Earth and Planetary Science Letters 325–326 (2012) 10–20

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Earth and Planetary Science Letters

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E–W extension at 19 Ma in the Kung Co area, S. Tibet: Evidence for contemporaneous E–W and N–S extension in the Himalayan orogen

Mayumi Mitsuishi a,⁎, Simon R. Wallis a, Mutsuki Aoya b, Jeffrey Lee c, Yu Wang d a Department of Earth & Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan b Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology (AIST), Central 7, Tsukuba 305-8567, Japan c Department of Geological Sciences, Central Washington University, Ellensburg, WA 98926, USA d Geologic Labs Center, China University of Geosciences, Beijing 100083, China article info abstract

Article history: Active faulting in southern Tibet consists of N–S trending extensional faults and linked strike-slip faults, Received 5 March 2011 which are an expression of regional E–W extension. A second type of extensional deformation associated Received in revised form 5 November 2011 with N–S movement is also recognized. This extension is expressed as a series of shear zones and normal Accepted 7 November 2011 faults in the High Himalayas – the Southern Tibetan Detachment System – and mid-crustal rocks exposed Available online xxxx in metamorphic domes. Reported constraints on the timing of movements associated with these two phases – – Editor: T.M. Harrison of extension indicate that N S extension predates the onset of E W extension. However, only a few studies have provided clear constraints on the timing of E–W extension and the extent to which the two kinemati- Keywords: cally distinct domains of extension were contemporaneous is unclear. southern Tibet The Kung Co fault in southern Tibet is a major N–S trending normal fault. The associated E–W extension is lo- normal faulting cally expressed as high-strain ductile deformation. Both field and microstructural observations show that this age of extension deformation occurred synchronously with granite intrusion. Previously reported U–Pb zircon dating shows remote sensing granite crystallization took place at around 19 Ma, implying that ductile E–W extension in the Kung Co area was also active at around 19 Ma. This is the oldest documented example of E–W extension in Tibet and shows that E–W extension was at least locally contemporaneous with N–S extension to the south at shallower crustal levels. Simultaneous mid-crustal N–S extension and upper crustal E–W extension may be explained by southward flow of Tibetan crust with a divergent radial component. © 2011 Published by Elsevier B.V.

1. Introduction (STDS), which emplaced an overlying mainly unmetamorphosed Tethy- an sequence onto metamorphic rocks of the high Himalayas (Burchfiel Active tectonics in the central part of the Tibetan Plateau is domi- et al., 1992; Burg and Chen, 1984). nated by a linked set of NE–SW and NW–SE trending strike-slip faults Age constraints indicate that the main phase of the N–S extension and N–S trending normal faults (Fig. 1)(Blisniuk et al., 2001; Taylor et took place between 23 and 12 Ma (Coleman and Hodges, 1995; al., 2003). The normal faulting acts to reduce the overall height of the Edwards and Harrison, 1997; Guillot et al., 1994; Murphy and Plateau despite the ongoing convergence of India with Asia and to- Harrison, 1999; Searle et al., 1997, 1999; Vannay et al., 2004; gether with the strike-slip faulting reflects a regional E–W extension Viskupic et al., 2005; Walker et al., 1999; Zhang and Guo, 2007). Lee (e.g. Armijo et al., 1986). The onset of normal faulting in central Tibet and Whitehouse (2007) propose that N–S mid crustal deformation in marks a time when the horizontal deviatoric stresses in the plateau the Mabja dome area dated at around 35 Ma is also related to move- changed from compressional to extensional and many workers indicate ments on the STDS, but deformation of a similar age in the Changgo this corresponds closely to the time when maximum elevation was area is interpreted by other workers as related to N–Sshortening obtained in the region (Cook and Royden, 2008; Molnar et al., 1993). (Larson et al., 2010). The onset of E–W extensional faulting is thought In southern Tibet there is also abundant evidence for an earlier phase to be between 17 and 7 Ma (Blisniuk et al., 2001; Cottle et al., 2009; of extension but in a N–S direction. This extension resulted in the forma- Dewane et al., 2006; Edwards and Harrison, 1997; Garzione et al., tion of a major set of E–W striking, north-dipping normal faults and 2003; Hager et al., 2009; Harrison et al., 1995; Hintersberger et al., shear zones known as the Southern Tibetan Detachment System 2010; Lee et al., 2011; Murphy et al., 2002; Stockli et al., 2002; Thiede et al., 2006) or even as young as 4 Ma (Mahéo et al., 2007)andsignif-

⁎ Corresponding author. icantly younger than movements on the STDS. Distinct kinematics and E-mail addresses: [email protected], different age ranges for the two types of extensional structures indicate [email protected] (M. Mitsuishi). that they are related to two distinct tectonic events (e.g. Beaumont et

0012-821X/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.epsl.2011.11.013 M. Mitsuishi et al. / Earth and Planetary Science Letters 325–326 (2012) 10–20 11

Fig. 1. Digital elevation model (Gtopo 30 data) of the Tibetan Plateau and major active faults. Location of faults modified from Blisniuk et al. (2001) and Taylor et al. (2003). Numbers are estimates of initiation age for E–W extension based on emplacement of N–S striking dykes (light-gray filled double triangles; Williams et al., 2001) and interpreted slip along normal faults (dark-gray filled double triangles), for Leo Pargil (Hintersberger et al., 2010; Thiede et al., 2006), Gurla Mandhata (Murphy et al., 2002), Thakkola (Garzione et al., 2003), Tangra Yumco (Dewane et al., 2006), Ama Drime Massif (Stockli et al., 2002), Xainza (Hager et al., 2009), Shuang Hu (Blisniuk et al., 2001), Yadong (Edwards and Harrison, 1997), Nyainqentanglha (Harrison et al., 1995), Gulu (Stockli et al., 2002), and Kung Co (this study) regions. The white-filled double triangles represent the active age for N–S extension associated with South Tibetan Detachment System for Zanskar (Walker et al., 1999), Sutlej (Vannay et al., 2004), Silving (Searle et al., 1999), Thakkola (Coleman and Hodges, 1995), Manaslu (Guillot et al., 1994), Langtang (Searle et al., 1997), Everest (Murphy and Harrison, 1999; Viskupic et al., 2005), Dinggye (Zhang and Guo, 2007), and Yadong (Edwards and Harrison, 1997) regions.

al., 2004; Blisniuk et al., 2001). There is, however, evidence that the two 2. Geology of Kung Co area phases are at least in part contemporaneous. Evidence for relatively young movements on the STDS comes from the Kula granite area. 2.1. Regional setting Here that STDS cuts b12.5 Ma granite, so slip in this area is younger than 12.5 Ma (Edwards and Harrison, 1997). These relationships are The southern boundary domain to the Tibetan Plateau is the confirmed by the presence of the ~12 Ma Wagyela granite structurally Himalayas. Within the Himalayan belt several major north-dipping beneath the STDS which displays a ductile normal slip fabric (Wu et al., thrust systems can be defined. The most prominent of these are the 1998). There is also a report of movement on the STDS as young as b Main Central Thrust (MCT) and Main Boundary Thrust (MBT). The 17.2 ka (Hurtado et al., 2001). The presence of ~18 Ma N–Strending hanging wall of the MCT consists of the High Himalayan metamorphic dykes provides further evidence for contemporaneous N–SandE–W sequence, which, in turn, has an upper boundary defined by a major extension. The age of the dykes partly overlaps with the main docu- north-dipping normal fault system, the STDS, which can be traced mented phase of N–S extension and their orientation suggests that em- roughly 2000 km along the length of the main Himalayan range placement may be related to the onset of E–W extensional stresses (Fig. 2)(Burchfiel et al., 1992). Both the Tethyan Himalayan sequence (Williams et al., 2001). These reports indicate that the two stages of ex- in the hanging wall of the STDS and the Higher Himalayan Sequence tensional deformation may have been contemporaneous and open the are thought to consist mainly of rocks that make up the pre- possibility that they are related. collisional Indian continental margin (e.g. Hodges, 2000; Le Fort, In this contribution, we present evidence for ductile E–W exten- 1996). sion in the Kung Co area of southern Tibet that took place at ~19 Ma Numerous granites are exposed in the Tethys Himalaya (Fig. 2)and and is associated with a major N–S trending normal fault. This ranks radiometric dating indicates that they can be divided into an early pre- with the oldest previously known examples of E–W extension and orogenic group with ages of formation around 560–500 Ma (Lee and is a rare documented example of ductile flow associated with E–W Whitehouse, 2007; Lee et al., 2000; Schärer et al., 1986) and a syn- extension. The revised older age for onset of E–W extension is con- orogenic younger group with ages around 35–10 Ma (Aoya et al., temporaneous with STDS related N–S ductile extension in the same 2005; Kawakami et al., 2007; Larson et al., 2010; Lee and region. This study reveals a close spatial and temporal association be- Whitehouse, 2007; Lee et al., 2006; Quigley et al., 2008; Schärer et al., tween the two extensional domains, which indicates that they are dif- 1986; Zhang et al., 2004a, 2004b). The Kung Co granite intruded ferent manifestations of a single large-scale tectonic event. We around 19 Ma (Lee et al., 2011) and belongs to the younger group. discuss the tectonics in the Miocene Himalayas that could have The Kung Co granite is cut by a N–S trending normal fault with a caused development of simultaneous N–S and E–W extension. very clear geomorphological expression. This normal fault also cuts 12 M. Mitsuishi et al. / Earth and Planetary Science Letters 325–326 (2012) 10–20

Fig. 2. Tectonic map of southern Tibet (modified from Burchfiel et al. (1992)) showing locations and zircon U–Pb ages of granite of the Kangmar dome (Lee et al., 2000; Schärer et al., 1986), Kuday dome (Zhang et al., 2004a, 2004b), Kouwu dome (Zhang et al., 2004a, 2004b), Mabja dome (Lee and Whitehouse, 2007; Lee et al., 2006), Kung Co granite (Lee et al., 2011), Malashan dome (Aoya et al., 2005; Kawakami et al., 2007), and Changgo and Kung Tang domes (Larson et al., 2010). Average stretching directions (−180°bθ≤180°) in gneiss domes are shown relative to the average of the Mabja dome with clockwise angles shown as positive. ITSZ = Indus-Tsangpo Suture Zone; STDS = South Tibetan Detachment System; MCT = Main Central Thrust; MBT = Main Boundary Thrust. Inset shows strains derived from GPS velocities (Allmendinger et al., 2007) and stretching lineation data (Brunel, 1986) plotted on the same DEM image as given in Fig. 1.

Cenozoic calcareous and siliciclastic Tethyan sedimentary units intrud- northwards towards the Indus–Tsangpo Suture Zone and is probably ed by the Kung Co granite and younger Quaternary sediments. continuous across the suture zone connecting with the Daggyai Tso fault making a total distance of about 250 km (Fig. 1). The fault has an 2.2. Lithological units overall NNW–SSE trend but locally segments that trend both N–Sand NW–SE are also present (Figs. 4 & 5). The fault dips to the W and has The Kung Co granite can be broadly classified into coarse-grained a normal displacement as shown by geomorphological features such two-mica granite containing K-feldspar phenocrysts, fine-grained as hanging valleys and triangular facets (Fig. 3e) on the eastern side two-mica granite (Fig. 3a), and aplite (Fig. 3b). Fine- and coarse- and the presence of a sedimentary basin on the western side. A normal grained varieties of granite commonly show gradational contacts. sense of displacement is also shown by the rotation of clasts in gravel Two independent studies report U–Pb zircon dating that has been cut by the fault (Fig. 3f). Slicken surfaces and slickenlines are locally ob- used to estimate the age of granite emplacement. Mahéo et al. served (Fig. 3g) and generally indicate dip slip motion. The master fault (2007) report a discordia lower intercept age of 22.0±2.7 Ma. Lee plane is exposed at the margin of the granite body (Armijo et al., 1986; et al. (2011) present a compatible but more closely constrained Lee et al., 2011; this study) where it dips 50° to 60° W, which is signif- concordia age of 19.06±0.15 Ma (2-sigma) with an MSWD of 1.6 icantly steeper than the slope of the triangular facets. that is based on the weighted average of 207Pb-corrected ages from Two separate studies have used thermo geochronology to investi- rims of zircon grains. gate the movement history of the Kung Co fault (Lee et al., 2011; The dominant lithology around the granite is calc schist. The calc Mahéo et al., 2007). Mahéo et al. (2007) interpret a U–Pb zircon age schist commonly contains cm-scale interlayers of pelitic schist and of 22.0±2.7 Ma as the emplacement age of the Kung Co granite. quartzite (Fig. 3c). A mappable unit of pelitic schist is present in They further interpret mica Ar–Ar and (U,Th)–He apatite dating to re- the southern part of the Kung Co area. The pelitic schist contains veal a period of subsequent rapid cooling lasting to 7.5 Ma. These data porphyroblasts of garnet, staurolite, andalusite and locally also silli- also indicate that there was a subsequent period of slow cooling until manite (Fig. 3d). The presence of andalusite and sillimanite indi- about 4 Ma. Mahéo et al. (2007) suggest that the temperature de- cates relatively high temperatures and low pressure and its crease up to 4 Ma simply reflects cooling of the pluton. They further restricted development adjacent to the granite implies that it note that simple extrapolation of the present-day (U,Th)–He apatite formed as the result of contact metamorphism by granite intrusion. age–height relationships implies an unrealistically high thermal gra- Mappable units of quartzite were not recognized in the field, but dient for the crust in the area at present. To account for this apparent analysis of remote sensing imagery from the north of the study contradiction, Mahéo et al. (2007) propose a phase of rapid faulting area reveals the presence of quartz-rich layers with km-scale folds and associated cooling after the youngest recorded age of 4 Ma. (Fig. 4). These authors do not exclude the possibility of earlier movements on the fault, but earlier movement would have been at relatively 2.3. The Kung Co fault high temperatures and they consider the lack of evidence for ductile deformation to indicate that the initial movement on the Kung Co The active Kung Co fault is one of the N–S trending normal faults fault was related to the post 4 Ma mentioned in the paper. that reflect the regional E–W extension in Tibet (e.g. Armijo et al., In a different study, Lee et al. (2011) utilize a more complete data 1986). The fault can be traced from south of the Kung Co granite set of zircon and apatite (U,Th)–He ages, which when combined with M. Mitsuishi et al. / Earth and Planetary Science Letters 325–326 (2012) 10–20 13

Fig. 3. Photographs showing geological features of the Kung Co lithologies. Kfs = K-feldspar; Qtzite = quartzite. (a) Boundary between coarse-grained two-mica granite with Kfs crystals up to several cm in length and fine-grained two-mica granite. (b) Aplite dykes intruding calc schist. (c) Thin pelitic schist and quartzite interlayers within the calc schist. (d) Andalusite porphyroblasts adjacent to the granite. (e) Granite triangular facets formed by normal faulting. (f) Outcrop photo showing rotated clasts in gravel cut by the Kung Co fault. (g) Two sets of slickenlines on a granite surface. The down-dip set is older. the mica Ar–Ar ages of Mahéo et al. (2007) show that rapid cooling to the east of the Kung Co fault is well exposed and particularly well began between ~12.5 Ma and 10.0 Ma and accelerated at 10 Ma. suited to geological analysis by remote sensing. These results provide strong evidence that there was a major phase The satellite sensor used in the present study is the Advanced of movement on the Kung Co fault at this time. Lee et al. (2011) Spaceborne Thermal Emission and Reflection Radiometer (ASTER) show that the initiation of fault movement was therefore much on the Terra satellite. It has three visible and near infrared bands older than the 4 Ma postulated by Mahéo et al. (2007). Using 2D ther- (VNIR), six shortwave infrared bands (SWIR) and five thermal infra- mal modeling, Lee et al. (2011) show a period of relatively slow red bands (TIR) with spatial resolutions of 15, 30 and 90 m, respec- cooling as micas closed to argon loss at ~15.5 Ma as temperatures tively (Yamaguchi et al., 1998). dropped below ~300 °C (see their Fig. 13). They attributed this slow The granite is leucocratic and contrasts with the much darker sur- cooling episode to the waning stages of thermal reequilibration fol- rounding sedimentary rocks (Fig. 3b, d). Information from the VNIR lowing emplacement of the Kung Co granite. 12.5 Ma–10 Ma there- can, therefore, be used to define the distribution of the Kung Co gran- fore marks a period when there was rapid displacement on the ite (Fig. 4a). The VNIR image of Fig. 4a comprises band 3 in red, band 2 fault, but leaves open the possibility that there were earlier substan- in green, and band 1 in blue. tial movements along the same tectonic boundary. The ASTER data can also be used to help define the distribution of different sedimentary units. The reflection spectra for the low-grade 3. Remote sensing and geological mapping metamorphic rocks of the Tethyan sequence are clearly different from those of the granite, Cenozoic Tethyan sediments, and Quaterna- Satellite images of the Kung Co area were used to complement ry sediments. This distinction is especially clear in the SWIR band field mapping and to produce a geological map of the area. The region ratio images, consisting of band ratios 7/6 in red, 6/5 in green, and 14 M. Mitsuishi et al. / Earth and Planetary Science Letters 325–326 (2012) 10–20

Fig. 4. ASTER satellite images used to help produce a geological map. (a) VNIR false-color image comprised of band 3 in red, band 2 in green, band 1 in blue. (b) SWIR band ratio image comprised of band 7/6 in red, band 6/5 in green, and band 6/4 in blue. (c) Gray-scale image of carbonate index. Carbonate Index (CI)=(band 13)/(band 14) (Ninomiya and Fu, 2002). (d) Gray-scale image of quartz index. Quartz Index (QI)=[(band 11)×(band 11)]/[(band 10)×(band 12)] (Ninomiya and Fu, 2002). The lighter colors correspond to an increase in the indexes for quartz and carbonate. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

6/4 in blue (Fig. 4b). In this image, the granite is red, low-grade meta- A geological map in the Kung Co area was constructed by combin- morphic rocks are blue, and Cenozoic Tethyan sediments and Quater- ing a series of remote sensing images with field observations (Fig. 5). nary sediments show a variety of colors. It is likely that the 7/6 and 6/ The Gyirong–Kangmar thrust is a major fault that extends from 4 band ratios in these images are most strongly influenced by the Kangmar through the Kung Co area and continues to the west (Burg presence of muscovite and chlorite, respectively (Perry, 1993). and Chen, 1984). The distribution of this fault in the study area is The meta-sedimentary rocks around the granite can be divided taken from Burg and Chen (1984). In addition to defining the distri- broadly into two lithologies based on the content of the carbonate bution of the granite body and faults, the ASTER images also show minerals. The main carbonate minerals show major differences in the presence of prominent layers of E–W striking quartz-rich rocks the strength of the reflection spectra for the TIR bands 13 and 14. in the north of the area that are distributed both the east and west Using this feature Ninomiya and Fu (2002) have proposed a carbon- of the Kung Co fault. The large-scale folds in the quartz-rich sedi- ate index: ments indicate high-strain ductile deformation occurred extensively in this area. The lack of major lateral offsets of these marker horizons CarbonateIndexðÞ¼ CI ðÞband13 =ðÞband14 across the fault is compatible with the down slip movement indicated by slickenlines. This index can be used for mapping the amount of the carbonate 4. Ductile deformation minerals. Ninomiya and Fu (2002) also propose an alternative expres- sion to examine the amount of quartz. 4.1. Deformation phases

QuartzIndexðÞ¼ QI ½ðÞband11 ðÞband11 =½ðÞband10 ðÞband12 In the Kung Co area, a strong foliation is developed in the metasedimentary schists surrounding the Kung Co granite. A set of Fig. 4c and d was constructed by using these two mineral indices. structures that formed during a phase of ductile deformation before Areas showing high CI values correlate with the distribution of calcar- development of the dominant schistosity can be defined on the eous rocks and metamorphosed equivalents observed in the field. basis of overprinting structures. These two stages of ductile deforma- Areas showing high QI values correlate with siliciclastic rocks and tion are referred to as D1 and D2 with associated foliations S1 and S2. their metamorphosed equivalents. There are also local bands in the A younger low-strain deformation stage, D3, can also be locally recog- north of the area that show especially high QI values and these repre- nized where post-D2 folds are developed. In general, therefore, al- sent quartz-rich rocks. On the ASTER images, these prominent layers most any tectonic foliation observed in the field can be classified as locally define km-scale folds. The distribution of the Quaternary sed- either S1 or S2. Both foliations can be observed in most outcrops iments is estimated using a combination of topography and digital with a folded S1 that has an axial planar S2 (Fig. 6a, b). The three images. phases of ductile deformation are overprinted by later phases of M. Mitsuishi et al. / Earth and Planetary Science Letters 325–326 (2012) 10–20 15

4.3. D2 deformation

D2 ductile deformation is the most widely developed deformation in the area. In the southern part of the Kung Co area, D2 structures are par- ticularly strongly developed in the pelitic schist. S2 dips steeply to the western quadrant with mean azimuth and dip of 264° and 53°, respec- tively (Fig. 7). The L2 stretching lineation is defined by strain shadows around porphyroblasts and the preferred orientation of elongate mineral grains. L2 plunges NW to SW with mean azimuth and plunge of 255° and 31°, respectively. Later deformation is only locally developed implying that the kinematic axes have not undergone major reorientation. The flow direction shown by the D2 structures therefore implies that D2 duc- tile deformation is associated with a component of E–Wextension. To further examine the kinematics of D2 deformation, the sense of shear was determined using structures developed on a variety of scales: (i) synkinematic rotation of andalusite porphyroblasts (Fig. 6c, d); and (ii) outcrop-scale shear bands and asymmetric lenses (Fig. 6e). The results show a consistent pattern of top to the W to top to the NW shear. D2 ductile deformation and later overprinting brittle normal faulting show similar kinematics. Lee et al. (2011) conclude that the similarity in orientation of the D2 foliation, fault plane, and fractures developed within the granite in this area shows that these structures record a history of progressive deformation during normal faulting from a depth where temperatures were high enough that granite underwent weak ductile deformation to shallow crustal levels where brittle deformation dominated.

5. Relationship between granite intrusion and ductile deformation

The relationship between granite intrusion and different stages of deformation can constrain the timing of the associated tectonic Fig. 5. Geological map of the Kung Co area incorporating field observations and the events. In our field studies we paid particular attention to structures results of remote sensing data analyses (Fig. 4). *1 Location of fault estimated from that could help to determine the relative timing of granite intrusion topographic map and Armijo et al. (1986). *2 Kangmar Gyirong Thrust (position and deformation. Features that were especially helpful include struc- approximate) after Burg and Chen (1984). tures of granitic dykes and contact metamorphic minerals. These fea- tures developed at the same time as the granite intrusion and observation of ductile deformation fabrics in the same region allows brittle fracturing and faulting. Fig. 7 shows stereographic plots of the us to determine if deformation occurred before or after intrusion. structural elements of D1, D2, and the brittle normal faulting. 5.1. Deformation of granitic dykes

4.2. D1 deformation and post-D1 metamorphism The contact zone between the granite and surrounding lithologies contains numerous granitic dykes (Figs. 3b, 8). The dykes commonly The oldest deformation event, D1, is best preserved farthest away crosscut the D1 tectonic fabrics and we saw no evidence that granite from the granite. The associated S1 generally dips at a low angle was affected by D1 deformation. However, granite dykes are locally (Fig. 7). The L1 stretching lineation on S1 is most commonly defined clearly affected by D2 folding (Fig. 8a) and they are also locally by iron oxide streaks associated with pyrite. D1 structures show con- boudinaged subparallel to the S2 foliation (Fig. 8b). These features in- siderable variation in orientation, which is likely to be due to the ef- dicate that granite was intruded after D1 and before or during D2 de- fects of subsequent deformation. formation. Undeformed granite, that crosscuts a well-developed S2 A microstructural criterion for differentiating between S1 and S2 is foliation within the pelitic schist, is also observed (Fig. 8c, d). These provided by porphyroblastic plagioclase, which is widely developed observations imply that granite intrusion continued at least locally in calc-schist units. The plagioclase grains commonly contain a well- after D2 deformation ceased, although it had already began before preserved straight internal foliation mainly defined by graphite and D2 was complete. The period of granite intrusion, therefore, overlaps muscovite inclusions. In samples that contain open D2 microfolds, that of the D2 deformation and the main phase of granite intrusion the trace of the folded S1 defines zigzag patterns due to the presence was at least in part contemporaneous with D2 deformation. Evidence of straight sections of S1 protected by overgrown plagioclase for high T deformation of the granite is given by the presence of (Fig. 6b). This microstructure indicates that D2 post-dates the growth chessboard subgrain microstructures in quartz (Fig. 8e) indicating of the porphyroblastic plagioclase. In addition, the straight internal the granite was deformed under high temperature D2 deformation foliation can be traced continuously into the external S1 implying (500–700 °C) (Passchier and Trouw, 2005). static, post-D1 growth of the host plagioclase. Therefore, the growth of plagioclase porphyroblasts occurred between D1 and D2. This rela- 5.2. Microstructures of contact metamorphic minerals tionship is useful for determining whether an observed foliation is S1 or S2, especially when an S2 overprinting an earlier S1 cannot be di- Contact metamorphism of pelitic schist around the granite body is rectly identified in outcrop. associated with the formation of andalusite and sillimanite. These 16 M. Mitsuishi et al. / Earth and Planetary Science Letters 325–326 (2012) 10–20

Fig. 6. Outcrop photograph and photomicrographs in the Kung Co area. Pl = plagioclase; Qtz = quartz; Bt = biotite; And = andalusite. (a) Representative mesostructures of calc schist showing S1 and S2. (b) Photomicrograph of axial planer S2 in pelitic schist with trace of folded S1; several plagioclase porphyroblasts that contain straight S1 composed by carbonate are indicated (crossed nicols). (c) Photomicrograph of an andalusite porphyroblast in pelitic schist showing the continuation of the external S2 with the rotated internal foliation (crossed nicols). The porphyroblast shows simultaneous extinction when viewed between crossed polars and there is no evidence for recrystallization or fracturing that could account for the observed curvature of the inclusions trails. (d) Enlargement of the boxed part of (c) highlighting the relationships of the porphyroblast and foliation. (e) Outcrop-scale shear bands developed within layers of quartzite interbedded with calc schist.

minerals form markers of contact metamorphism and their micro- most of the andalusite growth was synchronous with D2 deforma- structures can therefore be used to determine the relative timing be- tion. There is no evidence for the presence of andalusite during D1. tween ductile deformation and granite intrusion. Sillimanite is locally formed adjacent to the andalusite (Fig. 8j). Andalusite-bearing pelitic schist is present in the southern part of There is no microstructural evidence that the formation of sillimanite the study area. The porphyroblasts do not contain D1 fabrics and the is associated with the breakdown of muscovite by reaction with relationship with D1 could not be determined unequivocally. Howev- quartz. This implies pressures between 2 and 3.5 kbar and peak tem- er, microstructural studies suggest that S1 affected by D2 folds is lo- peratures between 500 and 650 °C; these P–T estimates are compati- cally preserved in the pressure shadows around andalusite ble with those given by Mahéo et al. (2007) for the contact porphyroblasts (Fig. 8f, g). Andalusite grains that are stretched and metamorphism. The strong alignment of the sillimanite parallel to boudinaged parallel to L2 are also locally well-developed (Fig. 8h, i) S2 indicates that it was affected by D2. Sillimanite growth post- showing pre-D2 or syn-D2 development. More information about dates andalusite formation, so sillimanite also formed during D2 the relationships between andalusite growth and deformation of the deformation. host rock are given by microstructural observations. Many andalusite The above microstructural observations show that contact meta- grains can be divided into an inclusion-free core and a rim with a morphic minerals grew during D2. This confirms the field observa- large numbers of inclusions (Fig. 8f, g). The minerals included in the tions indicating that granite intrusion was synchronous with D2. rim are of similar size and shape to the matrix and show an alignment continuous with the external S2 foliation but curved within the anda- 6. Discussion lusite grain (Figs. 6c, d & 8f, g). The external foliation shows no similar curvature. These observations show that the andalusite overgrows 6.1. Timing of the onset of E–W extensional deformation the S2 foliation whilst rotating indicating synkinematic growth dur- ing non-coaxial D2 deformation. The core regions are commonly in- Field relations, mesoscopic structures, and microstructures show clusion free and the relationship with deformation unclear, but that E–W ductile flow associated with extensional deformation M. Mitsuishi et al. / Earth and Planetary Science Letters 325–326 (2012) 10–20 17

Fig. 7. Stereonet plots showing orientations of D1 and D2 deformation structures, and normal fault structures measured in the outcrop. Geological map of the Kung Co area is based on field observations and image processing of ASTER satellite imagery. The fault slip data are also reported in Lee et al. (2011).

Fig. 8. Structural features of the Kung Co granite and andalusite in the surrounding pelitic schist formed by contact metamorphism. And = andalusite; Sil = sillimanite. (a) Outcrop photo showing granite dyke folded by D2. (b) Outcrop photo showing occurrence of boudinaged granite. (c) Field photo showing undeformed granite, that cross-cuts pelitic schist with a D2 foliation. (d) Field photo showing undeformed granite surrounding a partially detached sliver of pelitic schist that has a D2 foliation. (e) Photomicrograph of the chessboard subgrain microstructure in granite deformed by D2 (crossed nicols). (f) Photomicrograph of andalusite in pelitic schist (open nicol). Folds preserved in the strain shadow of the andalusite can be correlated with D1. The internal foliation of the porphyroblast is continuous with the external S2. (g) Enlargement of boxed part of (f) showing curved S2 within the andalusite porphyroblast and straight S2 outside. (h) Field photo showing boudinaged andalusite stretched parallel to L2. (i) Outcrop photo showing the D2 strain shadows around andalusite. (j) Photomicrograph of aligned sillimanite adjacent to the andalusite (open nicol). 18 M. Mitsuishi et al. / Earth and Planetary Science Letters 325–326 (2012) 10–20 occurred simultaneously with 19 Ma Kung Co granite intrusion in the the Kangmar area, the stretching directions associated with N–S mid Kung Co area. That is, ductile E–W extension was occurring in the crustal flow in the southern Tibetan metamorphic domes show a sim- Kung Co area at around 19 Ma—considerably older than most other ilar radial component to flow (Fig. 2, Table 1). estimates for the onset age of E–W extension. Ductile orogen- We propose, therefore, that southward spreading of Tibetan mid- parallel extension in the Himalayan orogen has also been reported crust has been associated with a radial component of flow since the from the Leo Pargil area at 17–14 Ma (Hintersberger et al., 2010; Miocene around 19 Ma, and this caused the onset of the observed Thiede et al., 2006) and the Ama Drime Massif area at ~13–12 Ma E–W extension at shallower levels (Fig. 9). Our proposal is consistent (Cottle et al., 2009)(Fig. 1). The presence of these similar instances with the observation that E–W extension is better developed in the of E–W ductile extension occurring in the early Miocene evolution south of the Bangong–Nujiang Suture than the north. Divergent crust- of the Himalayan orogen indicates that the results from Kung Co al flow is also seen in the SE of the India–Asia collisional domain to could represent a regionally developed phase of E–W ductile the south of the Sichuan Basin (Zhang et al., 2004a, 2004b). Exten- extension. sional shear zones related to orogen-parallel extension roughly per- pendicular to this flow have been documented (Jolivet et al., 2001) 6.2. Relationship with N–S extension of the Southern Tibetan and these may be analogous to the features we describe here. Detachment System 6.4. Comparison with other models Ductile deformation at mid crustal levels that occurred during the Himalayan orogeny is also reported from the Tethyan Himalayan Orogen-parallel extension due to the underthrusting of a rigid In- metamorphic domes (e.g. Aoya et al., 2006; Kawakami et al., 2007; dian continental margin (DeCelles et al., 2002; Kapp and Guynn, Lee et al., 2004, 2006; Quigley et al., 2008). In the metamorphic 2004) makes a similar prediction that extension will be concentrated domes, the extensional direction is dominantly N–S and generally in southern Tibet. However, this model does not account for the rela- interpreted as linked to the STDS associated with N–S extension tionship between the curvature of the Himalayan arc and extension (Fig. 2). The distinct timing and kinematics of deformation between direction that we highlight here. Large-scale underthrusting of a the N–S extension and E–W extension has meant that the two phases slab of Indian basement beneath Tibet is also likely to result in the de- have been treated as distinct with different tectonic causes (e.g. velopment of a significant surface slope (Jiménez-Munt and Platt, Coleman and Hodges, 1995). However, the age of the ductile E–W 2006); a feature which is not observed. Other models can also ac- flow reported in this contribution from the Kung Co area is slightly count for east–west extension in Tibet such as regional ductile flow older than the b17 Ma estimate for the age of movement on the of the mid crust into a region of weak crust (Cook and Royden, STDS reported from the Qomolangma detachment in the Everest 2008) and large-scale removal of the lithospheric mantle in an overall area (Murphy and Harrison, 1999), which is approximately 100 km north–south compressive regime (Molnar et al., 1993). However, to the south of Kung Co. These observations imply that a tectonic these models do not specifically address the stronger development model is required that incorporates both E–W and N–S flow occurring of extensional structures in the south. Our observations do not ex- at the same time in a relatively narrow region. clude either of these proposals for the regional tectonics. However, The ductile extensional deformation in the Kung Co area is spatially the evidence we present for linked upper crustal and mid-crustal de- closely associated with brittle faulting. The ductile brittle transition is formation does not support the idea that the upper crust can be generally at a level of a few kilometers. The structures developed in the treated as a rigid upper lid to a channelized flow (Beaumont et al., Kung Co area record this transition and suggest that at least the later 2004). We also note that the Kung Co fault shows a marked change part of the ductile deformation occurred at a similar depth. A shallow to very rapid movement at around 10 Ma (Lee et al., 2011), which is crustal level for the D2 deformation is confirmed by the low-pressure es- compatible with an additional tectonic process starting to operate at timates for the concomitant contact metamorphism (b3.5 kbar equiva- this time. lent to b13.2 km for ρ=2.7g/cm3). Contemporaneous N–Sductile extension is recorded in the cores of many of the Tethyan Himalayan domes (Kawakami et al., 2007; Lee et al., 2000, 2004). Pressure estimates 7. Conclusions in the domes show that the N–S ductile extension took place at deeper levels than that observed in Kung Co. This difference indicates that The Kung Co area in southern Tibet contains a major N–S trending there may be a link between structural level and kinematics of deforma- normal fault, associated with ductile deformation that records E–W tion: the deeper levels were associated with N–S flow, whereas the shal- extension with a normal sense of displacement. Field and microstruc- lower levels were associated with E–W flow. A similar change in tural observations show the ductile deformation is synchronous with stretching direction with crustal depth has been documented in the Sew- granite intrusion that occurred at 19 Ma. This is one of the oldest ard Peninsula of Alaska (Amato and Miller, 2004). documented examples of orogen-parallel E–W extension in Tibet. The age of ductile E–W extension reported here overlaps with N–S 6.3. Model to account for contemporaneous N–S and E–W extension ductile flow in the same region that is associated with extensional de- formation along the Southern Tibetan Detachment. Southward Copley and McKenzie (2007) present a model for mid-crustal flow spreading of mid-crust with a radial component can account for the beneath the Tibetan Plateau with spreading of the Tibetan crust southwards over the Indian shield. The Indian Shield is thought to ex- Table 1 tend up to 33°N beneath the Plateau (Jin et al., 1996), and most of the Details of average stretching lineation in gneiss domes. See Fig. 2 for the localities. N =

N–S faulting occurs to the south of this. In this model, gravitational number of data; θ = the azimuth of the vector mean relative to Mabja dome; d0 =95% flow over the Indian Shield is driven by the surface slope – or more confidence interval of the mean direction calculated following the procedure outlined precisely, the gravitational potential energy – of the Himalayan in Masuda et al. (1999).

Orogen. The curvilinear form of the Orogen results in a radial compo- Area N θ d0 References fl nent of ow, which causes orogen-parallel extension that could be re- Kangmar 109 +8.7° 9.0 Lee et al. (2000) lated to the E–W extension of southern Tibet. Recent divergent Kampa 66 −16.5° 11.5 Quigley et al. (2008) movement is shown clearly in strain patterns derived from GPS data Mabja 143 0° 5.0 Lee et al. (2004) (Allmendinger et al., 2007) and also in the pattern of stretching line- Kung Co 31 −94.9° 17.8 This study Malashan 56 +17.8° 14.8 Aoya et al. (2006) ations in the Himalayas (Brunel, 1986)(Fig. 2). With the exception of M. Mitsuishi et al. / Earth and Planetary Science Letters 325–326 (2012) 10–20 19

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Neotectonics of the Thakkhola graben This study was financially supported in part by the Grants-in-aid and implications for recent activity on the South Tibetan fault system in the central – fi Nepal Himalaya. Geol. Soc. Am. Bull. 113, 222 240. for Scienti c Research grants 13573011 and 20403014 awarded to Jiménez-Munt, I., Platt, J.P., 2006. Influence of mantle dynamics on the topographic SW and a grant from Central Washington University awarded to JL. evolution of the Tibetan Plateau: results from numerical modeling. Tectonics 25, We thank Y. Yamaguchi for advice on ASTER remote sensing analysis. TC6002. doi:10.1029/2006TC001963. Jin, Y., McNutt, M.K., Zhu, Y.S., 1996. Mapping the descent of Indian and Eurasian plates The authors are also grateful to the Earth Remote Sensing Data Anal- beneath the Tibetan Plateau from gravity anomalies. J. Geophys. 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