Extension in south-central British Columbia: mechanical and thermal controls

Abstract

Geological data indicate that the Omineca Belt, in the Canadian Cordillera, experienced a rapid episode of extension between 58 and 48 Ma following a compressional event and crustal thickening in the Early Tertiary. Restored crustal sections have been constructed using listric fault geometries to infer that the amount of Eocene extension affecting the southern Omineca Belt exceeded 50%. At the end of compression in the Early Tertiary the Omineca and Intermontane belts of southern British Columbia reached a crustal thickness of 60 km. Isostatic compensation implies that these belts formed elevated ranges; this is consistent with sediment deposition west of these ranges and the absence of in-situ sedimentary basins. The stress induced by topography and crustal thickening is calculated: the calculations show that the stress in homogeneously thickened lithosphere is small and that tensile stress in the crust is compensated by compression in the mantle. Removal of the mantle lithosphere induces a stress field that is tensile throughout the lithosphere. This local stress field combined with stress induced at the plate boundary by the oblique impingement of the Kula-Farallon ridge controlled the position and direction of crustal extension in south-central British Columbia. The present high heat flow on the west flank of the Omineca Belt, 80 mW m −2, is interpreted as the relict of Eocene extension following removal of the lithospheric mantle.