ALONG-STRIKE COMPARISON OF DIACHRONOUS PLATEAU DEVELOPMENT WITHIN THE CANADIAN CORDILLERA AND ITS MODES OF COLLAPSE BY OROGEN-PARALLEL AND OROGEN-NORMAL EXTENSION

Development of high-grade metamorphism and penetrative transposition fabrics within the hinterland of the Canadian Cordilleran appears to be strongly diachronous along the length of the orogen. An older and longer history of tectonism seems best preserved in the northern Canadian Cordillera that includes Permian to Late Cretaceous events, whereas its southern counterpart mainly preserves Early Jurassic to Late Cretaceous events. Regardless, a significant phase of orogenesis along the entire length of the Canadian Cordilleran orogen appears to coincide with Early Jurassic to mid-Cretaceous obduction of allochthonous terranes over the North American continental margin, which together with the imbrication of parautochthonous rocks, formed a foreland-propagating orogenic wedge.

The culmination of Cordilleran orogenesis resulted in a Cretaceous plateau system that may have extended along most of the length and width of the Canadian Cordilleran hinterland. The plateau system likely achieved a maximum thickness of at least 55-65 km, akin to that of the Altiplano and Tibetan plateaux, and was underpinned by plutonism, penetrative ductile deformation and high-grade metamorphism.

From north to south, the timing and modes of collapse of the plateau system and attendant exhumation of mid- to lower crustal levels appear to be quite different. In the north, the metamorphic hinterland changed from orogen-perpendicular wedge dynamics to orogen-parallel extension in the mid-Cretaceous. Rocks situated in the mid- to lower crust in the Middle Jurassic to Early Cretaceous were exhumed in the mid-Cretaceous along southeast-directed, orogen-parallel, extensional faults from beneath a supracrustal lid. Like the Himalayan orogen and eastern Alps, orogen-parallel extension in the northern Cordillera developed in an orthogonal plate convergence setting, simultaneous with, and bounded by, orogen-parallel strike-slip faulting that facilitated northwestward lateral escape of rocks normal to the direction of convergence. Conversely, collapse of the plateau system in the south was facilitated by Paleogene orogen-normal, crustal-scale extensional faulting that coincided with a transition to a dextral transtensional plate convergence setting between oceanic lithosphere and the North American plate.