Pliocene-to-present morphotectonics of the Dien Bien Phu fault in northwest Vietnam

The north- to northeast-trending Dien Bien Phu fault (DBPF) zone appears to the south of the Red River fault (RRF) zone, sharing the spatial alignment of the Xianshuihe–Xiaojiang fault (XXF) and extending over a distance of 150km from Yunnan, China, through northwest Vietnam into Laos. Although the DBPF is one of the most conspicuous active fault systems in Indochina, it is less studied than the RRF and XXF, and its quantified kinematics remain mostly unknown. Our detailed modern fault trace mapping, compiled from topographic maps, stereographic aerial photos, ASTER satellite imageries, and field reconnaissance reveals new information on the fault geometry, the slip magnitude and distribution along the fault, and the relationship between river-channel offset and fault activity. The geometry of the modern DBPF is complex, consisting of single strands and stepovers. Abundant geomorphic expressions along the DBPF illustrate that the modern fault is dominated by sinistral motion and the present left-lateral component of motion is also clearly demonstrated by the existence of numerous rivers offsets. Multiple offsets of geomorphic features along the fault are recognized and reconstructed, and the largest sinistral displacement on the DBPF is ca. 12.5km based on drainage network restoration. Because sinistral motion likely initiated approximately 5Ma, the most probable Pliocene-to-present average slip rate on the DBPF is on the order of 2.5mm/yr. Based on the slip rate, the DBPF could generate an earthquake with a magnitude greater than Mw 7 and a recurrence interval of 500 to 1000yr. The combined GPS velocity fields observed from northwest Vietnam and south China reveal approximately 2 to 3mm/yr left-lateral slip across the DBPF, and significant east–west extension exists in the western crustal block of DBPF. Although the boundary fault system of the active crust rotation around the Eastern Himalayan Syntaxis (i.e., XXF) does not cut the RRF, the Pliocene-to-present activity along the DBPF favors the possibility that the tectonic shear has been transmitted across the RRF and taken up on the DBPF. The results of this study suggest that the modern DBPF zone acts as a reactivated fault, with a different slip sense from its previous phase, and plays a role as an eastern boundary of the crustal deformation in northern Indochina.