The Paleoproterozoic Trans-Hudson Orogen: Large, Hot, but Only Moderately Thick?

English Abstract:

The composite Trans-Hudson Orogen (THO), which locally reaches up to 1,500 km in width, compares in size as well as duration of collision with the Grenville Orogen. That being said, in detail they are quite different entities. The most striking difference, notwithstanding local exceptions, is the lesser degree of crustal thickening, as well as the apparent absence of core complex structures and related syn- to post-collisional extensional shear zones observed in the THO in contrast to the Grenville Orogen. Assuming that the thermal gradient of the continental crust and convergence rates were similar at ca. 1.83 Ga and 1.10 Ga, other factors need to be considered in order to explain the observed differences. The most apparent empirical observation is the contrasting pre-collisional tectonic and magmatic accretion histories of both orogens. In the THO, widespread terrane and magmatic accretion shortly preceded terminal collision, resulting in structural and thermal weakening of the crust prior to collision. By contrast, in the Grenville Orogen, terrane and magmatic accretion predated collision by tens or hundreds of million years, allowing sufficient time for crust to cool prior to terminal collision. These contrasting starting conditions likely dictated how both orogens evolved with time, placing constraints on thermal and rheological boundary conditions. Another important factor, well constrained in the THO and much less so in the Grenville, is the size and shape of colliding continents and the presence of indenters, re-entrants, and oceanic freeboard. Particularly, the Ungava indenter (NE Superior Craton) was responsible for the highest degree of exhumation (by thrusting), as well as producing lateral escape of crustal blocks in the western Churchill Province, and southerly-directed crustal extrusion in the Core Zone, perhaps by tectonic overpressure.