THE PALEOPROTEROZOIC TRANS-HUDSON OROGEN: THE CASE FOR EARLY “MODERN-TYPE” ACCRETIONARY AND COLLISIONAL PROCESSES

The Trans-Hudson orogen in Canada records ca. 150 m.y. of ocean opening followed by ca. 90 m.y. of ocean closure, arc formation, tectonic and magmatic accretion, followed by 30 to 40 m.y. of continent-continent collision during the formation of the Supercontinent Nuna (Columbia). Early events related to plate convergence include outward growth of a pre-existing continental collage consisting of the Archean-age Slave and Rae cratons by successive accretion of Archean ribbon- and micro-continents (Hearne, Meta-Incognita, Sugluk and Sask cratons). Oceanic arc terranes that formed within and outboard of this collage (in the orogen internides) record a complex history of intraoceanic accretion, back-arc opening and inversion, tectonic accretion to continental margins, subduction flip, development of continental arc batholiths and obduction of oceanic crust. Observed collisional processes include development of pro-shear and retro-shear wedges, vergence inversion, vertical-axis rotation and crustal block extrusion. Overall, peak P-T data recorded throughout the orogen imply relatively modest crustal thicknesses during the collisional phase, except in areas where promontories or transpressional shear zones facilitated localized crustal uplift and exhumation of high- to medium-pressure granulites. Elsewhere, metamorphic facies range from prehnite-pumpelleyite to low-pressure granulite facies, with average mid-amphibolite facies rocks distributed across the width of the orogen. Core complexes or systems of large extensional detachment faults have yet to be identified. The very broad width of the orogen, the widespread presence of moderate pressures and temperatures, and the absence of significant tectonic exhumation by extension may indicate that the size and shape of the orogenic welt (broad with modest crustal thickness) may have been governed by thermal and rheological constraints acquired during the accretionary phase of the orogen.