Linking Paleogene rifting and inversion in the Northern Song Hong and Beibuwan Basins, Vietnam, with left-lateral motion on the Ailao Shan-Red River Shear Zone

Extrusion tectonics forced by plate collisions shape continents not only through lateral terrain displacement and mountain building but also through massive rifting and basin development. The rift system underneath the Gulf of Tonkin, Vietnam, constitutes a world-class example of how extrusion tectonics drives continental rifting and transtensional basin development. Rifting and the Song Hong and Beibuwan Basin evolution are compared with the development of the Ailao Shan-Red River Shear Zone (ASRRSZ) that accommodated the extrusion of Indochina forced by the Indian-Eurasia collision. Rifting occurred during later Eocene-Late Oligocene time forced by ASRRSZ left-lateral shearing. Latest Oligocene-earliest Miocene transpression and inversion brought rifting to a halt, after which left-lateral shearing decreased. Paleogene rift systems extended along the trail of the ASRRSZ now outlined by lower to midcrustal metamorphic core complexes. Most of these rift systems were probably inverted and removed during the latest Oligocene-earliest Miocene, however. The metamorphic core complexes are suggested to represent the lower to midcrustal roots of these transtensional rift basins exhumed by inversion. Rift termination in the northern Gulf of Tonkin and exhumation of the metamorphic core complexes coincided with cessation of Paleogene rifting along the western South China Sea, and a common causal mechanism is speculated.