Abstract

The Cowichan fold and thrust system on southern Vancouver Island comprises a linked system of folds and thrust faults in crystalline and sedimentary rocks of the Wrangellia terrane and its sedimentary cover, the Upper Cretaceous Nanaimo Group, and Upper Paleocene to Lower Eocene sandstones of the Chuckanut Formation. Apatite fission-track analysis indicates that cooling below the closure temperature of fission-track retention in apatite (100 ± 20 °C) for both the Island Plutonic Suite from the Wrangellian basement and the Nanaimo Group occurred at ~ 50–40 Ma (Middle Eocene). Thrusting is interpreted to have occurred immediately prior to this time in the Middle Eocene, but after post-Cretaceous regional burial of the Turonian to Maastrichtian Nanaimo Group and deposition of Upper Paleocene and Lower Eocene sandstones, which are involved in the deformation. These timing constraints support the hypothesized temporal linkage between contraction and uplift of Wrangellia during the Middle Eocene and continued margin-normal contraction of the Pacific Rim terrane (± the Crescent terrane) at ~ 45 Ma.

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