MIDDLE-LATE MIOCENE (GREATER-THAN-10 MA) FORMATION OF THE MAIN BOUNDARY THRUST IN THE WESTERN HIMALAYA

Three independent data sets from northwestern India and Pakistan sugge st initial displacement along >1000 km of the Main Boundary thrust pri or to 10 Ma, at least 5 m.y. earlier than previously reported. Regiona lly extensive changes in the depositional characteristics and rates of the foreland-basin fill between 11 and 9.5 Ma are interpreted to refl ect new hinterland loading due to the formation of the Main Boundary t hrust. Sediment-accumulation rates, sandstone-siltstone ratios, and th ickness and amalgamation of individual sandstone bodies all substantia lly increase after 11 Ma in well-dated stratigraphic sections from Pak istan to Nepal across the Indo-Gangetic foreland basin. In the Himacha l Pradesh reentrant of northwestern India, a newly discovered 8.7 Ma c onglomerate derived from the hanging wall of the Main Boundary thrust indicates that source-area uplift and denudation must have occurred pr ior to 9 Ma and probably prior to 10 Ma, assuming a gravel progradatio n rate of 3 cm/yr. Three apatite fission-track ages from structures at the leading edge of the Main Boundary thrust in the Kohat region of n orthwest Pakistan indicate that rapid cooling below similar to 105 deg rees C between 8 and 10 Ma followed bedrock uplift and erosion that be gan similar to 1-2 m.y. earlier. These data indicate that the Main Bou ndary thrust in the western Himalaya formed synchronously along strike in the middle-late Miocene, has a displacement rate of similar to 10 mm/yr, and has a displacement history that is coeval with late displac ement on the Main Central thrust.