Intense, Linear, Neogene Magmatism Related to Inherited Block Rotation Structures in the Western South China Sea

Abstract Most volcanoes exhibit circular or elliptical shapes, and volcanic cones in sedimentary basins along continental margins typically have irregular distributions. However, the continental margin of the South China Sea (SCS) hosts a unique feature: linear volcanic constructs surrounding the rigid Xisha Massif. These constructs are interpreted as the products of fissure eruptions, focused along underlying fault systems. We studied their distribution and characteristics using multibeam bathymetric data, 2D seismic profiles, and gravity/magnetic data sets. Our results show that these faults are formed in response to the incipient counter‐clockwise rotation of the rigid Xisha Massif during the Late Oligocene to Early Miocene—a rotation triggered by interaction with the southeastward‐escaping Indochina Block. This rotation generated faults with unusual orientations in the weak zones surrounding the massif, which were later exploited by magma to form the observed linear volcanic constructs. This study represents the first inference of incipient counter‐clockwise rotation of the Xisha Massif. It further reveals that the escape of Indochina Block exerted profound, long‐lasting influences on the stress field, faults activity and magmatism of the western SCS, persisting even after escape‐related motions ceased. More importantly, this work documents a case of en‐echelon linear volcanic constructs in a sedimentary basin, advancing the tectonic interpretive value of such features for intraplate shear extension and micro‐block interactions. The results may also be applicable to other areas with similar geological settings.