Tectonophysics 714–715 (2017) 117–124 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Ductile shearing to brittle thrusting along the Nepal Himalaya: Linking Miocene channel flow and critical wedge tectonics to 25th April 2015 Gorkha earthquake Mike Searle a,⁎, Jean-Philippe Avouac b, John Elliott a,1, Brendan Dyck a a Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK b Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA article info abstract Article history: The 25th April 2015 magnitude 7.8 Gorkha earthquake in Nepal ruptured the Main Himalayan thrust (MHT) for Received 29 January 2016 ~140 km east-west and ~50 km across strike. The earthquake nucleated at a depth of ~15–18 km approximating Received in revised form 1 August 2016 to the brittle-ductile transition and propagated east along the MHT but did not rupture to the surface, leaving half Accepted 6 August 2016 of the fault extent still locked beneath the Siwalik hills. Coseismic slip shows that motion is confined to the ramp- Available online 8 August 2016 flat geometry of the MHT and there was no out-of-sequence movement along the Main Central Thrust (MCT). Keywords: Below 20 km depth, the MHT is a creeping, aseismic ductile shear zone. Cumulated deformation over geological Gorkha earthquake time has exhumed the deeper part of the Himalayan orogen which is now exposed in the Greater Himalaya Nepal revealing a tectonic history quite different from presently active tectonics. There, early Miocene structures, in- Himalaya cluding the MCT, are almost entirely ductile, with deformation occurring at temperatures higher than ~400 °C, Channel flow and were active between ~22–16 Ma. Kyanite and sillimanite-grade gneisses and migmatites approximately Critical wedge 5–20 km thick in the core of the Greater Himalayan Sequence (GHS) together with leucogranite intrusions along the top of the GHS were extruded southward between ~22–15 Ma, concomitant with ages of partial melt- ing. Thermobarometric constraints show that ductile extrusion of the GHS during the Miocene occurred at muscovite-dehydration temperatures ~650-775 °C, and thus brittle thrusting and critical taper models for GHS deformation are unrealistic. As partial melting and channel flow ceased at ~15 Ma, brittle thrusting and under- plating associated with duplex formation occurred along the Lesser Himalaya passively uplifting the GHS. © 2016 Elsevier B.V. All rights reserved. 1. Introduction metamorphic isograds are inverted along the Main Central Thrust zone (MCT) and along the upper contact isograds are right way-up be- The Himalayan orogen is commonly interpreted as a crustal scale neath the South Tibetan Detachment (STD), an enigmatic low-angle, wedge, analogous to a critical taper (Yin and Harrison, 2000; Avouac, north-dipping normal fault (e.g. Burg and Chen, 1984; Searle, 2010, 2015; Bollinger et al., 2006), that formed as units detached from the 2015; Law et al., 2011; Cottle et al., 2015a). The GHS was exhumed in underthrusting Indian plate were accreted to the southern margin of the Oligocene - Miocene as a result of ductile shearing along the coeval Tibet since the India-Asia collision started about 50 Ma ago (Figs. 1,2). MCT and STD ductile shear zones, by a process known as channel flow, The upper crust of the Himalaya is represented by the so-called Tethyan the ductile extrusion of a mid-crustal layer of partially molten rocks (e.g. Himalaya, a sequence of Neo-Proterozoic to Cenozoic mainly sedimen- Beaumont et al., 2001; Grujic et al., 2002; Searle et al., 2003, 2010; Godin tary rocks showing intense folding and thrusting, crustal shortening et al., 2006; Law et al., 2011; Cottle et al., 2015a, 2015b). The southern- and thickening, but generally not metamorphosed (Corfield and most and structurally lower part of the Himalaya is the Lesser Himalaya, Searle, 2000). The middle crust is the Greater Himalayan Sequence comprising a series of south-vergent thrust sheets emplacing the (GHS) of highly metamorphosed and partially melted gneisses, Himalaya over the Siwalik foreland basin sediments along the Main migmatites and leucogranites all of which show Cenozoic metamor- Boundary Thrust (MBT). phism up to kyanite and sillimanite grade. Along the base of the GHS, This paper attempts to link the deformation in the Greater Himalayan hinterland (Early Eocene – Early Miocene metamorphism and deforma- tion) to the Lesser Himalaya foreland critical wedge (mid-Miocene – Recent) to the active thrusting as exemplified by the 25th April 2015 ⁎ Corresponding author. E-mail address: [email protected] (M. Searle). Gorkha earthquake rupture. The geometry of the Main Himalayan Thrust 1 Now at: School of Earth and Environment, University of Leeds, Leeds LS29JT, UK. (MHT), the basal detachment that ruptured during the 25th April 2015 http://dx.doi.org/10.1016/j.tecto.2016.08.003 0040-1951/© 2016 Elsevier B.V. All rights reserved. 118 M. Searle et al. / Tectonophysics 714–715 (2017) 117–124 84˚ 85˚ 86˚ 87˚ Tethyan Himalaya STD STD Manaslu MCT/CT Shisha Pangma Langtang Pokhara Cho Oyu 28˚Lesser Gorkha MCT/MT Everest Himalaya Greater Himalaya MBT Kathmandu Kathmandu Klippe Siwalik Hills Lukla MFT MCT/CT Phaplu MCT/RT 27˚ Tethyan Himalaya MFT STD - South Tibetan Detachment MBT Greater Himalaya MCT/CT - Chomrong Thrust Dunche Schist MCT/MT - Munsiari/Mahabharak Thrust MCT/RT - Ramgarh Thrust Lesser Himalaya MBT - Main Boundary Thrust Cities Line of Section Siwalik Group MFT - Main Frontal Thrust Mountains Fig. 2 Fig. 1. Digital Elevation Model (DEM) of the central Nepal Himalaya, showing main structures metamorphic grade across the Langtang – Ganesh Himalaya. Greater Himalayan Sequence (dark green) includes amphibolite facies gneisses and schists, migmatites and leucogranites. ASTER GDEM is a product of METI and NASA. Gorkha earthquake (Avouac et al., 2015; Elliott et al., 2016), is critical to (e.g. Beaumont et al., 2001; Grujic et al., 2002; Searle et al., 2003, the interpretation of the kinematic history the Himalaya. 2006, 2010; Jessup et al., 2006; Godin et al., 2006; Streule et al., 2010) Two major conflicts in Himalayan tectonics involve: (1) the discus- and proponents of the critical wedge taper model (e.g. DeCelles et al., sion between proponents of Channel Flow along the Greater Himalaya 2001; Kohn et al., 2004; Kohn, 2008; Webb et al., 2011; He et al., S Ganesh TIBET N STD Chitwan Himal MCT Tethyan hills Kathmandu sill sediments MT MT ky sill MFT MDT MBT klippe Gsz st STD 0 D 0 LESSER hunc Leucogranite SIWALIKS GT he sc hist GREATER Palaeozoic RT locked MHT HIMALAYA HIMALAYA 10 granite sills 10 Proterozoic MCT earthquake rupture Gneisses Archaean basement Depth (km) 20 MHT 20 V.E 1x 0 25 50 75 100 125 150 175 85.068E 85.169E 85.269E 85.373E 85.477E 85.577E 85.682E 85.788E 85.840E 27.125N 27.321N 27.524N 27.726N 27.925N 28.123N 28.320N 28.125N 28.615N Distance Along Profile NNE-SSW (km) Metamorphic Key Unmetamorphosed Tethyan Himalaya STD STD - South Tibetan Detachment 20-15 Ma Way-up metamorphic field gradient Leucogranites MCT - Main Central Thrust 20-15 Ma sill Sillimanite isograd Migmatites, sills MT - Munsiari / Mahabharat Thrust 20-15 Ma ky kyanite isograd Amphibolite facies GHS RT - Ramgarth Thrust 15-10 Ma st staurolite isograd MCT zone Greenschist facies MBT - Main Boundary Thrust 7- 0 Ma GSZ Galchi shear zone MDT - Main Dun Thrust Active GT Gorkha blind thrust Low-grade Lesser Himalaya MBT MFT - Main Frontal Thrust Active MHT Main Himalayan Thrust Siwalik sediments Fig. 2. Geological cross-section of the Langtang – Kathmandu Himalaya showing major structural units, metamorphic grade, thrust faults and extent of the rupture during the 25th April 2015 Gorkha earthquake. M. Searle et al. / Tectonophysics 714–715 (2017) 117–124 119 2015; Yu et al., 2015)(Fig. 3), and (2) whether ‘out-of sequence’ thrust- of active thrusts but these, although fairly common (Morley, 1988), ing has occurred in the past along the Main Central Thrust (e.g. Wobus are not as important as in-sequence (‘piggy-back’) thrusts. et al., 2003, 2005) or not (e.g., Lavé and Avouac, 2001; Bollinger et al., At the leading edge of a fold-thrust belt a ‘critical taper’ (e.g., Elliott, 2006; Herman et al., 2010; Nadin and Martin, 2012). 1976; Davis et al., 1983; Dahlen et al., 1984; Dahlen, 1990) develops The Langtang – Kathmandu profile across the Nepal Himalaya where material is continually added to the wedge by frontal accretion, (Figs. 1,2) is a well-exposed and well-studied transect where the ductile or underplating (Fig. 3). The shape of the critical wedge is governed and brittle structures have been mapped and there are P-T-t-D data (e.g. by various factors including the composition and density of the Reddy et al., 1993; Kohn, 2008). This was the area affected by the recent deforming rock, the geometry and slope of the basal detachment, fric- 25th April 2015 Gorkha earthquake and interpretation of the deep tional stress along the basal thrust fault, convergence rate, erosion structure of the MHT during this earthquake (Avouac et al., 2015; rate, and the nature of the backstop (e.g. Dahlen, 1984; Dahlen and Elliott et al., 2016) gives us a unique opportunity to correlate the geolog- Suppe, 1988; Malavieille, 2010). Whereas the pro-wedge side accom- ical history of older, deeper, hotter GHS rocks to Late Cenozoic brittle modates most of the crustal shortening, over-thickened wedges fre- thrust faulting beneath the Lesser Himalaya, and active thrust faulting quently result in backthrusts along the retro-wedge side. These along the MHT as deduced from the 25th April 2015 Gorkha earthquake.