Constraints on the Tectonic Evolution of the Northwestern Himalaya from Geochronologic and Petrologic Studies of Babusar Pass, Pakistan

Abstract

Petrologic, geochronologic, and structural studies of Indian-plate rocks in the footwall of the Main Mantle Thrust indicate that peak metamorphism occurred in thrust imbricates well before the final assembly of the Kohistan island-arc with the Indian plate. As the northern margin of the Indian plate was thickened by this early thrusting, the metamorphic rocks evolved along increasing pressure-temperature paths and, in some regions, reached temperatures sufficient for partial melting. It is difficult to constrain the timing of this early deformation and metamorphism, but $$^{40}Ar/^{39}Ar$$ hornblende ages and U/Pb ages of crosscutting leucogranite dikes suggest that in most areas peak metamorphism was earlier than ~40 Ma and in some areas earlier than 50 Ma. After peak metamorphism, in the interval from ~40 to 25 Ma, rocks of the Indian plate continued to be thrust southward, creating sharp metamorphic discontinuities at thrust boundaries. From 25 to 20 ma rocks of the Indian plate underwent rapid cooling, and data suggest that this rapid cooling occurred over large regions of the northern margin of the Indian plate, along as much as 300 km of the Main Mantle Trust. We tentatively suggest that the rapid cooling resulted from tectonic denudation of the Indian plate as Kohistan slid northward along later normal faults. Between 15 and 20 Ma, faulting ceased and the terranes were locked into their present structural position; since then these terranes have simply undergone uplift and erosion. North of the Babusar region, in the vicinity of Nanga Parbat, tectonic activity along the Main Mantle Thrust continued to the present. In this region, the Indian plate rocks are presently being thrust over the adjacent Kohistan terrane.