The Cowichan fold and thrust system, Vancouver Island, southwestern British Columbia

Abstract

The Cowichan fold and thrust system is a southwest-verging, linked thrust system formed by northeast-southwest, large-scale, crustal contraction involving the Upper Cretaceous Nanaimo Group and its Wrangellian basement. The geometry of the thrust system from plan and profile perspectives illustrates its thick-skinned structural style. The thrust system is interpreted to be a leading imbricate fan. Thrusting was most likely in sequence, creating an estimated minimum 20%-30% shortening at the basement/cover interface. Both fault-propagation and fault-bend folding are evident, the former being more common. Kinematic indicators and the geometry of the thrust system show no evidence of significant transpressional or transtensional displacement fields. Contraction postdates deposition of the Nanaimo Group and is indirectly dated as late Eocene. The sole fault of the thrust system is interpreted to rise from northwest to southeast on a series of deep lateral to oblique ramps and merge with the San Juan fault system. This explains the rapidly westward-thickening wedge of thrusted Wrangellian basement in the study area. In the east, the sole fault is interpreted to have locally permitted the Nanaimo Group to overthrust the San Juan Terrane. Investigation of the Cowichan fold and thrust system reveals how shortening during westward-progressing terrane accretion is accommodated in a forearc region dominated by a large slab of rigid, semicontinental crust.