PALEOGENE PULL-APART CORE- AND RIND-COMPLEXES, CANADIAN CORDILLERA

Paleogene translation, extension and magmatism in the central Canadian Cordillera, British Columbia are interpreted to be formed by plate motions expressed as a sequence of right steps of a dextral transform system. That stepping transform system permitted northwesterly directed crustal extension, thinning and local exposure of deeper crustal roots. The Paleogene structural regime throughout vast areas of central British Columbia is typical of the upper crustal sheets of low-angle detachments created by simple-shear crustal extension (here called a rind complex). Locally this crustal rind is punctuated with exposures of high to medium grade metamorphic core complexes. The magmatism includes moderate level granodiorite, high level peraluminous granite and porphyry, basaltic andesite and peraluminous rhyolite and dacite. The extensive volcanics have chemistry indicative of within plate and arc derivation. They range in age from 52 to 45 Ma (felsic 53-49 Ma, mafic 51-45 Ma). The area underlain by Eocene magmatism is broken by a dense set of northeasterly trending extensional faults. Local shallowly-dipping ductile shear zones have northwesterly- and southeasterly-directed extensional fabrics. They form the upper bounds to metamorphic core complexes with uplift ages of 50-42 Ma. Dated magmatism and structure show that extension in central British Columbia occurred 55 to 42 Ma. These structures are the same age as, and are interpreted to be a northern continuation of the Shuswap and Okanagan extensional complex of southern British Columbia and northern Washington State. These complexes are interpreted to have been generated by transfer of dextral motion from the Yalakom-Ross Lake fault system to the Southern Rocky Mountain Trench-Northern Rocky Mountain Trench fault system. In turn that system was cut by a discrete 43-35? Ma more northerly trending dextral fault system, also right stepping, with crustal extension and thinning, and which may not have generated new core complexes.