Interaction between magmatism and polygonal faults revealed by three-dimensional seismic data in the Zhongjiannan Basin, South China Sea

Abstract

The features of magmatism and polygonal faults are unveiled through a comprehensive analysis of high-resolution and reprocessed three-dimensional (3D) seismic data covering 1200 km2 in the Zhongjiannan Basin, South China Sea. These structures influence the hydrocarbons formation and migration, and the interaction between them is significant for hydrocarbon exploration. We identified magmatic bodies, including eight volcanoes, fifty sills, and a single igneous diapir, are mainly developed in three stages: 27.9–23 Ma (between T70 and T60), around 11.6 Ma (T40), and after 5.3 Ma (T30). The polygonal faults, characterized by linear, polygonal, and radial configurations, are extensively distributed within Miocene and Quaternary sediments. In the platform, polygonal faults manifest linear or polygonal attributes, while in the basin floor, they show a polygonal shape. Particularly, above volcanoes and the igneous diapir, the polygonal faults exhibit radial configurations. However, among volcanoes, few polygonal faults are discerned based on the coherence attribute slice. The controlling factors influencing the distribution of polygonal faults are the lithological disparity, temperature, and pressure. Radial fault formation is promoted by volcanoes and igneous diapir through differential compaction. The resultant vertical pathways formed by volcanoes and overlying radial faults play a crucial role in adjusting pressure differentials or water discharges. Consequently, these pathways disrupt the requisite conditions for the formation of polygonal faults among volcanoes. In the basin floor, magma intrusion occurs along the planes of polygonal faults, giving rise to inclined, spoon-shaped, and bowl-shaped sills. As weak zones, polygonal faults worked as connectors for sills at different depths. Hence, pre-existing polygonal faults are considered pathways facilitating magma migration from the lower Miocene to Pliocene strata, ultimately contributing to sill formation in shallow sediments. This study advances our understanding of the intricate interaction between polygonal faults and magmatism within sedimentary basins.