THE TRANS-HUDSON OROGEN OF NORTH AMERICA AND THE HIMALAYA-KARAKORAM-TIBETAN OROGEN OF ASIA, PART 2: STRUCTURAL AND THERMAL EVOLUTION OF THE UPPER PLATE

The collisional upper plate to the 1.8 Ga Trans-Hudson Orogen (THO) in NE Canada comprises a collage of Archean crustal blocks and Paleoproterozoic terranes that were assembled, deformed, and metamorphosed prior to terminal collision with the Superior plate. Early tectonothermal events include (1) accretion of Meta Incognita microcontinent to the Rae craton and consequent tectonic thickening of continental margin strata between 1880-1865 Ma, (2) pre- and post-accretion emplacement of Andean-type plutonic suites including the 1865-1848 Ma Cumberland batholith, (3) Grt-Crd-Sil metamorphism at mid-crustal depths (790-830°C; 6.9-8.5 kbar; 1849-1836 Ma), partial melting, and Crd-And metamorphism at higher crustal levels (550-600°C; 3.0-4.0 kbar; 1856-1835 Ma), (4) accretion of the intra-oceanic Narsajuaq arc to the upper plate between 1845-1836 Ma, and (5) emplacement of Andean-type plutons and granulite facies metamorphism along the leading edge of the upper plate between 1836-1820 Ma. Terminal collision is manifest by crustal-scale (re)-imbrication and Bt-Sil-Grt metamorphism (670-760°C; 5.0-7.8 kbar; 1820-1790 Ma).

In the Himalaya, the upper plate includes the vast Tibetan Plateau north of the Indus-Yarlung Tsangpo suture zone, and its extensions to the west, and the Karakoram and Hindu Raj - Hindu Kush Ranges. The geology of the plateau comprises mainly sedimentary and volcanic rocks, rarely deep crustal metamorphic rocks, whereas the Karakoram Range exposes extensive lower crustal rocks and pre- and post-collisional granites. Pre-collisional, 150-95 Ma Andean-type granite-diorites are related to subduction of the Tethyan ocean crust beneath the southern margin of Asia. Post-collisional granites include the Baltoro Bt-monzogranite and Grt-Bt-Ms leucogranites formed 25-18 Ma by melting of lower crust gneisses. St, Ky, and Sil metamorphism is sporadic and semi-continuous from 70-0 Ma along the southern margin of the Karakoram, following collision of the Kohistan arc during the Late Cretaceous, and India in the earliest Eocene.

Late thick-skinned folding and cross-folding in THO is constrained at 1758-1742 Ma. Himalayan manifestations might be the >1cm/year erosion rates in the Nanga Parbat syntaxis of Pakistan, and the Miocene-Pliocene Sil grade gneiss domes in the southern Karakoram.