Post-metamorphic cooling history of the Indian Plate crystalline thrust stack, Pakistan Himalaya

Abstract

A post-metamorphic Himalayan cooling history for the crystalline internal zones of the northern Indian plate is defined by amphibole and mica K-Ar and Ar-Ar geochronology. Hornblende, muscovite and biotite cooling ages from cover sequences metamorphosed during the Himalayan orogeny are 35–40, 30 to 23, and 29 to 23 Ma respectively. The mica ages, together with those derived from zircon and apatite fission track data demonstrate a cooling rate of about 30°C/Ma during the late Oligocene to early Miocene. This rapid cooling was initiated during the post-metamorphic development of the Indian Plate south-verging crustal-scale thrust stack. Most of the cooling occurred during the stripping of some 10 ± 2 km of overburden through a combination of erosion, recorded in the Miocene molasse sediments of the foreland basin, and major crustal extension within upper levels of the Indian Plate and the Main Mantle thrust zone. Both erosion and extension were the direct consequence of the evolution of the thrust stack.