TECTONICS, VOL. 23, TC5015, doi:10.1029/2003TC001564, 2004 Thermal structure and exhumation history of the Lesser Himalaya in central Nepal L. Bollinger,1,2 J. P. Avouac,1,2 O. Beyssac,3 E. J. Catlos,4 T. M. Harrison,4 M. Grove,4 B. Goffe´,3 and S. Sapkota5 Received 20 July 2003; revised 15 July 2004; accepted 19 July 2004; published 20 October 2004. 2 [1] The Lesser Himalaya (LH) consists of of up to 150 m /yr of LH rocks into the Himalayan metasedimentary rocks that have been scrapped off orogenic wedge. The steep inverse thermal gradient from the underthrusting Indian crust and accreted to across the LH is interpreted to have resulted from a the mountain range over the last 20 Myr. It now combination of underplating and post metamorphic forms a significant fraction of the Himalayan shearing of the underplated units. INDEX TERMS: 8102 collisional orogen. We document the kinematics and Tectonophysics: Continental contractional orogenic belts; 8122 thermal metamorphism associated with the Tectonophysics: Dynamics, gravity and tectonics; 9320 Information deformation and exhumation of the LH, combining Related to Geographic Region: Asia; KEYWORDS: Himalaya, thermometric and thermochronological methods with thermal structure, underplating, duplex, metamorphism, structural geology. Peak metamorphic temperatures thermochronology. Citation: Bollinger,L.,J.P.Avouac, estimated from Raman spectroscopy of carbonaceous O. Beyssac, E. J. Catlos, T. M. Harrison, M. Grove, B. Goffe´, and materialdecreasegraduallyfrom520°–550°Cbelowthe S. Sapkota (2004), Thermal structure and exhumation history of the Lesser Himalaya in central Nepal, Tectonics, 23, Main Central Thrust zone down to less than 330°C. TC5015, doi:10.1029/2003TC001564. These temperatures describe structurally a 20°– 50°C/km inverted apparent gradient. The Ar muscovite ages from LH samples and from the overlying crystalline thrust sheets all indicate the 1. Introduction same regular trend; i.e., an increase from about 3– 4Manear the front of the high range to about 20 Ma near [2] As early as the mid-nineteenth century, field geolo- the leading edge of the thrust sheets, about 80 km to the gists noticed the upward increase of metamorphic grade south. This suggests that the LH has been exhumed across the Himalaya [Mallet, 1874; Medlicott, 1864; jointly with the overlying nappes as a result of Oldham, 1883]. A number of field studies have since documented this apparently inverted gradient together with overthrusting by about 5 mm/yr. For a convergence the structural architecture of the range [e.g., Hodges et al., rate of about 20 mm/yr, this implies underthrusting of 1996; Peˆcher, 1975; Stephenson et al., 2000]. The inverted the Indian basement below the Himalaya by about gradient is most obvious within and close to the Main 15 mm/yr. The structure, metamorphic grade and Central Thrust (MCT) zone which marks the contact exhumation history of the LH supports the view that, between the High Himalayan Crystalline units (HHC) and since the mid-Miocene, the Himalayan orogen has the Lesser Himalaya (LH). Metamorphic grade increases essentially grown by underplating, rather than by upward from chlorite, to biotite, garnet, kyanite and frontal accretion. This process has resulted from sillimanite-grade rocks over a structural distance of a few duplexing at a depth close to the brittle-ductile kilometers [e.g., Peˆcher, 1989]. This observation has been transition zone, by southward migration of a interpreted either as evidence for a thermal structure with midcrustal ramp along the Main Himalayan Thrust recumbent isotherms [e.g., Le Fort, 1975], or as the result of postmetamorphic shearing of isograds [e.g., Brunel et al., fault, and is estimated to have resulted in a net flux 1979; Hubbard, 1996] and has stimulated a variety of thermal modeling [Harrison et al., 1998; Henry et al., 1997; Jaupart and Provost, 1985; Molnar and 1Laboratoire De´tection et Ge´ophysique, Commissariat a` l’Energie England, 1990; Royden, 1993]. The exhumation Atomique, Bruye`res le Chaˆtel, France. history of the crystalline nappes in the study area was 2Also at Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA. also studied by various authors [Arita et al., 1997; 3Ecole Normale Supe´rieure, Laboratoire de Ge´ologie, Paris, France. Macfarlane et al., 1992; P. Copeland, personal communi- 4Institute of Geophysics and Planetary Physics, University of California, cation]. By contrast, the metamorphic and exhumation Los Angeles, California, USA. history of the Lesser Himalaya remains poorly documented 5Department of Mines and Geology, National Seismic Center, Lainchur, Kathmandu, Nepal. mainly because of the poor mineralogy of these rocks and their supposed low metamorphic grade. However, this Copyright 2004 by the American Geophysical Union. information is critical to test models of mountain building 0278-7407/04/2003TC001564$12.00 in the Himalayas. Indeed, the LH thrust sheets form the TC5015 1of19 TC5015 BOLLINGER ET AL.: THERMAL STRUCTURE OF THE LESSER HIMALAYA TC5015 Figure 1. (a) Simplified geological map of Nepal with location of study area (box) shown in Figure 2 and of cross section AA0 (Figure 1b). (b) N018°E geological section across the Himalaya of west central Nepal. Abbreviations are as follows: DKl, Dadeldhura klippe; JKl, Jajarkot klippe; KKl, Kathmandu klippe; MFT, Main Frontal Thrust fault; MBT, Main Boundary Thrust fault; MCT, Main Central Thrust fault. All these faults are thought to sole into a basal thrust fault called here the Main Himalayan Thrust (MHT), which would correspond with the midcrustal reflector imaged below southern Tibet from the INDEPTH experiment [Hauck et al., 1998]. bulk of the Himalayan wedge that have been accreted to times, would have grown primarily by underplating near the the range over the last 10 Myr as suggested from brittle-ductile transition. structural and sedimentological investigations [DeCelles et al., 2000; Huyghe et al., 2001; Robinson et al., 2001; Schelling and Arita, 1991]. 2. Geological Setting [3] In this paper we focus our effort on the LH, most 2.1. Key Structural Features Across the Himalaya of specifically in the area of central Nepal, west of Kath- West Central Nepal mandu. To document the thermal structure, we take advan- tage of new temperature data obtained from the degree of [4] To the first order, the Himalaya has resulted from graphitization of carbonaceous material [Beyssac et al., crustal-scale thrusting on a few major thrust faults 2004], a technique which was found to be particularly (Figure 1). From north to south, the Main Central Thrust appropriate in the case of LH. We analyze these data in fault (MCT), the Main Boundary Thrust fault (MBT) and view of their structural setting, together with 40Ar/39Ar the Main Frontal Thrust fault (MFT) are thought to be cooling ages determined for white micas. We show that activated following a forward propagation sequence [e.g., these data can be interpreted from a kinematic model Gansser, 1964; Le Fort, 1975]. At present, deformation is according to which the Himalaya, since the late Miocene chiefly accommodated by thrusting along a major thrust 2of19 TC5015 BOLLINGER ET AL.: THERMAL STRUCTURE OF THE LESSER HIMALAYA TC5015 fault, the Main Himalayan thrust (MHT) that connects the Himalaya, the Pokhara-Gorkha anticlinorium [Peˆcher, MFT to a subhorizontal shear zone below the high range 1989], that cropped out to the surface between 5 and and southern Tibet [e.g., Lave´ and Avouac, 2000, 2001]. 11 Ma, as indicated from various geochemical indicators The MCT marks the contact between the High Himalayan of sediment provenance sampled in the Siwaliks [DeCelles crystalline units (HHC) and the Lesser Himalaya (LH). This et al., 2000; Huyghe et al., 2001; Meigs et al., 1995; major broad shear zone is thought to have been active Najman and Garzanti, 2000; Parrish and Hodges, 1996; approximately from 22 Ma [Coleman and Hodges, 1998; Robinson et al., 2001]. This structure has been interpreted Hodges et al.,1996]to4–8Ma[Catlos et al.,2001; as a hinterland dipping duplex [Brunel, 1986] and has Harrison et al., 1997]. On the basis of geomorphic criterion, been recognized on all sections across the Himalayas in namely the steepness of the front of the high range and Nepal. Some relatively detailed structural models of river knickpoints, and evidence for brittle deformation the duplex geometry have been derived from balancing posterior to the ductile fabric, some authors have suggested crustal-scale sections [DeCelles et al., 2001; Schelling and it might even be still active at places [e.g., Hodges et al., Arita, 1991]. The schists and phyllites of the LH show a 2004]. well-developed foliation and abundant microscale and [5] The deep structure of the crust beneath the High mesoscale folding, attesting to significant deformation by Himalaya has been investigated via reflection seismologic pure shear, a mechanism inconsistent with the geometric experiments [Alsdorf et al., 1998; Hauck et al., 1998; rules used to balance a cross section. The details of these Makovsky et al., 1996], gravimetry [Cattin et al., 2001; geometric and kinematic models may therefore be ques- Lyon-Caen and Molnar, 1985], magnetotelluric sounding tioned, but we believe the broad-scale structural geometry [Lemonnier et al., 1999], and through seismic reflection must imply some sort of duplex structure as depicted in the and oil well logs in the most frontal parts of the system section across west central Nepal (Figure 1b). Some lateral [Powers et al., 1998]. Together with the surface geology (E-W) variations in the geometry and degree of development investigations and balanced cross-section modeling of this duplex are suggested from the lateral succession of [DeCelles et al., 2001; Peˆcher, 1989; Schelling and Arita, HHC klippes and LH windows (Figures 1a and 2). 1991], these data suggest that the major thrust faults splay from the MHT, the main feature separating the Himalayan orogenic wedge from the underthrusting Indian plate 2.2.