In summary, investigations in the NW part of the Himalayan Orogen have provided structural, metamorphic, geochemical and geochronological data, which help to constrain the tectonic evolution of the area. These data suggest the following evolution (Fig. 14).
Figure 14. Tectonic reconstruction proposed for the India-Asia convergence
Tectonic reconstruction proposed for the India-Asia convergence in NW Himalaya since 110 Ma to 65 Ma, from the data compiled in this article.
1. Accretion of the Lhasa – Karakoram block to the Asian margin in the Early-Middle Cretaceous (1 a-b, Fig. 14). This accretion blocked the only free boundary of the Tethyan Ocean, with India commencing its convergence with Asia (2, Fig. 14).
2. Formation of an unique volcanic arc / back-arc above a N-dipping subduction zone, to the south of the Asian Karakoram margin. Arc volcanism was likely produced by interactions between slab melts and mantle after subduction of the Neo-Tethyan mid-oceanic ridge (3, Fig. 14). This arc was immature and intra-oceanic in the west, evolving to mature and possibly continental-based, as the Tibetan active margin to the east. This lateral evolution, which is supported by the evolution of sediments from marine in the west to clastic, continent-derived in the east suggest a subduction oblique to the Asian margin.
3. The Ladakh back-arc domain was subducted under the southern rim of Asia (3, Fig. 14). Evolution of subduction-derived magmatism from calc-alkaline to alkaline compositions in Asia suggests steepening of the slab by 88 Ma (4, Fig. 14).
4. Accretion of Ladakh-Kohistan Arc to the Asian rim occurred in the range 88-80 Ma (5, Fig. 14). South-vergent thrusts were cross-cut by calc-alkaline plutons and overlain by Andean-type volcanic rocks.
Figure 15. 3D sketch of post-collisional evolution in NW Himalaya orogen
A, horizontal shortening, and crustal-scale folding, within the Karakoram margin. B, Lithosphere-scale shearing. PT path obtained in the Nanga Parbat from Whittington et al. (1998), in the Karakoram margin from Rolland et al. (2001), and in the Karakoram Fault from Rolland & Pêcher (2001).
5. Subsequent collision of India with the Asian margin produced a lithosphere-scale partitioning between zones affected by horizontal shortening (A in Fig. 15) and zones of strike-slip deformation (B in Fig. 15). In both cases HT metamorphism has been observed due to advection processes within crustal-scale folds or lithosphere-scale faults, respectively. The evolution of magmatic rocks towards mantle composition over the past 20 Ma, suggests melting of the continental lithosphere following Indian slab detachment process.