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North-south extension in the Tibetan crust triggered by granite emplacement

¹ Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology, Central 7, Tsukuba 305-8567, Japan
² Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan
³ Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Hiroshima 739-8526, Japan
⁴ Department of Geological Sciences, Central Washington University, Ellensburg, Washington 98926, USA
⁵ Department of Earth Sciences, Faculty of Education, Okayama University, Okayama 700-8530, Japan
⁶ Department of Geology, China University of Geosciences, Beijing 100083, China
⁷ New Mexico Bureau of Mines and Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, New Mexico 87801-4796, USA

We combine zircon sensitive high-resolution ion microprobe U-Pb spot dating and mica ⁴⁰Ar-³⁹Ar plateau ages with field-geological and geochemical constraints from the Mala shan area of Southern Tibet to show that the deformed granite core of the North Himalayan metamorphic domes in this area is not Indian basement, but was intruded and deformed during the Himalayan orogeny. Microstructural observations reveal that a transition from top-to-the-south thrust-related to top-to-the-north extension-related deformation occurred during granite intrusion and related metamorphism. This suggests that intrusion triggered the onset of extensional tectonics in the Tibetan middle to upper crust. Expected positive feedback mechanisms between decompression melting leading to more intrusion and more extensional deformation suggest that this mechanism may have been important on a regional scale.

Key Words: north Himalayan gneiss domes • granite • age dating • extensional tectonics

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