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_Journal of Structural Geology 32 (2010) 70–85 Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com locate jsg Middle crustal ductile deformation patterns in southern Tibet: Insights from vorticity studies in Mabja Dome, Jackie Langille a,*, Jeffrey Lee a, Bradley Hacker b, Gareth Sewardb a Department of Geological Sciences, Central Washington University, Ellensburg, WA 98926, USA b Department of Earth Science, University of California, Santa Barbara, CA 93106, USA article info Article history: Received 24 November 2008 Received in revised form 13 June 2009 Accepted 9 August 2009 Available online 14 August 2009 Keywords: Channel-?ow Himalaya Mabja Dome Microstructures Middle crust Tibet Vorticity abstract Kinematic, kinematic vorticity (Wm), and deformation-temperature analyses were performed to test the hypothesis that mid-crustal rocks exposed in Mabja Dome, southern Tibet, were penetratively deformed within a southward-?owing mid-crustal channel during the late Eocene early Oligocene to early Miocene. Outcrop and thin-section kinematic indicators show a downward transition from mixed top-N and top-S shear in chloritoidand garnet-zone rocks, through dominantly top-S shear in garnetand kyanite-zone rocks, to solely top-S shear in staurolite-zone and deeper rocks. Along mineral elongation lineation-parallel transects, Wm in schists and orthogneisses decreases with structural depth from w0.80 (w40_ pure shear) to w0.55 (w63_ pure shear). Deformation temperature increases from w450 x14C in the chloritoid-zone to >700 x14C in the sillimanite-zone, coincident with peak metamorphic temperatures, indicating that Wm was recorded during peak metamorphism. These mid-crustal rocks thus exhibit deformational patterns characterized by: (1) locally opposing shear sense suggesting bulk pure shear at moderate structural depths; (2) a broad top-S shear zone above the Main Central Thrust; and (3) increasing pure shear with structural depth, suggesting an increase in lithostatic load. Our results from mid-crustal rocks exposed in the core of Mabja Dome yield patterns of ductile deformation in southern Tibet that de?ne non-ideal channel ?ow. ? 2009 Elsevier Ltd. All rights reserved. 1. Introduction The Himalayan orogen records Eocene to Holocene continental collision and convergence between the Indian and the Eurasian plates. Profound crustal shortening and thickening formed one of the most impressive orogenic belts on Earth: the Himalaya, with a length of w2500 km and 14 peaks over 8000 m in elevation, and the Tibetan Plateau, Earth’s largest plateau, which covers >5 ? 106 km2 and has an average elevation of w5000 m (Fielding et al., 1994). Extensive geologic and geophysical research over the last 15–20 years has focused on characterizing: (1) the development and outward growth of the Tibetan Plateau (e.g. Grujic et al., 1996, 2002; Vannay and Grasemann, 1998; Grasemann et al., 1999; Hodges et al., 2001); (2) the development of partial melt zones interpreted to reside in the present-day middle crust of Tibet (e.g. Nelson et al., 1996); (3) the development of structures along the southern margin of the plateau, including the broadly coeval South * Corresponding author at: Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA. E-mail address: jlangill@utk.edu (J. Langille). 0191-8141 $ – see front matter ? 2009 Elsevier Ltd. All rights reserved. doi:10.1016 j.jsg.2009.08.009 Tibetan Detachment System (STDS) and Main Central Thrust (MCT) that bound the high-grade Greater Himalayan sequence (GHS) (Fig. 1) (e.g. Grujic et al., 1996, 2002; Vannay and Grasemann, 1998; Grasemann et al., 1999); (4) southward extrusion of the GHS (e.g. Grujic et al., 1996, 2002; Vannay and Grasemann, 1998; Grasemann et al., 1999; Beaumont et al., 2001, 2004, 2006); and (5) focused erosion along the southern margin of the plateau (e.g. Burbank et al., 1996; Beaumont et al., 2001; Hodges et al., 2001). In aggregate, the results from these studies formed the foundation of the channel-?ow hypothesis (e.g. Beaumont et al., 2001; Hodges et al., 2001) (Fig. 2). For example, Grujic et al. (1996) used quartz microfabrics from Bhutan to demonstrate general non-coaxial ?ow of the GHS, and postulated that the GHS deformed as a wedge between the MCT and STDS; later, Grujic et al. (2002) reformulated this wedge model by postulating that the GHS deformed as a 10–15 km thick channel that extends >200 km northward beneath Tibet. Vannay and others (i.e. Vannay and Grasemann, 1998, 2001; Grasemann et al., 1999) used a combination of spatially varying kinematic vorticity numbers (de?ned below) and deformation temperatures, metamorphic pressure temperature (P–T) conditions associated with inverted isograds, and mica 40Ar 39Ar cooling ages from the base of the GHS in the Sutlej Valley to reach similar conclusions. J. Langille et al. Journal of Structural Geology 32 (2010) 70–85 71 N 85°E Indus-Tsangpo suture Gyirong-Kangmar Gangdise batholith Thrust 90°E Malashan Mabja LHS 70°E 40°N STDS MCT Everest 100°E GHS Kampa Tibet Subhimalayan Sequence 20°N India 500 km Tethys Himalaya Kangmar MBT 26°N kilometers 0 100 Fig. 1. Regional tectonic map of the south-central Himalayan orogen. Leucogranites (black) and highand low-grade metamorphic rocks (dark gray) of the North Himalayan gneiss domes, including Mabja Dome, are shown with respect to major geologic features such as the Main Boundary Thrust (MBT), Main Central Thrust (MCT), Greater Himalayan Sequence (GHS), Lesser Himalayan Sequence (LHS), and South Tibetan Detachment System (STDS). Inset map shows the regional location of the detailed map; box shows location of index map in Fig. 4. Modi?ed from Lee et al. (2004). These data and ideas have since been incorporated into a set of transient, plane-strain, ?nite-element models in which the GHS represents a 15–30 km thick, hot, low-viscosity middle-crust channel that extrudes southward from beneath Tibet toward the orogenic front during north–south convergence (Fig. 2; Beaumont et al., 2001, 2004, 2006). In these models, south-directed ?ow begins after the crust has been tectonically thickened and the middle crust experiences a reduction in viscosity as a consequence of partial melting due to mantle heat ?ux and crustal radiogenic heating. Flow and extrusion of the low-viscosity tabular body of middle crust is driven by a horizontal gravitational potentialenergy gradient produced by the topographic and crustal thickness differences between the Tibetan Plateau and its margins and enhanced focused erosion along the southern ?ank of the high Himalaya (e.g. Beaumont et al., 2001, 2004; Hodges et al., 2001). The low-viscosity channel is bounded above and below by normalsense (STDS) and thrust-sense (MCT) shear zones, respectively, that separate the channel from higher viscosity material above and below (Beaumont et al., 2001, 2004) (Figs. 1 and 2). Flow within a channel can range from pure Poiseuille ?ow (Fig. 3A) to a combination of Poiseuille and Couette ?ow (Fig. 3B). Poiseuille (or parabolic) ?ow develops between stationary rigid plates in which a horizontal gradient in lithostatic pressure and frictional resistance along the boundaries produces the greatest velocities in the center of the channel and decreasing velocities toward the top and bottom of the channel, leading to development of opposing shear sense. Poiseuille ?ow is characterized by a simple shear (large vorticity number (Wm)) at the top and bottom of the channel, general shear toward the center of the channel (decreasing Wm), and pure shear at the center of the channel (small Wm) (Fig. 3). Couette (or linear) ?ow develops between rigid plates moving relative to one another and is characterized by simple shear across the channel (e.g. White, 1974; Grujic, 2006) (Figs. 2 and 3). Thermal–mechanical models have been derived principally from geophysical data from southern Tibet and geological data from the Himalayan front. Absent are geological data north of the Himalaya, closer to the presumed source of ?owing crust. Data on the style, Wm, and spatial distribution of mid-crustal ?ow in southern Tibet are essential for testing the proposed link between mid-crustal channel-?ow and denudation-driven extrusion. Mabja Dome, southern Tibet, one of the North Himalayan gneiss domes (Fig. 1), is an ideal location for such investigations. This dome, w100 km north of the high Himalaya, provides excellent exposure of an originally w35-km thick sequence of middle crustal rocks that preserve mid-crustal deformational fabrics that predate doming and for which pressure temperature time data are well known (Lee et al., 2004, 2006; Zhang et al., 2004; Lee and Whitehouse, 2007). To document patterns of channel ?ow, we completed detailed kinematic, microstructural, and vorticity investigations on metamorphic mid-crustal rocks exposed in the core of Mabja Dome. Our MCT surface denudation STDS Everest Mabja upper crust 700 °C 800 °C middle crust mantle 0 km 100 V:H _ 1:1 Fig. 2. Schematic diagram of a southward-?owing low-viscosity middle crustal channel (gray) bounded by the STDS and the MCT. Poiseuille ?ow dominates within the channel and Couette ?ow beneath the channel (see Fig. 3). Predicted locations of middle crustal rocks exposed in Mabja Dome and of the Greater Himalayan sequence in the Everest region prior to exhumation are shown. Double-barbed arrows indicate velocity vectors; single-barbed arrows indicate relative sense of displacement; rain drops indicate erosion. Modi?ed from Beaumont et al. (2004) and Godin et al. (2006). 72 J. Langille et al. Journal of Structural Geology 32 (2010) 70–85 A S N Poiseuille flow STDS velocity profile caused by a pressure gradient velocity profile caused by shearing MCT Couette flow B S N Poiseuille flow velocity profile caused by a pressure gradient STDS velocity profile caused by shearing MCT Couette flow Fig Ключевые слова: figs, axis, institute, harrison vance, stds, ghs exposed, mct, york, himalaya, deformation temperature, type-i cross-girdles, garnet-zone rock, sequence, garnet, level, sedimentary history, geologic, peak metamorphism, garnet-zone, southward extrusion, a ax, feldspar texture, searle, langille, reverse, jamieson, harrison, mabja, journal structural, london, map, crust, orthogneiss, extension, sillimanite-zone rock, high himalaya, strain recorded, garnet porphyroclasts, mabja dome, metamorphism, vannay, rigid, ductile, data, journal structural geology, gneiss dome, aspect ratio, shear band, chloritoid, chloritoid-zone, top-n top-s, tibetan plateau, simpson, tibetan, velocity prole, crustal, kyanite-zone, recorded, schematic diagram, mixed top-n, lister, mid-crustal, quartz lpo, stu nitz, pure shear, shear sense, software, harris walker, elsevier, royal society, sample exhibited, earth, quartz, spatial distribution, mid-crustal rock, vorticity study, middle crust, table spear, kruhl, large component, sense, langille journal, plastic deformation, beaumont, angle, inclusion pattern, poiseuille, ?ow, rigid-grain technique, quartzite, kinematic, porphyroclasts, tethys himalaya, society, sch t-nd, stretching lineation, geological society, shear, grasemann grasemann, vorticity analysis, horizontal extension, everest region, wm, chloritoid isograd, deeper rock, modi?ed, tectonic evolution, mineral assemblage, kangmar dome, axis lpos, metamorphic, lee, hirth, single girdle, central, middle miocene, early stage, transitional, location, pressure, deformation, horizontal gradient, press, ramsay, middle, structural depth, pure, tullis, temperature, rock, grain, vorticity, deeper, mylonitic foliation, vertical thinning, uniform density, stable orientation, flow, ghs deformed, structural, tectonics, non-steady-state deformation, fabric, extrusion, north, contour, top-s, quartz lpos, tibet, american, journal, southern tibet, journal geophysical, cross, structural geology, velocity, increasing depth, structural level, increasing component, gray, top-s shear, increase, foliation, rigid grain, high, ductile deformation, component, tectonophysics, channel, everest massif, stretch parallel, himalayan, everest, dome, simple shear, lithostatic pressure, bounding plate, relative contribution, top-n, microstructures, southern margin, metamorphic grade, wallis, graphitic schist, hodges, slip, burg, science, dynamic recrystallization, garzanti, south, southward, t-s, exposed, stipp, ma, signicant component, nature, grujic, vertically thinned, ghs, law, wa, geophysical, depth, zone, strain shadow, metamorphic porphyroclasts, sch t-s, schist, top-n shear, basal, liu, lineation, history, opening angle, sample, lithostatic load, sch, passchier, couette, minimum, chloritoid-zone rock, southern, wang, geology, strain, geological, rock record, pattern, chen, continental, gneiss, grasemann, chloritoid-in isograd