The Cao Bang-Tien Yen Fault: Implications on the relationships between the Red River Fault and the South China Coastal Belt

Abstract

The Cao Bang Fault is a large NW trending fault zone which runs along the border between Vietnam and China and parallels the Red River Fault at a distance of about 160 km. The fault appears to have had a left-lateral responsible for the opening of various pull-apart basins. The Cao Bang basin, the most spectacular of these, had a polyphased evolution beginning with a 50 km wide diffuse zone marking a left-lateral releasing bend on which a large component of extension is observed. Deformation associated with this transtension increases from the centre of the deformed zone toward the edge, where ductile structures prevail. The basin evolved as perfect 85 sigmoid-shaped pull apart, concentrating the deformation in its central part. To the south, the Cao Bang Fault abuts the Cam Pha fault which runs parallel to the coast belt of the Bien Dong Sea, showing no offset of the latter, and thus ending into an extensional zone probably part of the Bac Bo (Beibu) Basin, meaning the fault was mostly active during the rifting stage of the Bien Dong Sea opening. The southern part of the Cao Bang fault also showed late margin-parallel extension associated with the wrench motion of the coastal belt. This indicates that the Cao Bang Fault shared the history of the Red River fault, with less finite wrench displacement and less exhumation. It however marks the opposite edge of a large 300 km wide crustal block, the Tonkin-Kunming Block, which accommodates differential motion between the Indochina and the South China blocks during the Tertiary. Advances in Natural Sciences Vol.4, No.4 (2003) (347-361)