The Structural Setting of the Song Ma Region, Vietnam and the Indochina-South China Plate Boundary Problem

The Song Ma-Song Da region of northern Vietnam contains the Song Ma Anticlinorium, a polydeformed polymetamorphosed, early Palaeozoic island arc/forearc terrane accreted to the South China plate in Siluro-Devonian times. The Song Ma Anticlinorium is not an Indosinian subduction zone and nor is the post-Triassic, post-Cretaceous Song Ma Fault. The Song Ma Fault is one of many northwest-trending, post-Cretaceous, high-angle reverse oblique-slip faults and thrusts responsible for shortening and strike-slip transposition of northern Vietnam. Folds penecontemporaneous with this faulting include the Song Ma Anticlinorium, which is formed probably of thrust-ramp folds. The faulting and penecontemporaneous folding developed during Oligocene sinistral strike-slip on the Song Hong Fault and was reactivated during Pliocene-Quaternary times. We conclude that the present plate boundary is a broad deformation zone between Da Nang and the Song Hong and that parts of the South China plate extend well to the south of the Song Hong Fault.