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

New geophysical data acquired over the buried crystalline basement of weste rn Canada provide constraints on the history of tectonic assembly of the we stern Canadian Shield in the interval 1.75-1.85 Ga. Specifically, these dat a provide new perspectives on the evolution of an Archean continental fragm ent (Hearne province) that was trapped in a tectonic "vise" between coeval orogenic belts that dipped beneath the Nearne province. The Trans-Hudson or ogen developed along the southeastern margin of the Hearne province as a se ries of ocean floor, oceanic are, and are marginal basins were telescoped a nd thrust obliquely beneath the Hearne. Along the northwest edge of the Hea rne, collapse and subduction of a narrow marginal basin, now marked by the subsurface extension of the Snowbird Tectonic Zone, led to formation of mag matic are and collision of order Proterozoic terranes. The Hearne province itself is characterized by regional granulite-grade metamorphism and eviden ce of extensive and pervasive partial melting of the crust. The internal ch aracter of the Hearne province seen on crustal seismic reflection profiles is that of a crustal-scale structural fan with reflection fabrics that verg e toward the bounding orogens. The deformation of the Hearne is predominant ly of Paleoproterozoic age and constitutes a thorough reworking of this for merly Archean crustal domain over a distance of more than 600 km across str ike. Entrapment and thermal weakening of the Hearne resulted from mechanica l coupling of inferred buoyant subduction-collision zones and removal of or modification of Archean lithospheric mantle that may have originally forme d the keel to the Hearne; Long-period magnetotelluric profiles show that an omalously conductive mantle lithosphere underlies the present day Hearne pr ovince, which is attributed to metasomatic modification of the subcontinent al 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 H earne province crust and subcontinental mantle, may be an example of the li thospheric consequences of opposing collisional polarity during assembly of continents.