Tectonic entrapment and its role in the evolution of continental lithosphere: An example from the Precambrian of western Canada

Abstract

New geophysical data acquired over the buried crystalline basement of western Canada provide constraints on the history of tectonic assembly of the western Canadian Shield in the interval 1.75–1.85 Ga. Specifically, these data provide new perspectives on the evolution of an Archean continental fragment (Heame province) that was trapped in a tectonic “vise” between coeval orogenic belts that dipped beneath the Hearne province. The Trans‐Hudson orogen developed along the southeastern margin of the Hearne province as a series of ocean floor, oceanic arc, and arc marginal basins were telescoped and thrust obliquely beneath the Hearne. Along the northwest edge of the Hearne, collapse and subduction of a narrow marginal basin, now marked by the subsurface extension of the Snowbird Tectonic Zone, led to formation of magmatic arc and collision of older Proterozoic terranes. The Hearne province itself is characterized by regional granulite‐grade metamorphism and evidence of extensive and pervasive partial melting of the crust. The internal character of the Hearne province seen on crustal seismic reflection profiles is that of a crustal‐scale structural fan with reflection fabrics that verge toward the bounding orogens. The deformation of the Hearne is predominantly of Paleoproterozoic age and constitutes a thorough reworking of this formerly Archean crustal domain over a distance of more than 600 km across strike. Entrapment and thermal weakening of the Hearne resulted from mechanical coupling of inferred buoyant subduction‐collision zones and removal of or modification of Archean lithospheric mantle that may have originally formed the keel to the Hearne. Long‐period magnetotelluric profiles show that anomalously conductive mantle lithosphere underlies the present day Hearne province, which is attributed to metasomatic modification of the subcontinental lithosphere following collisional thickening and delamination/convective removal of thickened lithosphere in the Proterozoic. Tectonic entrapment, as illustrated by the Proterozoic structural and thermal evolution of the Hearne province crust and subcontinental mantle, may be an example of the lithospheric consequences of opposing collisional polarity during assembly of continents.