Tectonic and magmatic evolution of the eastern Karakoram, India

Abstract

The Shyok suture zone separates the Ladakh terrane to the SW from the Karakoram terrane to the NE. Six tectonic units have been distinguished. From south to north these are: 1. Saltoro formation; 2. Shyok volcanites; 3. Saltoro molasse; 4. Ophiolitic melange; 5. Tirit granitoids; 6. Karakoram terrane including the Karakoram batholith. Albian–Aptian Orbitolina-bearing limestones and turbidites of the Saltoro formation tectonically lay over high-Mg-tholeiites similar to the tectonically overlie Shyok volcanites. The high-Mg tholeiitic basalts and calcalkaline andesites of the Shyok volcanites show an active margin signature. The Saltoro `molasse' is an apron-like, moderately folded association of red-green shales and sandstones that are interbedded with ~ 50 m porphyritic andesite. Desiccation cracks and rain-drop imprints indicate deposition in a subaerial fluvial environment. Rudist fragments from a polygenic conglomerate of the Saltoro `molasse' document a post-Middle Cretaceous age. The calcalkaline andesites of the Shyok volcanites are intruded by the Tirit granitoids, which are located immediately south of the Ophiolitic melange and belong to a weakly deformed trondhjemite-tonalite-granodiorite-granite suite. These granitoids are subalkaline, I-type and were emplaced in a volcanic arc setting. The subalkaline to calcalkaline granitoids of the Karakoram batholith are I-and S-type granitoid. The I-type granitoids represent a typical calcalkaline magmatism of a subduction zone environment whereas the S-type granitoids are crust-derived, anatectic peraluminous granites. New data suggest that the volcano-plutonic and sedimentary successions of the Shyok suture zone exposed in northern Ladakh are equivalent to the successions exposed along the Northern suture in Kohistan. It is likely that the Kohistan and Ladakh blocks evolved as one single tectonic domain during the Cretaceous–Palaeogene. Subsequently, collision, suturing and accretion of the Indian plate along the Indus suture (50–60 Ma) together with tectonic activity along the Nanga Parbat-Haramosh divided Kohistan and Ladakh into two arealy distinct magmatic arc terranes. The activity and a dextral offset along the Karakoram fault (Holocene–Recent) disrupted the original tectonic relationships.