ARTICLE IN PRESS EPSL-10123; No of Pages 11 Earth and Planetary Science Letters xxx (2010) xxx–xxx Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl Syncollisional extension along the India–Asia suture zone, south-central Tibet: Implications for crustal deformation of Tibet M.A. Murphy a,⁎, V. Sanchez a, M.H. Taylor b a University of Houston, 312 Science and Research Bldg. 1, Houston, TX 772004-5007, United States b Department of Geology, University of Kansas, Lawrence, KS 66045, United States article info abstract Article history: Crustal deformation models of the Tibetan plateau are assessed by investigating the nature of Neogene Received 15 October 2009 deformation along the India–Asia suture zone through geologic mapping in south-central Tibet (84°30′E). Received in revised form 18 November 2009 Our mapping shows that the suture zone is dominated by a system of 3 to 4 ENE-striking, south-dipping Accepted 20 November 2009 thrust faults, rather than strike-slip faults as predicted by models calling upon eastward extrusion of the Available online xxxx Tibetan plateau. Faults along the suture zone are not active, as they are cut by a system of NNW-striking Editor: T.M. Harrison oblique slip normal faults, referred to herein as the Lopukangri fault system. Fault-slip data from the Lopukangri fault system shows that the mean slip direction of its hanging wall is N36W. We estimate the net Keywords: slip on the Lopukangri fault by restoring components of the thrust system. We estimate that the fault has Himalaya accommodated ∼7 km of right-slip and ∼8 km of normal dip-slip, yielding a net slip of ∼10.5 km, and 6 km Tibet of horizontal east–west extension. The Lopukangri fault system is active and geomorphic offsets indicate extension right separations and westside-down dip-separation. The mapview curviplanar geometry and geomorphic Karakoram fault expression of the Lopukangri fault system is similar to faults and rift basins to its east and west. These extensional faults are en echelon in map view and encompass a region that is 200 km long (east–west) and 95 km wide (north–south). Assuming our results for the Lopukangri fault are applicable to the entire system, we estimate a maximum of 18% extension across the zone. All active faults in the system terminate southward adjacent to the India–Asia suture zone. Because the individual rift geometries are similar and suggest a common kinematic relationship, we propose that the extensional system formed as a trailing extensional imbricate fan at the southern termination of the central Tibet conjugate fault zone. Alternatively, the extensional system may terminate to the north and represent a group of isolated crustal tears. Both kinematic interpretations imply a semi-smooth north–south variation in the magnitude of east–west extension in southern Tibet, with higher magnitudes in the north along the Bangong–Nujiang suture zone than in the south along the India–Asia suture zone. Our results from southern Tibet show that deformation between southern Tibet and the Himalayas is broad (95 km wide) and best described as a continuum possibly since the Late Miocene. Conversely, the structural boundary between western Tibet and the Himalayas, which is defined by the Karakoram fault, is presently a discrete boundary, and probably has been since the Middle Miocene. We think this variation in the displacement gradient and age of these structural boundaries within the interior of Tibet is best explained by the fault patterns and strain history describing wholesale E–W stretching and N–S shortening of the Tibetan crust. © 2009 Elsevier B.V. All rights reserved. 1. Introduction in terms of the predicted magnitudes of displacement, slip rates, and lateral extent. One end-member category of models views the crust as a The prevailing view regarding internal deformation of the Tibetan mosaic of rigid blocks or microplates in which deformation is localized crust is that it is undergoing coeval east–west extension and north– along laterally extensive faults with long life-spans, large magnitudes of south shortening via north-trending normal faulting and strike-slip slip, and high slip rates (Peltzer and Tapponnier, 1988; Replumaz and faulting (e.g. Tapponnier et al., 2001; Taylor et al., 2003). Although all Tapponnier, 2003). Alternatively, continuum models of continental explanations of internal deformation of the Tibetan plateau recognize deformation view deformation as being “evenly” distributed through- these structural systems as essential elements to the puzzle, they differ out the crust partitioned along many faults with short life-spans, low magnitudes of slip, low slip rates, and relatively shorter lengths (England and Houseman, 1986; England and Molnar, 2005). ⁎ Corresponding author. Tel.: +1 713 743 3413. Central to differentiating between these end-member views are E-mail address: [email protected] (M.A. Murphy). the properties of two first-order Neogene structural components 0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.11.046 Please cite this article as: Murphy, M.A., et al., Syncollisional extension along the India–Asia suture zone, south-central Tibet: Implications for crustal deformation of Tibet, Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2009.11.046 ARTICLE IN PRESS 2 M.A. Murphy et al. / Earth and Planetary Science Letters xxx (2010) xxx–xxx within the Tibetan plateau, the right-slip Karakoram fault system (e.g. In this paper we present geologic mapping along the India–Asia Armijo et al., 1989; Murphy et al., 2000) and the central Tibet suture zone in south-central Tibet (84°30′E) in the vicinity of conjugate fault zone (Taylor et al., 2003). The Karakoram fault is one Lopukangri (7095 m) (Fig. 1). Our study takes advantage of an area of the longest faults within the Tibet–Himalayan orogen and serves as which straddles the India–Asia suture zone that coincides with the a critical component to several models explaining the deformation southern portion of a major rift in southern Tibet as well as the history of the orogen (Peltzer and Tapponnier, 1988; Armijo et al., hypothesized location of the Karakoram fault. The goals of this study 1989; McCaffrey and Nabelek, 1998; Murphy et al., 2000; Robinson, are 1) to assess the eastward extent of the Karakoram fault system 2009). Although several aspects of the fault are actively being along the India–Asia suture, and 2) evaluate the southern termination debated, one that stands to have a significant impact on several of southern Tibet (Lhasa block) graben. fronts is its lateral extent (Fig. 1). For example in the Mt. Kailas area, a significant component of the total slip along the Karakoram fault is 2. Geology of the Lopukangri area diverted to the Gurla Mandhata–Humla fault system (Ratschbacher et al., 1994; Murphy et al., 2002; Murphy and Copeland, 2005). Lacassin Geologic mapping was conducted during the summers of 2006 and et al. (2004) hypothesize that the remainder of the slip is fed along a 2008 at a scale of 1:100,000 (Fig. 2). The geologic framework of the continuation of the Karakoram fault along the India–Asia suture zone area can be viewed as consisting of two different components, each and facilitates wholesale rigid block eastward extrusion of the Tibetan with a unique deformational history. They are, a north-directed plateau (Tapponnier et al., 2001)(Fig. 1). This hypothesized geometry system of thrust faults, which we correlate to the Great Counter has been used to define boundary conditions for several models that Thrust system (Yin et al., 1999; Murphy et al., 2009), and the west- seek to explain long-term and short-term deformation of the Tibet– dipping Lopukangri fault system. Himalayan orogen (e.g. Tapponnier et al., 2001; Replumaz and Tapponnier, 2003; Chen et al., 2004; Meade, 2007). Although the 2.1. Great Counter Thrust Karakoram fault has been studied in some detail in western Tibet, its hypothesized eastward continuation into south-central Tibet has not Exposed primarily in the western portion of the mapped area is a been investigated in detail by field-based investigations. north-directed imbricate thrust fault system which defines the The central Tibet conjugate fault zone runs east–west and is surface trace of the India–Asia suture zone (Fig. 3). The thrust fault centered along the Bangong–Nujiang suture zone. This zone consists system is interformational and involves the Lower Triassic Qiongguo of right-slip and left-slip faults that define eastward-opening wedges Group, Upper Triassic Xiukang Group, Upper Cretaceous Xigaze (Taylor et al., 2003). Displacement along these faults has accommo- Group, and Oligocene–Miocene conglomerate, sandstone, and mud- dated east–west stretching as well as north–south shortening. Armijo stone (Liu, 1988). Five thrust faults were recognized in the field. The et al. (1989) and Taylor et al. (2003) show that the conjugate strike- southernmost thrust fault juxtaposes the Triassic Qiongguo and slip faults are kinematically linked to north-striking graben in Xiukang Groups (TSS — Fig. 1) in its hanging wall over the Cretaceous southern and northern Tibet via extensional stepover structures. In Xigaze Group in its footwall (Klm — Fig. 1). The Qiongguo and Xiukang southern Tibet, few graben can be shown to cross the India–Asia Groups are correlated to the Tethyan Sedimentary Sequence and are suture zone (Yin, 2000). One explanation may be that the rifts link composed of interlayered gray and green phyllite, and gray marble with strike-slip faults along the India–Asia suture zone. Alternatively, with numerous quartz-filled veins (Liu, 1988). The Xigaze Group in the rifts may simply tip out or terminate. The first scenario permits the study area is composed of a lower 1-km thick sequence of large magnitudes of extrusion relative to the latter case.