TECTONICS, VOL. 32, 540–557, doi:10.1002/tect.20020, 2013 Metamorphic field gradients across the Himachal Himalaya, northwest India: Implications for the emplacement of the Himalayan crystalline core Remington M. Leger,1,2 A. Alexander G. Webb,1 Darrell J. Henry,1 John A. Craig,1 and Prashant Dubey3 Received 28 August 2012; revised 23 January 2013; accepted 3 February 2013; published 31 May 2013. [1] New constraints on pressures and temperatures experienced by rocks of the Himachal Himalaya are presented in order to test models for the emplacement of the Himalayan crystalline core here. A variety of methods were employed: petrographic analysis referenced to a petrogenetic grid, exchange and net-transfer thermobarometry, Ti-in-biotite thermometry, and analysis of quartz recrystallization textures. Rocks along three transects (the northern Beas, Pabbar, and southern Beas transects) were investigated. Results reveal spatially coherent metamorphic field gradients across amphibolite-grade and migmatitic metamorphic rocks. Along the northern Beas transect, rocks record peak temperatures of ~650–780C at low elevations to the north of ~3210’ N; rocks in other structural positions along this transect record peak temperatures of <640C that decrease with increasing structural elevation. Rocks of the Pabbar transect dominantly record ~650–700C peak temperatures to the east of ~7755’ E and ~450–620C peak temperatures farther west. Peak temperatures of ~450–600C along the southern Beas transect record a right-way-up metamorphic field gradient. Results are integrated with literature data to determine a metamorphic isograd map of the Himachal Himalaya. This map is compared to metamorphic isograd map pattern predictions of different models for Himalayan crystalline core emplacement. This analysis excludes models involving large magnitude (>20–30 km) extrusion and permits (1) models involving small magnitude (<20–30 km) extrusion that is discontinuous along the orogen and (2) tectonic wedging models, in which the crystalline core was emplaced at depth between a sole thrust and a back thrust in the Early-Middle Miocene. Citation: Leger, R. M., A. A. G. Webb, D. J. Henry, J. A. Craig, and P. Dubey (2013), Metamorphic field gradients across the Himachal Himalaya, northwest India: Implications for the emplacement of the Himalayan crystalline core, Tectonics, 32, 540–557, doi:10.1002/tect.20020. 1. Introduction Daniel et al., 2003]. Development of inverted metamorphic field gradients in the Himalaya has been explained by [2] First-order aspects of Himalayan tectonics are debated, end-member and hybrid models involving postmetamorphic in particular the development and emplacement of the recumbent folding of isograds [Heim and Gansser,1939; crystalline core, i.e., the Greater Himalayan Crystalline Frank et al., 1973; Searle and Rex, 1989] and/or shearing complex (GHC) (Figure 1). Key features of this unit include fi of isograds [Jain and Manickavasagam,1993;Hubbard, an inverted metamorphic eld gradient extending through 1996; Vannay and Grasemann, 2001]; syn-metamorphic the base to its middle or upper structural levels, and shearing of isograds [Grujic et al., 1996; Daniel et al., two shear zones bounding the GHC from above and below 2003], including within the context of a channel flow [e.g., Le Fort, 1996; Vannay and Grasemann, 1998; [Jamieson et al., 2004; Larson et al., 2010]; emplacement of a crystalline “hot-iron” hanging wall [Le Fort,1975; Célérier et al., 2009a, 2009b]; basal accretion of large or Additional supporting information may be found in the online version of small thrust horses [Reddy et al., 1993; Corrie and Kohn, this article. 2011] commonly in association with surface erosion 1Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana, USA. [Royden, 1993; Herman et al., 2010]; shear heating 2Now at Department of Earth and Planetary Sciences, University of [Molnar and England, 1990]; and pre-Himalayan heating Tennessee, Knoxville, Tennessee, USA. by Early Paleozoic granite crystallization [Gehrels et al., 3Oil and Natural Gas Corporation of India, Mehsana, Gujarat, India. 2003]. Inverted metamorphic field gradients extend below fi Corresponding author: A. Alexander G. Webb, Department of Geology the GHC, and right-way-up metamorphic eld gradients and Geophysics, Louisiana State University, E235 Howe-Russell, Baton persist above it, such that the thermal field is commonly Rouge, LA 70803, USA. ([email protected]) continuous across the bounding shear zones and into the fl ©2013. American Geophysical Union. All Rights Reserved. anking units above and below [e.g., Bollinger et al., 0278-7407/13/10.1002/tect.20020 2004; Chambers et al., 2009]. 540 LEGER ET AL.: HIMACHAL METAMORPHIC FIELD GRADIENTS 70oE 80oE N 90oE 30oN China Zanskar shear zone Figures 4, 11 Figures 3, 8, 9, 10 India South Tibet detachment Bhutan Shimla Himachal Pradesh Almora / o 30 N Dadeldhura Myanmar klippe Pakistan India Kathmandu Nappe Bangladesh (Burma) Tethyan Himalayan Sequence: Quaternary Main Central thrust (MCT) < Precambrian - Cambrian Late Cretaceous - Tertiary < Ordovician - Mesozoic South Tibet detachment (STD) (includes Pulchauki Group) Asian and Indo-Burman plate rocks o Greater Himalayan Crystalline Great90 CounterE thrust Indian Craton complex Main Centralo thrust - Southo Tibet detachment Ophiolite / Ophiolitic 20 N 90 E Lesser Himalayan Sequence intersection line (white dash = buried, black melange dash = eroded) Figure 1. Geological Map of the Himalaya. The dashed line denotes the Indian state of Himachal Pradesh, boxes denotes the boundaries of Figures 3–5 and Figures 9–11. Modified from Webb et al. [2011a, 2011b]. [3] The GHC may have been emplaced as a northward- A. Wedge extrusion tapering wedge extruded to the south (Figure 2A) [Burchfiel E. Miocene - M. Miocene N and Royden, 1985; Grujic et al., 1996; Vannay and South Tibet ITS Grasemann, 2001]. Alternatively, these rocks may represent detachment Tibet a low viscosity channel of middle/lower crustal material THS driven south by the lateral pressure gradient from the high LHS GHC Tibetan Plateau to the Himalayan foreland [Nelson et al., Main 1996; Beaumont et al., 2001, 2004; Hodges et al., 2001; Central thrust Godin et al., 2006]. In both models (referred to as wedge B. Channel flow - focused denudation extrusion and channel flow-focused denudation, respectively), Early “tunnelling” stage: N the GHC was extruded to the surface during motion along the Eocene-Oligocene bounding faults (Figures 2A and B). In contrast, the tectonic ITS wedging model posits that the GHC was emplaced at depth Tibet THS as a southward-tapering wedge (Figure 2C) [Yin, 2006; Webb et al., 2007, 2011a, 2011b]. LHS [4] The GHC emplacement models make testable predic- tions for the distribution of structures and metamorphism in Localized exhumation stage: the orogen [e.g., Jamieson et al., 2004; Webb et al., 2011b]. E. Miocene - M. Miocene fl Wedge extrusion and channel ow-focused denudation South Tibet models predict that the GHC was emplaced at the surface in detachment the Early and Middle Miocene. At this time, the GHC would be bounded by the subparallel bounding shear zones, giving Main GHC these high grade rocks the appearance of a “pipe to the Central thrust ” surface in cross-sectional view (Figure 2). In map view, these C. Tectonic wedging models predict that the bounding faults and GHC consistently E. Miocene - M. Miocene N Great separate relatively low grade rocks to north and south. The Counter tectonic wedging model predicts that the bounding faults ITS thrust fi South Tibet Tibet merge to the south, de ning the leading edge of the GHC. THS detachment South of this fault merger (branch line), the GHC would LHS GHC be missing, and the two low grade units would be in direct Main thrust contact. Central thrust [5] In this study, we test the GHC emplacement models by quantifying regional variations in metamorphic grade across Figure 2. Models for the emplacement of the Greater the Himachal Himalaya of northwest India. Previous studies Himalayan Crystalline Complex (GHC), modified from of this region indicate significant along-strike variations Webb et al. [2011b]. 541 LEGER ET AL.: HIMACHAL METAMORPHIC FIELD GRADIENTS in structural geometry and metamorphic field gradient [Frank [e.g., Wyss et al., 1999; Vannay et al., 2004]; or (3) be et al., 1973, 1995; Bhargava et al., 1991; Epard et al., 1995; folded with a top-southwest anticline (the Phojal anticline) Fuchs and Linner, 1995; Thakur, 1998; DiPietro and Pogue, and merge with the MCT along the northern margin of the 2004; Vannay et al., 2004; Richards et al., 2005; Yin, 2006; Kullu window [Thakur, 1998; Yin, 2006; Webb et al., Webb et al., 2007, 2011b]. We present petrographic obser- 2007, 2011b]. vations, new geothermobarometric determinations obtained [8] Rocks of the MCT hanging wall to the southwest of via exchange (garnet-biotite) and net-transfer (garnet-biotite- the Kullu window are commonly interpreted as (1) divided muscovite-plagioclase) reactions [e.g., Thompson, 1976; Ghent into GHC rocks and Tethyan Himalayan Sequence rocks and Stout, 1981], and Ti-in-biotite thermometry [Henry et al., [e.g., DiPietro and Pogue, 2004], (2) grouped as all GHC 2005]. rocks [e.g., Frank et al., 1973; Vannay et al., 2004], or (3) grouped as all Tethyan Himalayan Sequence rocks 2. Regional Geology [Webb et al., 2007, 2011b]. [9] The different STD and MCT hanging wall concepts 2.1. The Himalayan Orogen are related and have direct implications for Himalayan [6] Himalayan geology is commonly described as a three- tectonic models. For example, if the STD and MCT merge layer stack with the crystalline core, the GHC, as the middle along the northern margin of the Kullu window, then these layer bounded by faults above and below [e.g., Hodges, structures bound the frontal tip of the GHC there and the 2000]. The base of the stack is the Lesser Himalayan MCT hanging wall farther southwest consists of Tethyan Sequence, the Tethyan Himalayan Sequence is the upper Himalayan Sequence rocks.