CONTRASTING TECTONIC STYLES IN THE NORTHERN GRENVILLE PROVINCE - IMPLICATIONS FOR THE DYNAMICS OF OROGENIC FRONTS

In western Labrador, tectonic elements of the parautochthonous belt of the northern Grenville orogen include a regional, mid-crustal, thin-s kinned, metamorphic fold-thrust belt structurally overlain by a thick- skinned thrust sheet with a high-pressure metamorphic signature. In ce ntral Ontario, the parautochthonous belt is composed of a major revers e-sense shear zone, along which deep crustal rocks have been uplifted and exhumed. Tectonic models for both areas involve crustal shortening by delamination in the lower crust and tectonically driven exhumation up a crustal-scale ramp. The fold-thrust belt northwest of the ramp i n Labrador is a result of the development of critical topography in th e thrust wedge at the top of the ramp and the accretion of thrust slic es to the base of the wedge during its propagation onto the foreland. The fold-thrust belt is absent in Ontario because syntectonic erosion of the thrust wedge prevented the development of critical topography.