Pliocene-to-present morphotectonics of the Dien Bien Phu fault in northwest Vietnam Kuang-Yin Lai a, b , Yue-Gau Chen a, ⁎, Doãn Đình Lâm c a Department of Geosciences, National Taiwan University, No. 1, Section 4th, Roosevelt Road, Taipei 106, Taiwan b Institute of Earth Sciences, Academia Sinica, No. 128, Section 2nd, Academia Road, Taipei 115, Taiwan c Institute of Geological Sciences, Vietnamese Academy of Science and Technology, Hanoi, Viet Nam abstract article info Article history: Received 29 March 2011 Received in revised form 25 May 2012 Accepted 27 May 2012 Available online 1 June 2012 Keywords: Morphotectonics Dien Bien Phu fault Active fault River offset Slip rate Earthquake potential 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 dis- tance of 150 km 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 informa- tion 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 recog- nized and reconstructed, and the largest sinistral displacement on the DBPF is ca. 12.5 km based on drainage network restoration. Because sinistral motion likely initiated approximately 5 Ma, the most probable Pliocene-to-present average slip rate on the DBPF is on the order of 2.5 mm/yr. Based on the slip rate, the DBPF could generate an earthquake with a magnitude greater than M w 7 and a recurrence interval of 500 to 1000 yr. The combined GPS velocity fields observed from northwest Vietnam and south China reveal ap- proximately 2 to 3 mm/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 dif- ferent slip sense from its previous phase, and plays a role as an eastern boundary of the crustal deformation in northern Indochina. © 2012 Elsevier B.V. All rights reserved. 1. Introduction There are a number of north- to northeast-striking faults in north- ern Indochina, which may have developed as a result of roughly east– west trending compression as a consequence of the India–Eurasia col- lision in the early Cenozoic (Fig. 1; Tapponnier et al., 1982, 1986). In the easternmost part of the deforming zone, the Dien Bien Phu fault (DBPF) is morphologically conspicuous and appears immediately south of the Red River fault (RRF) zone, extending over 150 km from south Yunnan through northwest Vietnam into Laos and per- haps connecting to the Nan (Nan-Uttaradit) suture in Thailand (Leloup et al., 1995; Wang et al., 1998; Lepvrier et al., 2004). Although the DBPF zone is one of the active fault zones in Indochina, the quan- tified kinematics of this fault remain mostly unknown. According to the proposed concept of tectonic extrusion in south- east Asia (Tapponnier et al., 1982, 1986), the present north- to northeast-striking faults in northern Indochina, including the DBPF, were a set of right-lateral strike-slip faults that developed in the mid-Tertiary in response to the left-lateral action of the RRF. Due to the progressive collision between the Indian and Eurasian plates, the second phase of extrusion was induced ca. 20 Ma (Tapponnier et al., 1982, 1986). However, a number of models have been proposed to argue that tectonic rotation around the Eastern Himalayan Syntaxis in the southeast margin of the Tibetan Plateau became the major ki- nematic force rather than two-phase extrusion (England and Houseman, 1989; Houseman and England, 1993; King et al., 1997; Wang et al., 1998; Chen et al., 2000). The rotation of the tectonic block is bounded by the Xianshuihe–Xiaojiang fault (XXF) system Geomorphology 173–174 (2012) 52–68 ⁎ Corresponding author at: Department of Geosciences, National Taiwan University, Taiwan. Tel.: +886 2 2369 7648; fax: +886 2 2364 4625. E-mail address: ygchen@ntu.edu.tw (Y.-G. Chen). 0169-555X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2012.05.026 Contents lists available at SciVerse ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph