HR: 16:20h
AN: T24A-02    [Abstracts]
TI: Early, steep subduction of India beneath Asia required by timing of UHP metamorphism
AU: * Leech, M L
EM: mary@geo.stanford.edu
AF: Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115 United States
AU: Klemperer, S L
EM: sklemp@stanford.edu
AF: Geophysics Department, Stanford University, Stanford, CA 94305-2215 United States
AU: Singh, S
EM: sandeep-singh@lycos.com
AF: Department of Earth Sciences, Indian Institute of Technology, Roorkee, 247667 India
AU: Jain, A K
EM: akj_rke@yahoo.co.in
AF: Department of Earth Sciences, Indian Institute of Technology, Roorkee, 247667 India
AB: Ultrahigh-pressure (UHP) rocks in the NW Himalaya are the youngest on earth, and allow testing of critical questions of UHP formation and exhumation and the India-Asia collision. The Tso Morari Complex (TMC) is a UHP subduction zone complex in eastern Ladakh in the western Himalaya, south of the Indus-Tsangpo suture zone. New U-Pb SHRIMP data from zircon shows the TMC has a Proterozoic protolith with a Pan-African history, and defines three stages in the Eocene history of the TMC, hence constraining the timing of collision, subduction, and exhumation in the western Himalaya. Our precise new single zircon U-Pb ages indicate that UHP eclogite-facies metamorphism occurred at 53.3$\pm$0.7 Ma followed by a HP eclogite-facies event at 50.0$\pm$0.6 Ma, and amphibolite-facies metamorphism at 47.5$\pm$0.5 Ma. Retrograde zircon growth may result from dehydration reaction-related fluids evolved during exhumation; the intermittent presence of these fluids would allow zircon crystallization during exhumation that would document the changing P-T conditions. Today India subducts beneath Tibet at an angle of only c. $10\deg$; at this low angle, rocks must travel over 500 km along the subduction thrust to reach the depth of c. 100 km required for UHP metamorphism. Initial collision of India with Asia, based on stratigraphic and paleomagnetic data, is widely cited as being at 55$\pm$1 Ma. Here we show that Indian rocks reached UHP depths at 53.3$\pm$0.7 Ma. Given the fast India-Asia convergence rate of 69 km/m.y., our result implies an earlier onset of collision (c. 57 Ma) and that UHP rocks must have traveled a short, hence steep path into the mantle. We infer that early continental subduction was at a steep angle, probably vertical, comparable to modern continental subduction in the Hindu Kush.
DE: 9320 Asia
DE: 8102 Continental contractional orogenic belts
DE: 3660 Metamorphic petrology
DE: 1035 Geochronology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting