From intra-oceanic convergence to post-collisionnal evolution: the India-Asia convergence in NW Himalaya, from Cretaceous to present

Y. Rolland

The NW part of the Himalayan Orogen (Kohistan, Ladakh and Karakoram, in Pakistan and India) has been investigated to reconstruct the successive stages of convergence of two continents (India and Asia) over the past 110 Ma, from oceanic to post-collisional settings. The intra-oceanic stages of the convergence have been reconstructed from study of the preserved Tethyan Arc series of the Kohistan-Ladakh Terrane. Geochemical and lithological data indicate continuation of the Kohistan intra-oceanic arc in the west into an Andean arc on the Tibetan continental margin in the east. Adakitic and Nb-Ta-Ti rich lavas appear to be present, interlayered with basalts to andesites in the intra-oceanic arc series. Along with major, trace and isotopic (Sr, Nd and Pb) data of basalt to andesite lavas, adakitic magmatism suggests complex interactions between crustal melts and mantle. In the tectonic context of the Neo-Tethys Ocean at 110 Ma, the Kohistan-Ladakh Arc system may have formed following the subduction of the Neo-Tethys mid-oceanic ridge, similar to a model which has been proposed for the initiation of Oman ophiolite obduction. This ocean-ridge subduction could be triggered by the initiation of fast northward drift of India in the Middle Cretaceous period.

The post-collisional evolution comprises two stages: 1. A phase of crustal thickening by SW nappe stacking in a NE-SW shortening context, associated with Barrovian-type metamorphism (650°C, 10 kbar) between 60 and 37 Ma. 2. A phase of ongoing north-south shortening, characterised by tectonic partitioning between: a. An E-W band of domes that crosscuts the former structures, in a N-S shortening and vertical extrusion context, linked to HT granulite facies metamorphism (800°C, 6 kbar) and magmatism showing mantle affinities, formed during the last 20 Ma. This granulite grade metamorphism could be linked to advective heat input from the asthenosphere due to detachment of the Indian slab by 20 Ma. b. The Karakoram strike-slip fault, which accommodates the lateral extrusion of Tibet. Granulites exhumed within the fault zone suggest that it could be a lithospheric-scale fault, with a total dextral offset of approximately 300 km deduced from correlation between the Lhasa and Karakoram blocks.