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

We combine zircon sensitive high-resolution ion microprobe U-Pb spot dating and mica [sup.40][Ar-.sup.39]Ar plateau ages with field-geological and geochemical constraints from the Malashan 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. Keywords: north Himalayan gneiss domes, granite, age dating, extensional tectonics.