Geological Evolution of the Karakoram Terrane since Neoproterozoic

Abstract

The Karakoram Terrane (KT) in the western margin of the Himalayan-Tibetan orogen preserves the record of the Tethyan oceanic lithospheric subduction beneath the Asian Plate and metamorphism linked to the subduction of the Indian slab and the India-Asia collision. This terrane is also significant for understanding the changes in tectonics over time in the continent-continent collision zones related to mantle dynamics, slab underthrusting, or large-scale strike-slip faults. Despite a long geological research history over the past 140 years, this terrane's magmatic, petrogenetic, deformation, and uplift history remains enigmatic. This is mainly because of non-consensus over the distribution, source, and processes for the emplacement of variably occurring I-and S-type granitoids and changing tectonic processes through time. Thus, to resolve these issues, this paper reviews the KT's magmatic, deformation, and uplift history. Apart from its Neoproterozoic basement, two main stages of Andean-type magmatism have been identified in the KT. Stage 1 happened during ∼160-100 Ma, forming hornblende (Hbl)-biotite (Bt)-bearing and Bt-bearing granitoids. The field evidence, hybrid isotope signatures, and thermodynamic whole-rock major element modeling show that the formation of these granitoids involves assimilation with upper crustal rocks. The Stage 2 Hbl-Bt bearing Pangong Transpression Zone (PTZ) granitoids in the eastern KT were formed during ∼83-56 Ma; they show juvenile isotopic signatures and, thus, their origin from the mantle. The major element thermodynamic modeling of these granitoids suggests that these rocks were formed due to fractional crystallization. Our compilation and analysis of geochemical, isotopic, and geochemical modeling suggest that these granitoids belong to the Ladakh magmatic arc rather than the Karakoram Batholith granitoids of the KT, contrary to earlier beliefs. The origin of Eocene-Oligocene leucogranites of the KT remains controversial with the variable inferred processes of their origin, like water-flux melting of granitoids, dehydration melting of metapelites, and slab-breakoff, shearing or compression-induced partial melt generation. This study assesses these scenarios by compiling geochronological, geochemical, and isotopic data to understand the role of these processes in partial melt generation over spatial and temporal scales. Our assessment suggests that the monzogranites and hbl-bearing leucogranites are generated as a result of slab-breakoff and KF shearing-induced melting of the existing igneous sources, while the two-mica±grt leucogranites are sourced from the melting of supracrustal metapelitic rocks due to crustal thickening. This paper provides the compilation of cooling rates from all the published mid-to-low temperature thermochronometric ages to assess the different phases of the uplift of the KT on spatial and temporal scales. Our compilation suggests that the KT was uplifted mainly in three phases: (a) during the Early-Late Cretaceous as evidenced by pre-India-Asia collisional metamorphism and crustal thickening, (b) lithospheric-delamination driven uplift as a result of the slab breakoff of the subducting Indian lower crust during the Oligo-Miocene, and (c) in its final phase because of the underthrusting of the Indian Plate beneath the KT during the Pliocene. The interplay of tectonic uplift and glacial erosion carved the present-day topography of the KT into steeper hill slopes and relief with steeper river channel gradients since the Plio-Quaternary.