TECTONIC UNROOFING OF THE CENTRAL WOLVERINE METAMORPHIC COMPLEX, NORTH-CENTRAL BRITISH COLUMBIA

The Wolverine metamorphic complex (WMC) consists of siliciclastic and basaltic rocks belonging to the basal Windermere Supergroup. The WMC underwent upper amphibolite facies metamorphism during Mesozoic crustal thickening related to the obduction and accretion of Slide Mountain terrane and the Insular Superterrane onto the western paleomargin of North America. Peak metamorphism of the WMC was accompanied by partial melting and the development of tight to isoclinal NE-vergent folds that are axial planar to a transposition foliation. Garnet-biotite thermometry and garnet-aluminosilicate-quartz-plagioclase barometry indicate peak metamorphic conditions of 790-830 °C and 8.8-10.5 kbars. The absence of retrograde muscovite indicates a near-isothermal decompression path from 8.8-10.5 to 4 kbars, corresponding to ~18 km of exhumation prior to cooling below approximately 650°C. Juxtaposition of Middle Jurassic greenschist-facies rocks against upper amphibolite-facies rocks of the WMC that record 40Ar/39Ar cooling ages of 53-49 Ma suggests the WMC remained deeply buried until it was rapidly exhumed in the Eocene along the west-directed Wolverine fault zone.

Southwest-vergent deformation that is observed both to the north and south of the WMC is conspicuously absent at the latitude of this study; this is reconciled by suggesting that the WMC represents a diffuse NE-vergent basal shear zone that formed beneath a SW-vergent panel of rocks that was translated northeastward during Mesozoic contraction. Eocene extension along the Wolverine fault zone placed the NE-vergent panel of high-grade rocks within the WMC against low-grade upper crustal levels that were exhumed in the Middle Jurassic prior to the development of a SW-vergent prowedge. This tectonic unroofing of the WMC is attributed, in part, to a reduction in the internal strength of a critically tapered wedge due to significant devolatilazation and partial melting of middle to lower crustal levels. This also coincided with widespread Eocene extension in the southern Canadian Cordillera that is attributed to an increase in the obliquity of convergence at the plate margin and the onset of transtension and right-lateral strike-slip faulting.