EVOLUTION OF ARC-CONTINENT COLLISION IN WOPMAY OROGEN, NORTHWESTERN CANADA: UPPER PLATE EXTENSION TO LOWER PLATE BREAKOFF

Wopmay orogen is a Paleoproterozoic orogenic belt formed by the obduction of Hottah terrane, an arc-bearing microcontinent, onto the western margin of the Archean Slave craton at about 1.88 Ga. Early bimodal magmatism, sedimentation, and consequent high-T/low-P metamorphism that occurred on the leading edge of the upper plate are attributed to extension and asthenospheric upwelling during roll-back of the lower plate as the arc encountered progressively older oceanic crust and lithosphere near the passive margin of Slave craton. The upwelling culminated in regional heating of the crust, melting, and generation of the peraluminous-metaluminous Hepburn batholith within the upper plate just prior to thrusting onto the Slave craton. Rocks of the Hepburn batholith and the early bimodal suite are now preserved as part of a large klippe occupying the core of the orogen. Earlier workers considered the supracrustal rocks of the klippe to be rift-related facies of Slave craton, but geological mapping, Pb and Nd studies, and sequence stratigraphy clearly demonstrate that they are exotic and part of Hottah terrane.

A 200 km long, orogen-parallel swarm of mafic sills, located at the passive-margin shelf-slope break of the lower plate, is demonstrably syncollisional as the sills cut orogenic flysch yet were folded, thrust eastward, and metamorphosed. We relate the sills to extension and magmatism caused by breakoff of the oceanic lithosphere as the outer edge of the lower plate entered the trench. Slab failure not only leads to subduction and recycling of much of the rift-facies assemblage and its subjacent basement, but may also be responsible for thick-skinned deformation and inverted metamorphism.