Abstract

Three independent data sets from northwestern India and Pakistan suggest initial displacement along >1000 km of the Main Boundary thrust prior to 10 Ma, at least 5 m.y. earlier than previously reported. Regionally extensive changes in the depositional characteristics and rates of the foreland-basin fill between 11 and 9.5 Ma are interpreted to reflect new hinterland loading due to the formation of the Main Boundary thrust. Sediment-accumulation rates, sandstone-siltstone ratios, and thickness and amalgamation of individual sandstone bodies all substantially increase after 11 Ma in well-dated stratigraphic sections from Pakistan to Nepal across the Indo-Gangetic foreland basin. In the Himachal Pradesh reentrant of northwestern India, a newly discovered 8.7 Ma conglomerate derived from the hanging wall of the Main Boundary thrust indicates that source-area uplift and denudation must have occurred prior to 9 Ma and probably prior to 10 Ma, assuming a gravel progradation 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 northwest Pakistan indicate that rapid cooling below ∼105° C between 8 and 10 Ma followed bedrock uplift and erosion that began ∼1–2 m.y. earlier. These data indicate that the Main Boundary thrust in the western Himalaya formed synchronously along strike in the middle-late Miocene, has a displacement rate of ∼10 mm/yr, and has a displacement history that is coeval with late displacement on the Main Central thrust.

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