma-poor margins.