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_Journal of Structural Geology 32 (2010) 900-919 Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Kinematic evolution of the Ama Drime detachment: Insights into orogen-parallel extension and exhumation of the Ama Drime Massif, Tibet-Nepal Jackie M. Langille a,*, Micah J. Jessup a, John M. Cottle b, Dennis Newell c, Gareth Sewardb a Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA b Department of Earth Science, University of California, Santa Barbara, CA 93106, USA c Earth and Environmental Sciences, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Article info Article history: Received 13 June 2009 Received in revised form 25 March 2010 Accepted 11 April 2010 Available online 20 April 2010 Keywords: Himalaya Microstructures Vorticity techniques Quartz fabrics Two-feldspar geothermometry Detachment dynamics Abstract The Ama Drime Massif is a north-south trending antiformal structure located on the southern margin of the Tibetan Plateau that is bound by the Ama Drime and Ny?nno Ri detachments on the western and eastern sides, respectively. Detailed kinematic and vorticity analyses were combined with deformation temperature estimates on rocks from the Ama Drime detachment to document spatial and temporal patterns of deformation. Deformation temperatures estimated from quartz and feldspar microstructures, quartz c-axis fabrics, and two-feldspar geothermometry of asymmetric strain-induced myrmekite range between 400 and 650°C. Micro- and macro-kinematic indicators suggest west-directed displacement dominated over this temperature range. Mean kinematic vorticity estimates record early pure shear-dominated flow (49–66% pure shear) overprinted by later simple shear (1–57% pure shear), high-strain (36–50% shortening and 57–99% down-dip extension) dominated flow during the later increments of ductile deformation. Exhumation of the massif was accommodated by at least 21–42 km of displacement on the Ama Drime detachment. Samples from the Ny?nno Ri detachment were exhumed from similar depths. We propose that exhumation on the Ny?nno Ri detachment during initiation of orogen-parallel extension (11–13 Ma) resulted in a west-dipping structural weakness in the footwall that reactivated as the Ama Drime detachment. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Continental collision between the Indian and Asian plates from the Eocene to Holocene resulted in profound crustal shortening and thickening that produced the Himalaya and Tibetan Plateau. Previous studies (e.g., Grujic, 2006; Grujic et al., 1996, 2002; Vannay and Grasemann, 1998; Grasemann et al., 1999; Hodges et al., 1992, 2001) have largely focused on the southward propagation or extrusion of the Greater Himalayan Series (GHS) from the Eocene to middle Miocene during continental convergence (Fig. 1). Southward flow was accommodated by coeval movement on the South Tibetan detachment system (STDS) on top of the GHS and the Main Central thrust zone (MCTZ) at the bottom (Fig. 1) (Nelson et al., 1996; Searle et al., 2006). After the middle Miocene, a transition from south-directed mid-crustal flow to orogen-parallel extension occurred in the Himalaya that resulted in the formation of north-south striking normal faults, graben, and domes that often offset or reactivate the STDS and/or the MCTZ (Fig. 1) (Murphy et al., 2002; Kapp and Guynn, 2004; Murphy and Copeland, 2005; Theide et al., 2005, 2006; Jessup et al., 2008a; Jessup and Cottle, in press). Structural, geochronologic, and thermochronometric data from the Leo Pargil dome, the Gurla Mandhata core complex, and the Ama Drime Massif (ADM) demonstrate that faults and shear zones that accommodated crustal shortening (i.e., the STDS and MCTZ) between the Eocene and early Miocene are now inactive and are therefore no longer capable of accommodating south-directed mid-crustal flow (Cottle et al., 2007; Murphy, 2007). The ADM is a ~30-km-wide north-south striking antiformal structure that narrows towards the north and is located ~50 km northeast of Mount Everest. It is bound by the north-south striking Ny?nno Ri detachment (NRD) on the eastern flank and the north-south trending Ama Drime detachment (ADD) on the western flank (Figs. 1 and 2) (Jessup et al., 2008a). To the north, the NRD transitions into the Xainza-Dinggy? graben that offsets the STDS (Burch?el et al., 1992; Zhang and Guo, 2007) and is kinematically linked to east-west extension in the interior of the plateau (Taylor et al., 2003). Structural, petrologic, and geochronologic data indicate that the antiformal structure of the ADM is the result of evolving mid-crustal flow along the southern margin of the Tibetan Plateau (Jessup et al., 2008a; Cottle et al., 2009a). This study presents new kinematic, microstructural, and vorticity data from three transects across the ADD on the western flank of the ADM along with one transect from the NRD on the eastern flank. These data provide new constraints on the role of strain partitioning and shear zone development associated with orogen-parallel extension in a convergent setting. We integrate these new data with existing data to propose a model for the evolution of the ADM. 2. Geologic setting 2.1. Regional geology The geology in the Mount Everest region, southwest of the ADM can be grouped into three main fault-bounded litho-tectonic units. In order of increasing structural position these include (from south to north); the Lesser Himalayan Series (LHS), the GHS, and the Tibetan Sedimentary Series (TSS). These units are separated by three north-dipping fault systems; the Main Boundary thrust (MBT), the MCTZ, and the STDS (Fig. 1). The MCTZ and the MBT are inferred, based on geophysical data, to sole into the Main Himalayan thrust (MHT) to the north beneath the Tibetan Plateau (Nelson et al., 1996; Searle et al., 2006). Below the MHT (35–75 km), the lower crust is composed of Archean Indian shield granulite facies rocks (Searle et al., 2006). The ~20-km-thick LHS, bound between the MBT below and the MCTZ above, consists of Paleoto Mesoproterozoic metamorphosed clastic sediments and gneiss (Brook?eld, 1993; Pognante and Benna, 1993; Goscombe et al., 2006). The Neoproterozoic to Cambrian GHS is bound at the base by the MCTZ and at the top by the STDS. The upper GHS is separated from the lower GHS by the High Himalayan thrust (HHT) (Goscombe et al., 2006). The lower GHS is bound between the HHT and the base of the MCTZ. The upper GHS structurally overlies metapelitic schists and the Ulleri and Num orthogneisses in the lower GHS (within the MCTZ) (Searle et al., 2008). In contrast, others (e.g., Goscombe et al., 2006) place the Ulleri orthogneiss in the LHS. The GHS is composed of a ~28-km-thick section of metapelitic rocks, augen gneiss, calcsilicates, and marble that was metamorphosed to amphibolite facies and intruded by Miocene sills and dikes (Hodges, 2000; Searle et al., 2003; Viskupic et al., 2005). South of Mount Everest, in the Dudh Kosi drainage, the maximum age for movement along the MCTZ is constrained by 40Ar/39Ar hornblende and 208Pb/232Th monazite geochronology and indicates that amphibolite-facies metamorphism of hanging wall rocks occurred at ~22 ± 1 Ma (Hubbard and Harrison, 1989) and potentially as early as 24–29 Ma (Catlos et al., 2002). The GHS in the Everest region experienced an early kyanite-grade event (550–680°C and 0.8–1.0 GPa) at ~38.9 ± 0.9 Ma that is a record of crustal thickening (Pognante and Benna, 1993; Cottle et al., 2009b). Kyanite-grade metamorphism was overprinted by a high-temperature low-moderate-pressure sillimanite-grade event (650–750°C and 0.4–0.7 GPa) associated with decompression melting and granite emplacement between ~28.0 and 22.6 Ma (Pognante and Benna, 1993; Simpson et al., 2000; Searle et al., 2003; Viskupic et al., 2005; Jessup et al., 2008b; Cottle et al., 2009b). 40Ar/39Ar biotite ages from the GHS are <14 Ma, suggesting that metamorphism in the interior portion of the GHS had ceased by this time (Viskupic et al., 2005). Timing constraints on the STDS in the Mount Everest region suggest that the system was active until ~18–16 Ma (Hodges et al., 1992; Murphy and Harrison, 1999; Searle et al., 2003). In the Dzakaa Chu section of the STDS (Figs. 1 and 2), U-Pb geochronology conducted on a leucogranite dike that crosscuts the mylonitic fabric within the lower part of the STDS suggests that fabric development in this section of the shear zone occurred at <20 Ma (Cottle et al., 2007). In the Dinggy? graben, the ductile portion of the footwall rocks ~100 m below the STDS brittle detachment were active until ~15–16 Ma (Leloup et al., 2010). The structurally highest unit, the TSS, consists of Proterozoic to Jurassic pre-, syn-, and post-rift sedimentary rocks, a Jurassic to Cretaceous passive continental margin sedimentary sequence, and an upper Cretaceous to Eocene syn-collisional sedimentary sequence (Gansser, 1964; Le Fort, 1975; Gaetani and Garzanti, 1991; Brook?eld, 1993; Liu and Einsele, 1994; Garzanti, 1999). Middle Miocene to Holocene north-south striking normal faults cut these units (Fig. 2). J.M. Langille et al. Journal of Structural Geology 32 (2010) 900-919 Ключевые слова: mylonite zone, range, evolution, yo ri, quartz grain, two-feldspar geothermometry, southern margin, mica, aspect ratio, chen, non-steady-state deformation, pure shear, footwall rock, granulitized eclogite, hacker, tibet, law, qtz, footwall, tibetan plateau, kangmar dome, vorticity analysis, grasemann, simple shear, ductile deformation, searle, dzong tso, metamorphic, transect, sedimentary history, tullis, nrd, society, myrmekite, hanging wall, ductile, data provide, grain shape, technique, geological society, mctz, wa, deformation temperature, gns t-ec, ma, journal structural, earth, eastern, rock exposed, axis, qtz t-w, shear zone, fabric, foliation, rolfo groppo, ama drime, southern, dynamic recrystallization, estimate derived, sample, high, elsevier, asymmetric myrmekite, fault, orogen-parallel extension, kinematic indicator, dominated, exhumation, angle, stable orientation, transects, ?ow, upper ghs, exposed, eastern limb, pure, massif, american, extension, cross, benna simpson, stormer, structural data, liu, temporal variability, science, harrison, opening angle, pattern, mount everest, location map, myrmekite blade, mineralogy, vorticity estimate, temperature range, kinematic evolution, t-wc, langille, plastic deformation, extrusion, st?nitz, kapp, deformation, plane, grain, theide, tectonophysics, opening, top-to-the-west, record, quartz fabric, drime, ama, sense shear, compositional map, drag fold, himalayan, ulleri orthogneiss, london, nepal, top-to-the-south shear, slip, journal structural geology, journal, cottle, geochronologic data, jessup, fabric development, location, relative contribution, kinematic, biotite age, north, mabja dome, xypolias, study, pung chu, table, temperature, himalaya, murphy, yo, rigid-grain technique, mylonitic fabric, ri, tectonic evolution, data, channel, dome, wallis, gneiss, orogen-parallel, assumed dip, eastern ank, protolith age, sense, sharkha transect, axis fabric, stipp, sangkar fault, zone, lister, estimate, top-to-the-east sense, structural geology, garzanti, ghs, strain, lpo, assumed pressure, wang, adm, dzakaa chu, everest, eastewest extension, langille journal, tectonics, structural, exhumation history, geological, nekvasil, rock, deformed, middle miocene, transect deform, west, rhomb hai, grujic, shape, accurately dene, geology, tibetan, western ank, sharka transect, add, lineation, brittle fracturing, ductile extrusion, lee, geothermal gradient, feldspar, detachment, hodges, wm, lombardo, quartz, feldspar porphyroclasts, structural evolution, strain-induced myrmekite, passchier, brittle detachment, orogen, minimum depth, kali, structural boundary, recrystallization, chu, south, porphyroclasts, gbm recrystallization, vorticity, down-dip extension, hai, map, analysis, history, top-to-the-west displacement, shear