Basement geology and tectonic evolution of the Vancouver region

Abstract

The Vancouver region is underlain by three different basements, which form the Coast Mountains to the north, the Vancouver Island Ranges to the west, and the Cascade Ranges to the southeast. These elevated areas are separated by Georgia Depression and Fraser Lowland, low regions in which the Late Cretaceous and younger sedimentary cover is preserved. The basements contain much of the record of the crustal evolution of the region, which can be divided into three stages. (1) During the pre-accretion stage (>100 Ma), the crustal block forming Vancouver Island and southwestern Coast Mountains was in uncertain paleogeographic relationship to the North American plate margin, and separated from it by basinal terranes in southeastern Coast Mountains. (2) The syn- and postaccretion stage (100-40 Ma) was initiated when the western block was accreted to the plate margin, an event accompanied and followed by crustal thickening, uplift, and erosion centred in southeastern Coast Mountains. (3) During the last 40 million years, the continental Cascade magmatic arc formed on the North American plate margin above a subduction zone, the surface trace of which was, and is west of Vancouver Island. The regional physiography, probably formed in the last 10 Ma, may be related to stress distribution in the plate margin and thermal expansion in the magmatic arc; it was also influenced by the basements, as topographic depressions coincide with basement boundaries.