Geological evolution of the Karakoram ranges

Michael P. Searle(*)
(*) Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK. E-mail: mikes@earth.ox.ac.uk

The Karakoram terrane in Northern Pakistan is geologically equivalent to the Qiangtang terrane of central Tibet but unlike Tibet shows extreme topographic relief, high uplift-exhumation and erosion rates and exposes deep crustal metamorphic and igneous rocks of the lower crust. The Karakoram terrane has been above sea-level since Early to Mid-Cretaceous, the age of the youngest marine sedimentary rocks. During the Late Jurassic to Mid-Cretaceous northdipping subduction of Tethyan oceanic lithosphere beneath the Karakoram resulted in widespread Andean I-type granite-granodiorite magmatism (Hunza, Hushe plutonic units) with possible concomitant low-pressure andalusite-sillimanite grade Buchan-type metamorphism. The Kohistan intra-oceanic island arc developed above a second north-dipping subduction zone to the south during the Aptian-Eocene (~120-45 Ma) and was accreted to the southern margin of Asia concomitantly with obduction onto India during the Late Cretaceous. Final closure of Neo-Tethys and collision of India with Asia occurred at ca. 50 Ma. Continuous crustal thickening occurred across the southern Karakoram resulting in widespread regional Barrovian facies kyanite- and sillimanite-grade metamorphism with U-Pb zircon and monazite ages ranging from Latest Cretaceous (~63 Ma) to Pliocene times (4-3 Ma). Crustal melting resulted in intrusion of several sets of leucogranite dykes (Hunza dykes) at ~50 Ma and 35 Ma. The major phase of crustal melting in the Baltoro plutonic unit spanned at least 13 million years from 26-13 Ma (Late Oligocene to Mid-Miocene), and resulted in intrusion of a large-scale biotite monzogranite to garnet two-mica leucogranite intrusion of batholithic proportion. Some extra heat contribution from the upper mantle may have been required to account for the large size of the granite batholith and the lamprophyric dykes intruded around the margins. The youngest metamorphism and partial melting is known from the deep crustal metamorphic-migmatite gneiss domes (Dassu dome) of Pliocene age in the Baltoro region and the Late Miocene Sumayar leucogranite in Hunza. Geological and U-Pb geochronological data show that the Karakoram Range was a dynamic active mountain range with semi-continuous crustal thickening, deformation, metamorphism and partial melting for at least the last 63 million years and continues to this day.

Karakoram, Tectonics, U-Pb ages.