HR: 17:45h
AN: T24A-08    [Abstracts]
TI: Quantifying Metamorphism and Exhumation in the Kumaun Lesser Himalaya, North West India
AU: * Celerier, J
EM: julien.celerier@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Harrison, M
EM: mark.harrison@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Dunlap, W
EM: jim.dunlap@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Beyssac, O
EM: Olivier.Beyssac@ens.fr
AF: Laboratoire de Geologie, Ecole Normale Superieure, Paris, 75231 France
AU: Beyssac, O
EM: Olivier.Beyssac@ens.fr
AF: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AU: Webb, A
EM: webb@ess.ucla.edu
AF: Department of Earth and Space Sciences, University of California Los Angeles, Los Angeles, CA 90095 Australia
AU: Yin, A
EM: yin@ess.ucla.edu
AF: Department of Earth and Space Sciences, University of California Los Angeles, Los Angeles, CA 90095 Australia
AB: The Himalaya have been described as the `world's best natural laboratory in which to study processes associated with large-scale continent-continent collision' and as such play a key role in our understanding of both modern and ancient accretionary orogens. It is, therefore, somewhat surprising that quantitative descriptions of the conditions of metamorphism, patterns of deformation and exhumation are currently largely restricted to a narrow strip of amphibolite grade rocks known as the Greater Himalayan Crystallines (GHC). The GHC, though coinciding with the regions of highest relief and what we instinctively think of as the Himalayan mountains, actually only account for ~20% of the exposure. Owing to perceived and real problems related to understanding the petrogenesis of rocks of low metamorphic grade, quantitative constraints concerning the evolution of the other 80% of the rock volume are lacking. Here we integrate metamorphic and thermochronological data with structural observations to document and decipher the complex internal deformation of the Lesser Himalaya of Kumaun, India.
The Lesser Himalaya (LH) are composed of Proterozoic to Palaezoic metasedimentary rocks of Indian continental affinity which have been accreted to the mountain range since the early Miocene and define one of four principal tectonometamorphic units of the Himalayan mountains. The sequence is bounded to the north by the Main Central Thrust (MCT), a major crustal thickening structure which places the GHC over the LH, and to the south by the Main Boundary Thrust, which places the LH over the Tertiary foreland basin. Raman Spectroscopy of Organic Material has been used to constrain the conditions of metamorphism within the metasedimentary pile. Samples of graphitic schist and slate collected along 5 parallel transects reveal that adjacent to the MCT, peak metamorphic temperatures are in the vicinity of 550°C}. Down structural section from the MCT, thermal gradients are relatively steep, in the range of 30°C}-50°C}/km. Beyond ~25km south of the MCT, and towards the contact with the foreland basin, metamorphic temperatures fall below the range of the technique (~330°C}).
⁴⁰Ar/³⁹Ar thermochronolgy of white micas, separated from Lesser Himalayan quartzites, reveal a range of cooling ages from Pliocene to Early Miocene. We find that these cooling ages, the first of their kind from the Lesser Himalaya, define a trend of younger ages upsection, i.e. towards the MCT, with the youngest cooling age of 4.3 Ma coming from an MCT footwall quartzite directly adjacent to the thrust contact. Samples within ~15km south of the MCT are characterized by cooling ages less than 10 Ma, whilst ages range between 10 and 26 Ma beyond ~15km south (downsection) of the MCT. Many of the cooling ages obtained fit chronological frameworks proposing episodic, out of sequence, movement along the MCT, yet there are also several cooling ages which fall outside the Miocene and Pliocene MCT motion paradigm. These cooling ages, and the continum which they define, are particularly interesting as they suggest that exhumation of the Lesser Himalayan metasedimentary pile has been continuous since the Miocene and independent of motion on the MCT.

DE: 1090 Field relationships (3690, 8486)
DE: 1140 Thermochronology
DE: 3652 Pressure-temperature-time paths
DE: 3660 Metamorphic petrology
DE: 8012 High strain deformation zones
SC: Tectonophysics [T]
MN: Fall Meeting 2005