Time-temperature pathways of himalayan and trans-himalayan crystalline rocks: A comparison of fission-track ages

Abstract

Fission-track (FT) dates of apatate and zircon recording, respectively, paleo-temperatures of 100–120°C and ∼230°C provide a useful tool for understanding the thermotectonic history of the Himalaya, which is the world's loftiest and most dynamic orogenic belt, brought about by the continent-to-continent collision of the Indian and Asian plates. Systematic FT ages are now available for the crystalline rocks of the Higher Himalaya and Trans-Himalaya in Pakistan and N.W. India, and some FT ages have been reported from southern Tibet, the Karakorum and the Kunlum. These FT data are synthesized and discussed here in order to assess the contribution of FT thermochronology to Himalayan geology. FT zircon and apatite ages from the Higher Himalayan Crystalline rocks are confined to the Neogene. For example, in the Zanskar region of N.W. India and the Babusar-Kaghan region of Pakistan, zircon ages are 13–17 Ma and apatate ages range from 4–10 Ma (mostly ∼6 Ma) indicating an exhumation of at least 8 km for these rocks since the Middle Miocene and average unroofing rates of ∼ 6 mm yr −1. In the northwestern Himalaya, overall the FT ages become younger towards the Nanga Parbat Massif, which is the promontory of the Indian plate and has experienced a rapid Quaternary uplift and exhumation. Other areas of young uplift in the Himalayan include antiformal domes in Zanskar, such as the Kishtwar Window. The bulk of the Cretaceous-Paleocene Trans-Himalayan crystalline rocks, which are situated to the north of the Indus-Tsangpo Suture Zone and were generated from the consumption of the Tethyan ocean beneath Asia, seem to have cooled rapidly through paleo-temperatures of 200–500°C in the Eocene as a result of uplift and erosion that affected the Trans-Himalayan Batholith after the India-Asia collision. Apatite ages from these rocks in western Kohistan, southern Ladakh, and Lhasa are Early-Middle Miocene. In the Karakorum (western Tibet) and the Kunlun (northern Tibet), Miocene apatite ages demonstrate young tectonics of the ranges beyond the Himalaya related to the India-Asia convergence.