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In this research, pre-Cenozoic deformation characteristics will be presented for the Ca River fault zone based on field observations and geological mapping conducted in the Muong Xen area, Nghe An, northwest of the Ca River fault zone. | Vietnam Journal of Earth Sciences, 38(2), 166-177, DOI: 10.15625/0866-7187/38/2/8599 Vietnam Academy of Science and Technology (VAST) Vietnam Journal of Earth Sciences http://www.vjs.ac.vn/index.php/jse Pre-Cenozoic tectonic deformation Muong Xen area, Nghe An province phases in the Nguyen Ba Minh*1, Nguyen Van Vuong2 1 2 General Department of Geology and Minerals of Vietnam University of Science, Vietnam National University, Hanoi Received 6 April 2016. Accepted 23 May 2016 ABSTRACT The Ca River fault zone, located in the North-Central Vietnam, is one of the most complicated deformation zones with very low grade metamorphism but high intense folding. Investigations of deformation conducted in the Muong Xen area, northwest of the Ca River fault zone, show that, the Ca River fault zone had undergone at least three deformation phases before the Cenozoics. The first (oldest) deformation phase is characterized by recumbent, reverse folds and overthrust faults in NW-SE direction. The second deformation phase is characterized by large scale Zshaped folds that deformed oder structures. Those structures are covered by a pre-Late Triassic angular unconformity. The third deformation phase refolds previously formed structures and shows similar characteristics of folding as the first phase. These structures were subsequently deformed by Cenozoic tectonics. Keywords: Ca River fault zone; Pre-Cenozoic defomation; Recumbent folds; Z-shaped folds; Pre-Late Triassic angular unconformity. ©2016 Vietnam Academy of Science and Technology 1. Introduction The Ca River fault zone is located in Nghe An, Ha Tinh provinces, and partly in northern Quang Binh province. Geologically, the fault zone is limited by route 48 faults in the north and Rao Nay fault to the south. Up to date, a number of studies on structural tectonics in this area have been conducted (Nguyen, 1987; Phan, 1995; Bui et al., 1996; 2001; 2002; 2005; 2008; Lepvrier et al., 1997; 2003; 2008; Cao et al., 1998; .
