Penecontemporaneous polygonal faulting triggered by sand overloading onto unconsolidated clays in the northern South China Sea

Abstract

Layer-bound polygonal fault systems are commonly found in fine-grained sediments in continental margin basins worldwide, but their origin remains a dispute. This study focuses on the structural characteristics of polygonal fault systems newly discovered in the Middle Miocene mudrocks of the Yinggehai Basin in the northern South China Sea. Our analysis reveals that the layer-bound normal faults constitute a complex network with straight or curvilinear fault traces and frequent orthogonal intersections in the Meishan Formation. We also observe structural depressions filled with syn-deformation deposits, with their long axis aligned parallel to the structural contours. Our findings suggest that the polygonal faults formed during rapid loading of fine-grained sands as gravity-flow deposits onto unconsolidated clays, resulting in density inversion and fault nucleation. Fault propagation was facilitated by continuous sediment deposition and ceased when sedimentation ended in the faulted interval. This study offers new insights into early sediment deformations in the northern South China Sea region and contributes to our understanding of the genesis of polygonal fault systems.