THE TRANS-HUDSON OROGEN OF NORTH AMERICA AND THE HIMALAYAN OROGEN OF ASIA, PART 1: STRUCTURAL AND THERMAL EVOLUTION OF THE LOWER PLATE
The structural and thermal evolution of the lower plate Superior margin during continental collision involves (1) early, thin-skinned thrusting and consequent regional metamorphism (400-575°C; 6.3-9.1 kbar; 1820-1814 Ma), (2) out-of-sequence thrusting and regional Ky grade metamorphism (585-720°C; 7.7-9.8 kbar; 1814-1795 Ma), and (3) amphibolite facies re-equilibration (675-775°C; 7.0-8.9 kbar; 1795-1758 Ma), partial melting and leucogranite formation. The crustal evolution of the Indian plate along the Himalaya involves (1) early, deep subduction of thinned continental crust to UHP eclogite depths (~680°C; 28 kbar; 49-46 Ma), (2) regional Ky grade metamorphism (550-680°C; 10-12 kbar; 35-32 Ma), and (3) widespread, regional Sil ± Crd grade metamorphism (650-770°C; 3.7-4.5 kbar; 30-16.5 Ma) associated with partial melting and leucogranite formation.
In the Himalaya, the shallow depths of high-temperature metamorphism and melting are consistent with mid-Miocene ductile flow of an Indian plate mid-crustal channel, southward from beneath southern Tibet to the Greater Himalaya. This zone is bounded by crustal scale shear zones, the Main Central Thrust, with its inverted and compressed metamorphic isograds along the base, and the South Tibetan Detachment system of low-angle, normal sense shear zone along the top. In contrast, the thermal evolution of the lower plate in Trans-Hudson Orogen appears to be primarily a consequence of the thermal relaxation of crustal isotherms in tectonically thickened crust without recourse to a mid-crustal channel.