Syncollisional Extension Along the India–Asia Suture Zone, South-Central Tibet: Implications for Crustal Deformation of Tibet

ARTICLE IN PRESS EPSL-10123; No of Pages 11 Earth and Planetary Science Letters xxx (2010) xxx–xxx

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Earth and Planetary Science Letters

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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

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. Moreover, the interbedded limestone (Klm) and siltstone sequence and an upper latter case implies a strong north–south gradient in the east–west 1.5-km thick sequence of interbedded fine-grained sandstones, stretching of the Tibetan plateau. siltstones, and mudstones (Ksh). The southernmost thrust fault

Fig. 1. Map modiﬁed from Taylor and Yin (2009) showing extensional faults and suture zones in southern Tibet. The inset boxes (Figs. 2 and 5) show the location of our study area which is located at the juncture between the hypothesized continuation of the Karakoram fault and the southern termination of the Lopukangri rift. Abbreviations: DF — Dangardzang fault; GM — Gurla Mandhata; KF — Karakoram fault; LK — Lopukangri; NQTL — Nyainqentanghla; TF — Tibrikot fault; and TK — Takkhola graben.

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Fig. 2. Geologic map of the Lopukangri area. Lower hemisphere equal area stereoplots show fault-slip data from the main faults discussed in the text. See Fig. 1 for location.

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Fig. 3. Geologic cross-sections through the Lopukangri area. Cross section locations are shown on the geologic map in Fig. 2. strikes WNW and dips moderately to the south. Fault zones are is dominated by vein quartz and green altered andesite. Tcg2 lies characterized by 10 meter wide zones composed of interlayered red structurally above Tcg1. It is a matrix supported cobble–pebble and green cataclasites, fractured mudstone, and boulder, cobble, conglomerate. The matrix is medium- to coarse-grained sandstone. pebble, gravel conglomerate. Clast compositions include pyroxenite, Clasts lie parallel to bedding and chiefly consist of granite with white marble, and serpentinized basalt. In addition, 500 to 1000 m significant amounts of gray quartzite and vein quartz. Tcg3 is the long lenses of serpentinized mafic and ultramafic rocks crop out structurally highest map unit. It is composed of interbedded reddish- locally along the fault zone. Shear sense indicators and fault striations brown siltstone, mudstone, medium-grained sandstone, and pebble indicate NNE-directed thrusting (Fig. 2). To the north of this thrust, conglomerate. The conglomerate is locally clast-supported and another thrust juxtaposes a 1-km thick interbedded limestone and resistant to weathering. The matrix consists of medium-grained siltstone sequence in its hanging wall over a 1.5-km thick sequence of sandstone. Clasts are exclusively sedimentary. Gray sandstone and red interbedded fine-grained sandstones, siltstones, and mudstones. The mudstone are the two most dominant lithologies. In addition, we hanging wall siltstone locally contains concentrated accumulations of identified gray limestone and green chert which was only recognized disarticulated pelecypods. in this unit. The Cretaceous Xigaze Group (Klm + Ksh) is thrust northward Two intraformational thrust faults separate Tcg1, Tcg2, and Tcg3 over a thick sequence of interbedded cobble, gravel, pebble (Fig. 2). They strike NW, dip moderately to the south, and contain conglomerate and coarse-grained sandstone interpreted to be striations that indicate northeast-directed thrusting. Fault zones Tertiary in age (Liu, 1988). We divide the Tertiary stratigraphy into range from 1 to 3 m thick and contain 1 to 10 cm thick bedding three map units based on lithology and structural position. The parallel high strain zones characterized by clay gouge and cataclasite. contacts between the map units are faulted therefore the stratigraphic relationship between them is uncertain. However, the structurally 2.2. Lopukangri fault system lowest map unit (Tcg1) lies nonconformably on coarse-grained granite that we correlate to Gangdese batholith rocks (Liu, 1988) The Lopukangri fault system extends ∼100 km from the southern (Fig. 3). Therefore it is the stratigraphically lowest unit, at least locally. Lhasa block into the Tethyan Himalaya physiographic province to the Tcg1 is a thick sequence of interbedded cobble, gravel, pebble south. In the vicinity of Lopukangri (7095 m) the Lopukangri fault conglomerate and coarse-grained sandstone. The clast composition consists of three west-dipping subparallel faults. In the mapped area

M.A. Murphy et al. / Earth and Planetary Science Letters xxx (2010) xxx–xxx 5 the fault system strikes N09°E and cuts across the Great Counter of the Lopukangri rift. Approximately 30 km due north of the Thrust at 29°50′N in the northeastern part of the study area (Figs. 2 Lopukangri fault shown in Fig. 2 is the northern continuation of the and 3). The greatest amount of throw on the fault system is on the fault system. At this location, the Lopukangri fault system bounds a easternmost fault. We refer to this fault as the Lopukangri fault. The north-northwest-trending valley in the Lhasa block. The width of the footwall of the Lopukangri fault contains two thick (∼1–2 km), east- valley is relatively narrow along its southern segment and the valley dipping tabular calc-alkaline granite bodies that are interpreted as width increases to the north. The southern segment trends NNW, is Gangdese arc rocks (Liu, 1988)(Fig. 4). Structurally beneath these approximately 4–6 km wide, and ∼34 km long. Prominent range- plutonic rocks is a N1.5-km thick section of metamorphic rock that bounding faults are indicated by fault scarps and deflected stream includes foliated garnet biotite schist, amphibolite, calcsilicate gneiss, channels located along the eastern margin of the valley. The northern banded gray marble, and leucocratic dikes and sills (Fig. 4). The Great part of the valley displays an S-shaped geometry due in large part to Counter Thrust is exposed in the hanging wall of the Lopukangri fault the range-bounding fault changing to a more northwesterly strike, system. As discussed later, we interpret metamorphic rock units and the active basin-bounding fault switching to the western margin. exposed in the southern portion of the Lopukangri footwall as the This change in fault geometry implies a polarity change to an east- metamorphic equivalent to the rock units involved in the Great dipping geometry and the development of an accommodation zone Counter Thrust system exposed in the hanging wall. Brittle deforma- between the southern and northern portions of the valley. Addition- tion within the Lopukangri fault system is distributed over a 4-km ally, the northern valley becomes wider at this location with a valley wide zone. Brittle faults synthetic to the Lopukangri fault strike NNE width of approximately 12 km. The northern extent of the Lopukangri and dip moderately to the west. The maximum density of striation fault is unknown. measurements on these faults is N60W/48 (Fig. 2). Brittle faults antithetic to the Lopukangri fault strike NE and dip moderately to the 2.3. Active faulting east. The maximum density of striation measurements on these faults is S43E/55. Mineral stretching lineations along the Lopukangri fault North of the India–Asia suture zone, the strike of the Lopukangri are defined by elongated quartz and feldspar porphyroclasts and yield fault system curves to the northwest (Fig. 5). Normal and right-slip a mean slip direction of N36W/40 (Fig. 2). faults form the eastern margin of a crescent-shaped basin that drains The southernmost portion of the mapped area consists of northward. Three generations of alluvium deposits were interpreted interlayered buff medium-grained sandstone, gray phyllite, and from ASTER imagery and reconnaissance field mapping and are slate. The phyllite contains abundant foliation parallel quartz-filled denoted as oldest, younger and youngest based on reflectance veins. To the north, these rock units lie structurally above a N1.5-km characteristics (Fig. 5). The youngest deposits have higher reflectance, thick sequence of interlayered calcsilicate rocks and slate which are in appearing bright yellow to white, whereas the oldest deposits have fault contact with a tabular granite body that is mappable throughout lower reflectance appearing off-white to buff yellow. The youngest much of the Lopukangri massif. We correlate the interlayered buff alluvium deposits and streams are right-laterally offset throughout medium-grained sandstone and gray phyllite unit to the Upper the eastern margin of the valley, thus characterizing an active margin Triassic Xiukang Group exposed in the hanging wall of the Lopukangri compared to the western side of the valley where offsets are more rift in the northwestern portion of the mapped area (TBGMR, 1982, subtle. As interpreted by fault scarps in the most reflective deposits, 1992). Based on composition and structural position we interpret the younger en echelon fault segments appear to cut alluvium deposits by interlayered calcsilicate and shale unit to represent the metamor- stepping basinward (Fig. 6). The fault segments trend N to N20W and phosed equivalent of Xigaze Group rocks exposed in the Great range in length from a few kilometers to 10s of kilometers. Counter Thrust in the northwest portion of the mapped area. Prominent fault scarps and deflected stream channels suggest the We used ASTER (Advanced Spaceborne Thermal Emission and Lopukangri fault system is an active structure. Along strike from the Reflection Radiometer) and the three arc-second Shuttle Radar south, three active to recently active strands of the Lopukangri fault Topography Mission digital elevation model (SRTM DEM 90 m system were identified based on offset terrace risers and deflected resolution) data to extend the ground controlled mapping shown in stream channels (Fig. 6). We interpret the disrupted Quaternary Fig. 2 to the north (Fig. 5). Our focus was to evaluate the spatial alluvium at the base of the range, as a range front fault scarp that is the pattern and style of active deformation, as well as the geomorphology northern extension of the Lopukangri fault system mapped to the

Fig. 4. Photograph looking east across the Lopukangri fault system showing amphibolite-grade metamorphic rocks and Cretaceous–Tertiary granitoids exposed in its footwall and Tethyan sedimentary rocks (TSS) exposed in its hanging wall.

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Fig. 5. Map of the Lopukangri rift showing faults and Quaternary surficial units mapped in the field and interpreted from Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) scenes. To the east and west are structures we correlate to the Lopukangri rift based on fault geometry and basin morphology. south. A prominent E-trending terrace riser in the foreground of Fig. 6 northward tilting of the basin floor could result in erosion and (offset 2) is truncated at the range front when projected upstream to northward migration of the northern terrace riser producing the the east. We interpret the catchment most likely related to the apparent dextral offset from the point source of the active west development of this terrace riser, as the present day catchment for the flowing stream. An additional fault strand is located to the west and active west flowing stream channel. If this correlation is correct it further into the basin as indicated by subtle fault scarps observed in implies a dextral fault separation of 850±150 m. Alternatively, the ASTER imagery (Fig. 6,offset1).Thedeflection of the

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Fig. 6. Grayscale ASTER scene (bands 3–2–1) of the northern portion of the Lopukangri rift showing offset geomorphic features (1,2, and 3) that indicate right separation and westside-down dip-separation. White arrows indicate offset features. Black arrows show relative motion of the offset geomorphic features across the interpreted faults. See Fig. 5 for location. southernmost stream (offset 3) may further indicate ∼480±100 m of The geometric relationship between the Great Counter Thrust and right-lateral separation. The uncertainties stated are based on the the Lopukangri fault system is determined by our field mapping. The differences in reconstructing offsets of the stream system. We best-fit plane to fault plane measurements of the southernmost thrust interpret the kinematics for this fault strand as dominated by in the Great Counter Thrust on the westside of the Lopukangri fault is apparent right-slip motion, as indicated by the dextral deflections of N60W/55SW. We assume the Great Counter Thrust fault zone on the the stream channels and the subtlety of the fault scarps. east side of the Lopukangri fault zone has the same orientation. The The geomorphic relationships described above suggest ∼850 m of mean orientation of the Lopukangri fault system where it intersects right separation along the easternmost fault strand and ∼500 m of with the Great Counter Thrust is N09E/45NW. The best-fit plane of the right separation on the westernmost strand, resulting in a cumulative Great Counter Thrust is projected onto this plane and restored in a right-lateral separation of 1.35 km. Because the westside of the active direction parallel to the mean slip direction on the Lopukangri fault faults lie within the basin, we infer that they have a westside-down system as defined by ductile shear sense indicators (N36W). Using dip-slip component. The inferred fault kinematics is consistent with this method we calculate 6.9 km of right-slip, 8.2 km of normal dip- that observed along the Lopukangri fault system in the mapped area slip and 10.6 km of net slip. This slip estimate results in ∼6kmof to the south. horizontal extension in a direction oriented N36W.

2.4. Magnitude of slip 3. Discussion

We estimate the net slip on the Lopukangri fault system by 3.1. Lateral extent of the Karakoram fault restoring the Great Counter Thrust across the fault system (Fig. 2). We make three general assumptions in our restoration: (1) The total The mapping presented here shows that strike-slip faults do not offset of the Great Counter Thrust is a result solely of movement along occur along the India–Asia suture zone in the study area (84°30′E). the Lopukangri fault system, (2) The Lopukangri fault system has Rather, our observations show that the India–Asia suture zone in the maintained a constant slip direction throughout its evolution, and (3) vicinity of Lopukangri is deﬁned by a system of thrusts that correlate The Great Counter Thrust was planar and continuous across the with respect to tectonic position, geometry, and kinematics to the Lopukangri area prior to slip on the Lopukangri fault system. Great Counter Thrust system as mapped in southeastern and

8 M.A. Murphy et al. / Earth and Planetary Science Letters xxx (2010) xxx–xxx southwestern Tibet (Ratschbacher et al., 1994; Yin et al., 1999; eastward translation. As will be discussed in more detail later, the Murphy et al., 2009). Fault kinematics from the thrust system show observation that the Karakoram fault does not extend along the entire NNE-directed thrusting, rather than east–west right-lateral shear as length of southern Tibet (Lhasa block) suggests that strain is better predicted by models advocating eastward lateral extrusion of the described as stretching/lengthening towards the east from a fixed Tibetan plateau (Fig. 7A) (e.g. Peltzer and Tapponnier, 1988; position on the immediate west side of the Karakoram fault. This Tapponnier et al., 2001). Since the thrust system is cut by the could support a strain partitioning model such as that proposed by Lopukangri fault, the suture zone in the study area is not a site of McCaffrey and Nabelek (1998) and Styron et al. (2009) which predicts active faulting. the magnitude of strike-slip to increase westward from the central Because the Karakoram fault does not extend east to Lopukangri Himalaya due to oblique convergence along the arcuate Himalayan (84°30′E) southern Tibet (Lhasa block) could not have undergone thrust belt. wholesale eastward lateral translation/extrusion (Tapponnier et al., This result also impacts boundary conditions used in kinematic 1982; Replumaz and Tapponnier, 2003). Lateral extrusion requires models that fit GPS velocity data (Chen et al., 2004; Thatcher, 2007; coeval movement on conjugate fault systems that extend along the Meade, 2007). Chen et al. (2004) and Meade (2007) define the India– entire length of the region being extruded resulting in translation of Asia suture zone in western and south-central Tibet as a boundary to a the block with no internal deformation (Fig. 7A). Extrusion models block that reproduces the kinematics from GPS observations. call upon the left-slip Altyn Tagh fault and right-slip Karakoram fault Interestingly, Thatcher (2007) also fit the kinematics from GPS to facilitate eastward extrusion of the Tibetan crust. Because the observations without using the suture as a boundary in this part of Karakoram fault only extends as far east as Gong Co, only regions to the orogen. Although we think that the boundaries used by Thatcher the west (westernmost Tibet) could have undergone wholesale (2007) may be more accurate, the ability to fit the GPS observations in

Fig. 7. End-member models describing crustal deformation within the Tibetan plateau. Areas in gray represent that which is deforming in each of the models. Black faults are those operating within the context of the models and gray faults are directly related to the deformation prescribed. A. Wholesale lateral extrusion model in which eastward translation of Tibetan crust occurs via coeval movement along the Karakoram and Altyn Tagh faults (Tapponnier et al., 1982; Peltzer and Tapponnier, 1988). This model requires that the Karakoram fault extends across southern Tibet. B. Pure-shear stretching model describes eastward lengthening of Tibetan crust via conjugate strike-slip faulting (Taylor et al., 2003). Faults bordering and within the Tibetan plateau are not laterally extensive resulting in semi-smooth variations in the deformation ﬁeld. Abbreviations: CTCFZ — Central Tibet conjugate fault zone; LK — Lopukangri; LGF — Longmu Co-Gozha Co fault; NATF — Northern Altyn Tagh fault; and QB — Qaidam basin.

M.A. Murphy et al. / Earth and Planetary Science Letters xxx (2010) xxx–xxx 9 both cases raises the possibility that rigid crustal blocks may not be et al., 2009)(Fig. 1). Although the magnitude of slip along these the most accurate representation of active deformation of Tibetan strike-slip faults in Nepal is unknown, we suspect that it is low crust (Flesch and Bendick, 2008). (b10 km) since individual fault segments are short, b10 km (Nakata, Lacassin et al. (2004) and Murphy and Burgess (2006) present 1989; Styron et al., 2009) and a ductile strike-slip shear zone has not interpretations of satellite imagery that suggest the Karakoram fault been observed. Therefore, we think that these faults could not have extends along the India–Asia suture zone to the eastern end of Gong contributed much to eastward extrusion of Tibetan crust. Co (82°19′E). East of this, mountain fronts are sinusoidal, not linear, and no clear geomorphic indication of active faulting is obvious along 3.2. Southern termination of Southern Tibetan (Lhasa block) rifts the suture. This implies that the block boundary along the suture zone used by Replumaz and Tapponnier (2003) and Chen et al. (2004) The Lopukangri fault is one of several structures that lie adjacent to describing long-term and short-term deformation, respectively, are the India–Asia suture zone between 83°45′E and 85°45′E(∼200 km) inaccurate. Murphy and Burgess (2006) show that an active right- (Fig. 7). We have identiﬁed 3 structures to the east of Lopukangri and lateral fault lies along the northern margin of the Gurla Mandhata 2 structures to its west that can be correlated to the Lopukangri fault massif, ∼35 km to the south of the India–Asia suture zone and may be based on geometry, dimensions, and geologic and geomorphic linked to the Karakoram fault via a right-stepover daylighting along characteristics. They are located within the Gangdese shan and the eastern end of Mapam Yumco (Fig. 1)(Lacassin et al., 2004). The involve Gangdese arc rocks. These structures are en echelon and eastward continuation of this active right-slip fault is interpreted to individually have a characteristic curviplanar mapview geometry link to another right-stepover structure transferring strike-slip (concave towards the WSW) with basins at their southeastern end. deformation into the Himalayan thrust belt in western Nepal and Their northwestern ends are not associated with basins nor localized possibly linking with the active right-slip Tibrikot fault in the Dolpo surface uplifts. As discussed in detail below, this can be explained by region of Nepal (Nakata, 1989; Murphy and Copeland, 2005; Styron either a decreasing displacement northward along strike and

Fig. 8. Kinematic interpretations describing the function of the Lopukangri-like faults. A. Extensional and strike-slip fault systems in southern and western Tibet. Light gray colored regions represent portions of southern Tibet with little internal deformation. The Lunggar rift is interpreted to form the boundary between two “rigid” blocks. The southern portion of the boundary near Lopukangri that encloses the smaller rift systems is interpreted to be a zone of deformation. The dashed box indicates the area represented in the models that follow. B. The trailing extensional imbricate fan model shows the Lopukangri fault system as one of several oblique extensional faults that the Lunggar rift terminates into. This model predicts that the magnitude of extension gradually decreases from north to south. C. The crustal tear model shows the southern en echelon extensional system to terminate to the north and south. The predicted extension pattern consists of two extensional highs separated by very little extension akin to a crustal scale accommodation zone.

10 M.A. Murphy et al. / Earth and Planetary Science Letters xxx (2010) xxx–xxx terminating at their northern ends or that the northwestern portion is 3.2.2. Northward and southward tapering tear dominated by strike-slip displacement (Fig. 9). All structures display a Alternatively, the extensional system may not be linked to faults relatively short-length (b95 km) compared to rifts to the north (Fig. within southern Tibet as proposed above. Rather, individual faults 9). The system encompasses a zone that is ∼200 km long (east–west) may terminate at both their southern and northern ends indicating and 95 km wide (north–south). Yin (2000) also recognized this that they represent a group of crustal tears (Fig. 9C). The extension system of short rifts and notes that they have a characteristic spacing pattern implied by this kinematic interpretation is comprised of a of 46±7 km. Applying the extension estimate calculated for the northern zone with high extension and a southern zone with lesser Lopukangri fault to the entire system results in 18% horizontal extension. Extension is small between the fault tip of the northern extension. We consider this a maximum value since Lopukangri is the zone and the fault tip of the southern zone. longest of these structures and presumably has the largest displacement. A rapid cooling event beginning at 8±1 Ma is recognized in the footwall of the easternmost structure bounding Daga Co (Harrison et 3.3. Implications for continental tectonics al., 1994) suggesting that exhumation enhanced by normal faulting at this locality and possibly the entire extensional system was underway Both kinematic interpretations, Trailing extensional imbricate fan in the Late Miocene. and crustal tear, imply that east–west stretching and north–south The Lopukangri fault cuts the India–Asia suture zone, indicating shortening resulting from conjugate faulting is restricted to more that it does not link with strike-slip faults along the suture zone. In central regions of the Tibetan plateau (Fig. 7B), an interpretation fact, no faults were identified along the southern end of the supported by analysis of the eastward component of the GPS-derived Lopukangri fault in which it could have merged into thus suggesting velocity field (Taylor and Yin, 2009) using data presented in Zhang that the fault terminates/tips out. All other faults in the system et al. (2004). If the models described above are applicable to the terminate north of the India–Asia suture zone. The observation that northern Tibetan plateau, then eastward stretching of the Tibetan extensional fault systems in south-central Tibet terminate rather than plateau would show a maximum centered on the Bangong–Nujiang merge with other structures indicates that the magnitude of extension suture zone and decrease smoothly to zero southwards to the India– in southern Tibet is variable with low magnitudes in the southern Asia suture zone and northwards to the Jinsha suture zone. The crustal portion (this study) and highest magnitudes in the north along the tear model predicts this pattern is more localized in central Tibet than Bangong–Nujiang suture zone (Taylor et al., 2003; Kapp et al., 2008) the trailing extensional imbricate model. Both interpretations imply (Fig. 9). Below we propose two explanations for the regional that the interior of the Tibetan plateau is undergoing pure-shear kinematics of the extensional system described above. deformation, albeit at a regional scale (Figs. 7B and 9). Locally, our results from southern Tibet show that a discrete structural boundary has not existed between the southern Lhasa block 3.2.1. Trailing extensional imbricate fan and Himalaya possibly since the Late Miocene. Rather, the extensional All graben in the zone described above display a more east–west system described here indicates that the boundary is broad (95 km strike along their northern margins than their southern margins, wide) and that extensional strain tapers at the southern end to zero giving rise to a curviplanar mapview geometry that is concave resulting in a semi-smooth gradient. This implies that crustal towards the WSW. At Lopukangri, the rift valley along the west- deformation between southern Tibet and the Himalayas is best trending northern segment is very narrow suggesting little if any described as a continuum. Conversely, the structural boundary extension. Resolving the kinematics from the southern segment of the between western Tibet and the Himalayas, which is defined by the Lopukangri fault on to this northern, west-trending segment predicts Karakoram fault, is presently a discrete boundary with a large step-like that it is dominated by right-slip. We interpret this fault system increase in the magnitude of displacement across the fault (Murphy geometry and the inferred kinematics to represent a trailing and Copeland, 2005). This structural boundary has probably operated extensional imbricate fan (Fig. 9B). Locally this is supported by the in this fashion since the Middle Miocene (Phillip et al., 2004; Valli et al., significant right-slip component observed along the Lopukangri fault. 2007). We think this variation in the displacement gradient and age of We interpret that the western end of the imbricate fan merges with these structural boundaries within the interior of Tibet is best the Lunggar rift (Kapp et al., 2008). explained by the fault patterns and strain history highlighted in Fig. 8.

Fig. 9. Topography of south-central Tibet generated from 90 meter resolution Shuttle Radar Topography Mission data (SRTM). Black indicates topographically high regions. Light- colored basins (features A–F) embedded within the high topography are rifts that we suggest are broadly similar to the Lopukangri rift geometry, and geologic and geomorphic characteristics suggesting a genetic relationship.

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4. Conclusions England, P., Molnar, P., 2005. Late Quaternary to decadal velocity fields in Asia. J. Geophys. Res. 110, B12401. doi:10.1029/2004JB003541. Flesch, L.M., Bendick, R., 2008. A comment on “Present-day kinematics at the India–Asia Our study of the deformation style along the India–Asia suture collision zone”. Geology 36, e160. doi:10.1130/G24443C.1. zone in the Lopukangri area yields the following results: Harrison, T.M., Copeland, P., Kidd, W.S.F., Lovera, O.M., 1994. Activation of the Nyainqentanghla shear zone: implications for uplift of the southern Tibetan plateau. Tectonics 14, 658–676. 1) The India–Asia suture zone at 84°30′Eisdefined by a NNE-directed Kapp, P., Taylor, M., Stockli, D., Ding, L., 2008. Development of active low-angle normal imbricate thrust belt (Great Counter Thrust), rather than strike- faults systems during orogenic collapse. Geology 36, 7–10. slip deformation as hypothesized by models calling upon whole- Lacassin, R., Valli, F., Arnaud, N., Leloup, P.H., Paquette, J.L., Li, H., Tapponnier, P., Chevalier, M.-L., Guillot, S., Maheo, G., Xu, Z., 2004. Large-scale geometry and offset sale lateral extrusion of the Tibetan plateau during convergence of the Karakoram fault, Tibet. Earth Planet. Sci. Lett. 219, 255–269. between the Indian subcontinent and Asia. Liu, Z.Q., 1988. Geologic Map of the Qinghai-Xizang Plateau and Its Neighboring Regions 2) The suture zone is cut by an oblique normal fault referred to here (Scale at 1:1,500,000), Chengdu Inst. of Geol. and Miner. Resour., Geol. Publ. House, Beijing, 1988. as the Lopukangri fault system. This observation shows that the McCaffrey, R., Nabelek, J., 1998. Role of oblique convergence in the active deformation suture zone is not a site of active faulting in the vicinity of the of the Himalayas and southern Tibet plateau. Geology 26, 691–694. study area. Meade, B.J., 2007. Present-day kinematics at the India–Asia collision zone. Geology 35, 81–84. 3) The slip direction along the Lopukangri fault is NW. We estimate Murphy, M.A., Burgess, W.P., 2006. Geometry, kinematics, and landscape characteristics ∼10 km of net slip (6 km of horizontal extension) along the of an active transtension zone, Karakoram fault, Southwest Tibet. J. Struct. Geol. 28, Lopukangri fault system based on a piercing line defined by 268–283. Murphy, M., Copeland, P., 2005. Transtensional deformation in the central Himalaya components of the imbricate thrust belt. and its role in accommodating growth of the Himalayan orogen. Tectonics 24. 4) The mapview curviplanar geometry of the Lopukangri fault system Murphy, M.A., Saylor, J.E., Ling, D., 2009. Landscape evolution of southwest Tibet based is similar to the fault geometry to its east and west. These NNW- on integrated paleoelevation reconstructions and structural history. Earth Planet. – striking extensional faults are en echelon and encompass a region Sci. Lett. 282, 1 9. Murphy, M.A., Yin, A., Kapp, P., Harrison, T.M., Lin, D., Guo, J.H., 2000. Southward that is 200 km long (east–west) and 95 km wide (north–south). propagation of the Karakoram fault system, southwest Tibet: timing and All faults in the system terminate in the south, north of the suture magnitude of slip. Geology 28, 451–454. zone, with the exception of the Lopukangri fault which terminates Murphy, M.A., Yin, A., Kapp, P., Harrison, T.M., Manning, C., Ryerson, F., Ding, L., Guo, J., ∼ – 2002. Structural evolution of the Gurla Mandhata detachment system, southwest 20 km south of the India Asia suture zone. This observation Tibet: implications for the eastward extent of the Karakoram fault system. Geol. indicates that the magnitude of extension in southern Tibet (Lhasa Soc. Amer. Bull. 114, 428-447. block) is variable, with higher extension magnitudes in the north Nakata, T., 1989. Active faults of the Himalaya of India and Nepal. In: Malinconico, L.L., Lillie, R.J. (Eds.), Tectonics of the Western Himalaya: Geol. Soc. Amer. Special Paper, than the south. vol. 232, pp. 243–264. 5) We propose that the extensional system described in this Peltzer, G., Tapponnier, P., 1988. Formation and evolution of strike-slip faults, rifts, and contribution represents a trailing extensional imbricate fan basins during the India–Asia collision: an experimental approach. J. Geophys. Res. 93, 15,085–15,117. which is linked to the north and west to faults within the central Phillip, R.J., Parrish, R.R., Searle, M.P., 2004. Age constraints on ductile deformation and Tibet conjugate fault zone. Alternatively, the extensional system long-term slip rates along the Karakoram fault zone, Ladakh. Earth Planet. Sci. Lett. may terminate to the north and represent a group of northward 226, 305–319. Ratschbacher, L., Frisch, W., Liu, G., Chen, C., 1994. Distributed deformation in southern and southward crustal tears. and western Tibet during and after the India–Asian collision. J. Geophys. Res. 99, 6) Applying our kinematic interpretation to the northern Tibetan 19,917–19,945. plateau, implies that eastward stretching of the Tibetan plateau Replumaz, A., Tapponnier, P., 2003. Reconstruction of the deformed collision zone would show a maximum centered on the Bangong–Nujiang suture between India and Asia by backward motion of lithospheric blocks. J. Geophys. Res. 108. doi:10.1029/2001JB000661. zone and decrease smoothly to zero southwards to the India–Asia Robinson, A.C., 2009. Geologic offsets across the northern Karakorum fault: implications suture zone and northwards to the Jinsha suture zone. for its role and terrane correlation in the western Himalayan–Tibetan orogen. Earth – 7) Our results from southern Tibet show that deformation between Planet. Sci. Lett. 279, 123 130. Styron, R., Taylor, M., Murphy, M., 2009. Himalayan Orogen-parallel Extension from southern Tibet and the Himalayas is broad (90 km wide) and best GPS Geodesy and Structural Geology. The 5th International Symposium on Tibetan described as a continuum possibly since the Late Miocene. Plateau/The 24th Himalaya–Karakorum–Tibet Workshop, Beijing, China. August – Conversely, the structural boundary between western Tibet and 11 14. fi Tapponnier, P., Peltzer, G., Le Dain, A.Y., Armijo, R., Cobbold, P., 1982. Propagating the Himalayas, which is de ned by the Karakoram fault, is extrusion tectonics in Asia: new insights from simple experiments with plasticine. presently a discrete boundary, and probably has been since the Geology 10, 410–415. Middle Miocene. We think this variation in the displacement Tapponnier, P., Zhiqin, X., Roger, F., Meyer, B., Arnaud, N., Wittlinger, G., Jingsui, Y., 2001. Oblique stepwise rise and growth of the Tibet Plateau. Science 294, 1671–1677. gradient and age of these structural boundaries within the interior Taylor, M.H., Yin, A., 2009. Active structures of the Himalayan–Tibetan orogen and their of Tibet is best explained by the fault patterns and strain history relationships to earthquake, contemporary strain field, and Cenozoic volcanism. describing wholesale E–W stretching and N–S shortening of the Geosphere 5, 199–214. Taylor, M.H., Yin, A., Ryerson, R., Kapp, P., Ding, L., 2003. Conjugate strike-slip faulting Tibetan crust. along the Bangong–Nujiang suture zone accommodates coeval east–west extension and north–south shortening in the interior of the Tibetan Plateau. Tectonics 22, 22. TBGMR (Tibetan Bureau of Geology and Mineral Resources), 1982. Regional Geology of Acknowledgments Xizang (Tibet) Autonomous Region: Beijing, Geological Publishing House, Geological Memoirs, ser. 1, no. 31, 707 p. (in Chinese with English summary). This study was supported by a grant from the U.S. National Science TBGMR (Tibetan Bureau of Geology and Mineral Resources), 1992. Geologic Map of Xizang Autonomous Region: Beijing, Geological Publishing House, scale 1:500 000. Foundation (grant EAR 0711527 to Murphy and EAR 0809408 to Thatcher, W., 2007. Microplate model for the present-day deformation of Tibet. Taylor). The clarity of this manuscript benefited from constructive J. Geophys. Res. 112, B01401. doi:10.1029/2005JB004244. reviews by Alex Webb and an anonymous reviewer. Valli, F., Arnaud, N., Leloup, P.H., Sobel, E.R., Mahéo, G., Lacassin, R., Li, H., Tapponnier, P., Xu, Z., 2007. Twenty million years of continuous deformation along the Karkorum fault, western Tibet: a thermochronologic analysis. Tectonics 26. doi:10.1029/ References 2005TC001913. Yin, A., 2000. Mode of Cenozoic east–west extension in Tibet suggests a common origin Armijo, R., Tapponnier, P., Han, T., 1989. 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