Seismic characteristics and evolution of post-rift igneous complexes and hydrothermal vents in the Lingshui sag (Qiongdongnan basin), northwestern South China Sea

The study of morphology, distribution, and characteristics of igneous complexes has great significance to the understanding of magma plumbing processes, geodynamics, and tectonic evolution of continental margins. Previous studies concentrated partly on the magma-rich rifted basins, where the lateral magma transport mainly affects the igneous complexes' connection and distribution. However, due to seismic wave shielding effects of the large shallow magmatic bodies, the underlying igneous complexes and their corresponding magma plumbing systems in the magma-poor rifted margins are still in debate. In this study, 2D/3D seismic data and well data are utilized to describe the morphology, architecture, and spatial-temporal distribution of igneous complexes in the Lingshui sag of the Qiongdongnan basin, northwestern South China Sea margin. The identified igneous complexes include 98 intrusive sills and feeder dykes beneath some of the isolated sills. Twenty-six cone-shaped mounds that overlie intruded sills through internal disturbed conduits were also described. Drilled well samples and seismic expressions suggest that these mounds are hydrothermal vents. A uniform Bottom Mounds Horizon of these vents suggests that they probably formed at the same time. Constrained by biostratigraphic data and sedimentation rate of underlying and overlying sedimentary layers, the magma emplacement was dated to the middle Miocene (ca. 14.6 Ma). Most of the hydrothermal vents are distributed along the F2 fault zone and have direct linkage with the underlying sills, while the large sill complexes that are connected with limited vents are mainly present above the hyperextended continental crust, where the crust thins to 6-10 km. The sills intruded into different layers, from the lower Oligocene to the lower Miocene and the emplaced depth of sills is 1.2-6.3 km, whether or not they feed any vents above. Unlike most of the large volume and laterally linked sills found in the magma-rich rifted margins, the scattered distribution of sills at different levels indicates that dykes probably play an important role in magma transport, which might coexist with numerous polygonal or small faults and interference reflections. This work highlights the critical role of basin structures in controlling the distribution of post-rift igneous complexes in magma-poor margins, including thinned continental crust, sedimentary thickness, and faults.