POST-RIFT BURIED VOLCANOES AND IGNEOUS PLUMBING SYSTEMS ALONG A CONTINENTAL RIBBON: INSIGHTS FROM THE XISHA MASSIF, NORTHWESTERN MARGIN OF THE SOUTH CHINA SEA

Abstract

possible to investigate the size, age, and geographical distribution of the buried volcanoes from multi-beam, single-, and multi-channel seismic data collected beneath the South China Sea (SCS) Xisha massif, which we argue is a continental ribbon. These data made it evident that the Middle Miocene volcanoes frequently generated massive volcanic fields that erupted along the rift fault zones, in contrast to the Early Miocene volcanoes, which typically built clusters of small-volume volcanic cones in the half-graben. Details include the presence of numerous volcanoes above and to the side of the dome-shaped main edifice that constitutes the middle Miocene volcanic field. Intrusive sills beneath volcanoes are isolated and have a dispersed distribution pattern at different levels, whereas dykes beneath volcanoes are numerous and have vertical zones of disruption (VZD) that connect to underlying faults or extend through the sediments to the crust. The relationship between the volcanoes and intrusions suggests that shallow igneous plumbing systems within the Xisha massif are most likely dyke domains. The Xisha massif has favorable conditions, including a relatively thin sedimentary sequence over a slightly extended continental crust (20–28 km) that might provide enough magma pressure for an igneous plumbing system that is primarily fed by dykes. Additionally, rifted faults in the upper crust and possibly sub-vertical foliations in the basement rock mass were thought to be viable routes for magma transport vertically. We emphasize the importance of crustal structure on the continental ribbon in controlling igneous plumbing styles and the distribution of post-rift volcanic systems along magma-poor continental margins, including crustal thickness, pre-existing faults, heterogeneous basement, and sediments.