THE TRANS-HUDSON OROGEN OF NORTH AMERICA AND THE HIMALAYAN OROGEN OF ASIA, PART 1: STRUCTURAL AND THERMAL EVOLUTION OF THE LOWER PLATE

Collision of the Superior plate with the composite Churchill plate ca. 1.8 billion years ago led to formation of the Trans-Hudson Orogen of North America, possibly the largest and most recognizable continental collision recorded in the first 2,200 million years of Earth history. Similarly, collision of the Indian plate with Asia ca. 50 million years ago resulted in the formation of the Himalaya, the Karakoram-Hindu Kush mountain ranges and the Tibetan Plateau, arguably the largest continental collision in the last 450 Ma of Earth history. Both orogens share key tectonic, structural and petrological collisional traits, which suggest that the India-Asia collision zone is an instructive present-day analogue for collisional orogens in the Paleoproterozoic.

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.