GEOLOGICAL NOTES Jurassic Dextral Movement along the Dien Bien Phu Fault, NW Vietnam: Constraints from 40 Ar= 39 Ar Geochronology Te-Hsien Lin, Ching-Hua Lo, 1 Sun-Lin Chung, Pei-Ling Wang, 2 Meng-Wan Yeh, 3 Tung-Yi Lee, 3 Ching-Ying Lan, 4 Nguyen Van Vuong, 5 and Tran Tuan Anh 6 Department of Geosciences, National Taiwan University, Taipei, Taiwan ABSTRACT The NNE-SSW-trending Dien Bien Phu fault, which dextrally displaces the NW-SE-trending Song Ma suture, in northwestern Vietnam, is widely considered one of the most seismically active faults in Indochina. In order to better understand the fault’ s activity, this study reports new 40 Ar= 39 Ar geochronological data for mica schists and mylonites from several areas along the Dien Bien Phu fault, showing 40 Ar= 39 Ar ages of 194–212 Ma for mica schists and 158–198 Ma for mylonites. These ages suggest that after the collision of the South China and Indochina blocks, dextral shearing deformation initiated along the Dien Bien Phu fault in the Jurassic, significantly earlier than previously thought. In light of the relevant tectonic events in Indochina, the successive suturing/collision of the Indochina, Simao, and Sibumasu blocks may have been responsible for the initiation of dextral shearing along the Dien Bien Phu fault, which in turn resulted in offset of the Song Ma suture. Online enhancement: color version of figure 2. Introduction The collision between India and Eurasia, which led to the activation of major faults and plate reor- ganization in East Asia, represents the most sig- nificant tectonic event of the Cenozoic Earth (for a recent review, see Burchfiel 2004). In this context, many workers have investigated the NW-SE- trending transverse fault systems in Southeast Asia (fig. 1) since the collision-extrusion tectonic model was proposed by Tapponnier et al. (1982). Recent GPS data suggest that the Tibetan extru- sion involved clockwise rotation (e.g., Zhang et al. 2004), meaning that crustal flow is now bounded to the east by a sinistral strike-slip system con- sisting of the Xianshuihe, Xiaojiang, and Dien Bien Phu faults (fig. 1). However, geological corre- lations indicate that the Dien Bien Phu fault has dextrally offset the Song Ma belt (Fontaine and Workman 1997; Lepvrier et al. 2004; fig. 1), implying a complicated movement for the fault. This complexity has apparently been magnified by the multiple collision events involved in the amalgamation of the South China, Indochina, Sibumasu, and Simao blocks during the Indosinian Orogeny (Carter et al. 2001; Carter and Clift 2008). In order to better understand the fault activities, we undertook a detailed 40 Ar= 39 Ar geochronologi- cal study of the Dien Bien Phu fault. Mesozoic ages obtained for mica fish in mylonites from two areas along the Dien Bien Phu fault suggest that the fault is long-lived and that it may have dextrally offset the Song Ma suture during the Jurassic. Using both our own and previously published age and struc- tural data, we suggest that the successive suturing/ collision of the Indochina, Simao, and Sibumasu blocks probably played an important role in con- trolling dextral movement along the Dien Bien Phu fault. Manuscript received June 25, 2008; accepted October 29, 2008. 1 Author for correspondence; e-mail: loch@ntu.edu.tw. 2 Institute of Oceanography, National Taiwan University, Taipei, Taiwan. 3 Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan. 4 Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan. 5 Department of Geology, National Vietnam University, Hanoi, Vietnam. 6 Institute of Geology, Vietnam Academy of Science and Technology, Hanoi, Vietnam. [The Journal of Geology, 2009, volume 117, p. 192–199] © 2009 by The University of Chicago. All rights reserved. 0022-1376/2009/11702-0006$15.00 DOI: 10.1086/595965 192