The Akato Tagh bend along the Altyn Tagh fault, northwest Tibet 1: Smoothing by vertical-axis rotation and the effect of topographic stresses on bend-fl anking faults Eric Cowgill† An Yin Department of Earth and Space Sciences, University of California, Los Angeles, California 90095, USA J Ramón Arrowsmith Department of Geological Sciences, Arizona State University, Tempe, Arizona 85287, USA Wang Xiao Feng Zhang Shuanhong Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing, China ABSTRACT shortening into the two inside corners of a INTRODUCTION restraining double bend may cause both the To better understand the mechanics of bend and the fault to undergo vertical-axis The active, left-lateral Altyn Tagh fault restraining double bends and the strike-slip rotation, thereby reducing the bend angle system (Fig. 1) (Molnar and Tapponnier, faults in which they occur, we investigated and smoothing the trace during progressive 1975, 1978; Tapponnier and Molnar, 1977) the relationship between topography and deformation. Such vertical-axis rotation may is the largest strike-slip fault system within bedrock structure within the Akato Tagh, help explain why fault trace complexity is the Indo-Asian collision zone and is thought the largest restraining double bend along inversely related to total displacement along to have played a central role in accommodat- the active, left-slip Altyn Tagh fault. The strike-slip faults. Second, we calculate four ing India’s ongoing indentation into Eurasia bend comprises a ~90-km long, east-west independent age estimates for the Akato Tagh (Peltzer and Tapponnier, 1988). Although striking central fault segment fl anked by bend, all of which are much younger than the active deformation is concentrated along one two N70°E-striking sections that parallel Altyn Tagh system. We use these estimates in principal trace, the Altyn Tagh fault, this trace the regional strike of the Altyn Tagh system. a companion study to postulate that the Altyn is fl anked by a distributed set of secondary The three segments form two inside corners Tagh and similarly multi-stranded strike-slip strike-slip and oblique-slip structures (Fig. 1) in the southwest and northeast sectors of the systems may evolve by net strain hardening. (Cowgill et al., 2000; Jolivet et al., 2001; uplift where they link. We fi nd that both the Third, comparison of the Akato Tagh with Liu, 1988; Xinjiang Bureau of Geology and topography and bedrock structure of the other restraining double bends highlights sys- Mineral Resources, 1993). Thus, the Altyn Akato Tagh restraining bend are strongly tematic differences in the style of borderland Tagh system constitutes a northeast-striking asymmetric. The highest and widest parts of faulting and we speculate that these variations left-lateral shear zone that is at least 100 km the uplift are focused into two topographic result from different states of stress adjacent wide (Cowgill et al., 2003; Jolivet et al., 2001). nodes, one in each inside corner of the bend. to the bends. Strike-slip dominated bends Although several major strike-slip systems are σ Structural mapping of the western half of such as the Akato Tagh may form where H = characterized by similarly broad zones of fault- σ σ σ σ σ the bend suggests the southwest node coin- 1, v = 2, and h = 3 whereas thrust-domi- ing, it remains unclear how these zones evolve cides with a region of anomalously high, nated bends like the Santa Cruz bend along during progressive deformation and what this σ σ σ bend-perpendicular shortening. We also fi nd the San Andreas fault form when H = 1, h evolution implies about the mechanical behav- σ σ σ that partitioned, transpressional deforma- = 2, v = 3. This hypothesis predicts that the ior of the continental lithosphere. In particular, tion within the Akato Tagh borderlands is style of faulting along a restraining double does the breadth of these zones increase during absorbed by bend-parallel strike-slip faulting bend can evolve during progressive deforma- progressive deformation, and if so, does this and bend-perpendicular folding, unlike the tion, and we show that either weakening of expansion indicate that the continental litho- thrusting reported from many other double borderland faults or growth of restraining sphere deforms by strain-hardening processes bends. Synthesis of these results leads to three bend topography can convert thrust-domi- (Cowgill et al., 2004; also see Holdsworth et implications of general signifi cance. First, we nated bends into strike-slip dominated uplifts al., 2001)? show that focusing of bend-perpendicular such as the Akato Tagh. Between 85°E and 95°E longitude, the principal active trace of the Altyn Tagh system †Present address: Department of Geology, Uni- Keywords: restraining bend, Altyn Tagh constitutes an en échelon set of fi ve, northeast- versity of California, Davis, California 95616, USA; fault, strike-slip systems, Tibetan Plateau, striking straight fault sections punctuated by e-mail: [email protected]. fault strength, topographic stress. four shorter, right-stepping, east-west trending GSA Bulletin; November/December 2004; v. 116; no. 11/12; p. 1423–1442; doi: 10.1130/B25359.1; 12 fi gures. For permission to copy, contact [email protected] © 2004 Geological Society of America 1423 COWGILL et al. Figure 1. Location of the Akato Tagh restraining double bend along the Altyn Tagh fault within the Indo-Asian collision zone. (A) Simplifi ed map of major faults in the Indo-Asian collision zone. Box outlines Fig- ure 1B. (B) Shaded relief image of the central Altyn Tagh range. Known and inferred strands within the Cenozoic Altyn Tagh fault system as compiled from our mapping and interpretation of CORONA satellite imagery along with previous reports (Chinese State Bureau of Seis- mology, 1992; Cowgill et al., 2000; Liu, 1988; Meyer et al., 1998). Active trace of the Altyn Tagh fault has four restraining double bends (Sulamu Tagh, Akato Tagh, Akatengneng Shan, and Aksai). Locations of the prominent mountain ranges Eastern and Western Kunlun Shan (EKS and WKS) are shown. Box outlines Figure 2. Base map was generated from the GTOPO30 1 km digital elevation data using GMT soft- ware developed by Wessel and Smith (1991). segments in the Sulamu Tagh, Akato Tagh, Restraining double bends such as those we present an integrated bedrock and neotec- Mangnai, and Aksai regions (Fig. 1B). We along the Altyn Tagh fault are key features tonic investigation of the 90 km-long Akato follow Crowell (1974) in referring to such along strike-slip faults because fault-parallel Tagh range, the largest double restraining bend geometric complexities as restraining double motion outside the bend produces convergence along the Altyn Tagh fault. Cowgill et al. (2004) bends because the fault trace bends fi rst to the within the double bend (e.g., Crowell, 1974). investigates the age of the active trace relative to right and then to the left when followed along Thus, analysis of the amount and timing of the Altyn Tagh system as a whole to explore the strike. Because the straight segments are sys- deformation within the bend will yield insight question of whether or not strain hardening is tematically right stepping, there are no releasing into the history of strike-slip faulting along the important in producing geometrically complex, double bends of equivalent size to the transpres- adjacent fault segments. Bends and stepovers multi-stranded strike-slip faults like the Altyn sional jogs along the Altyn Tagh fault. Flanking are also thought to exert a fi rst-order control Tagh fault system. The present study lays the each restraining double bend are mountain on the initiation and extent of earthquake rup- foundation for Cowgill et al. (2004) by explor- ranges that are anomalously high relative to the tures (e.g., Du and Aydin, 1995 and references ing the relationship between the topography and surrounding regions (Figs. 1B and 2), suggest- therein; Nielsen and Knopoff, 1998; Segall and bedrock structure of the bend. In particular, we ing that the double bends are localized sites of Pollard, 1980; Wesnousky, 1988). In this study address three questions. (1) What is the spatial active horizontal shortening and rock uplift. and a companion paper (Cowgill et al., 2004) distribution of anomalous topography within 1424 Geological Society of America Bulletin, November/December 2004 THE AKATO TAGH BEND ALONG THE ALTYN TAGH FAULT, NW TIBET 1 Figure 2. Overviews of the Akato Tagh restraining double bend. (A) Structural map of the Akato Tagh restraining bend compiled from our 1:100,000 structural mapping (Fig. 4) and analysis of CORONA imagery in addition to the active fault map of the Altyn Tagh fault (Chinese State Bureau of Seismology, 1992). Deformation of the strike-slip borderlands within the bend is partitioned into fault-perpendicular folding and fault-parallel strike-slip faulting. Inset shows locations of western, central, and eastern segments of the Akato Tagh bend and the positions of the two inside corners in the southwest and northeast sectors of the uplift. Polygon outlines Figure 4. (B) Photo-mosaic of peaks in the south- west inside corner, view to northeast. The Altyn Tagh fault is located behind the snow-capped peaks. Note that elevations increase from west to east, corresponding to areas that lie outside of, and within, the Akato Tagh restraining bend, respectively. Photo location as in part A. the bend? (2) What structures produced this we demonstrate, an important implication of structural context of the central Altyn Tagh fault topography, and in particular, are the areas that such asymmetry is that the principal fault trace and the Akato Tagh uplift. We then examine the fl ank the double bend cut by thrusts, similar to should undergo counterclockwise, vertical axis topography of the bend.