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Thermal structure and exhumation history of the Lesser Himalaya in central Nepal

Bollinger, L. and Avouac, J. P. and Beyssac, O. and Catlos, E. J. and Harrison, T. M. and Grove, M. and Goffé, B. and Sapkota, S. (2004) Thermal structure and exhumation history of the Lesser Himalaya in central Nepal. Tectonics, 23 (5). Art. No. TC5015 . ISSN 0278-7407. http://resolver.caltech.edu/CaltechAUTHORS:20110111-091600174

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Abstract

The Lesser Himalaya (LH) consists of metasedimentary rocks that have been scrapped off from the underthrusting Indian crust and accreted to the mountain range over the last ~20 Myr. It now forms a significant fraction of the Himalayan collisional orogen. We document the kinematics and thermal metamorphism associated with the deformation and exhumation of the LH, combining thermometric and thermochronological methods with structural geology. Peak metamorphic temperatures estimated from Raman spectroscopy of carbonaceous material decrease gradually from 520°–550°C below the Main Central Thrust zone down to less than 330°C. 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 same regular trend; i.e., an increase from about 3–4 Ma near the front of the high range to about 20 Ma near the leading edge of the thrust sheets, about 80 km to the south. This suggests that the LH has been exhumed jointly with the overlying nappes as a result of overthrusting by about 5 mm/yr. For a convergence rate of about 20 mm/yr, this implies underthrusting of the Indian basement below the Himalaya by about 15 mm/yr. The structure, metamorphic grade and exhumation history of the LH supports the view that, since the mid-Miocene, the Himalayan orogen has essentially grown by underplating, rather than by frontal accretion. This process has resulted from duplexing at a depth close to the brittle-ductile transition zone, by southward migration of a midcrustal ramp along the Main Himalayan Thrust fault, and is estimated to have resulted in a net flux of up to 150 m^2/yr of LH rocks into the Himalayan orogenic wedge. The steep inverse thermal gradient across the LH is interpreted to have resulted from a combination of underplating and post metamorphic shearing of the underplated units.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1029/2003TC001564 DOIUNSPECIFIED
http://www.agu.org/pubs/crossref/2004/2003TC001564.shtmlPublisherUNSPECIFIED
Additional Information:© 2004 American Geophysical Union. Received 20 July 2003; revised 15 July 2004; accepted 19 July 2004; published 20 October 2004. This research was funded by the Commissariat à l’Energie Atomique and by the PNRN Centre National pour la Recherche Scientifique program. We thank Department of Mines and Geology, National Seismic Center, for fieldwork assistance. Thermochronological work was accomplished with the invaluable aid of Eric Cowgill. We are most grateful to the Associate Editor, Darrel Cowan, and two anonymous reviewers for their comments and suggestions, which greatly helped improve this manuscript. Auxiliary material for this article contains a data set with thermometrical, structural, and thermochronological data for central western Nepal Lesser Himalayas.
Group:Caltech Tectonics Observatory, Caltech Tectonics Observatory. Indo-Asian Collision Zone
Funders:
Funding AgencyGrant Number
Commissariat à l’Energie Atomique (CEA) (France)UNSPECIFIED
PNRN Centre National pour la Recherche Scientifique programUNSPECIFIED
Subject Keywords:Himalaya; thermal structure; underplating; duplex; metamorphism; thermochronology
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Other Numbering System NameOther Numbering System ID
Caltech Tectonics ObservatoryUNSPECIFIED
Record Number:CaltechAUTHORS:20110111-091600174
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20110111-091600174
Official Citation:Bollinger, L., J. P. Avouac, O. Beyssac, E. J. Catlos, T. M. Harrison, M. Grove, B. Goffé, and S. Sapkota (2004), Thermal structure and exhumation history of the Lesser Himalaya in central Nepal, Tectonics, 23, TC5015, doi:10.1029/2003TC001564
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:21684
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:01 Feb 2011 00:13
Last Modified:26 Dec 2012 12:50

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