APATITE FISSION-TRACK DATING OF THE COWICHAN FOLD-AND-THRUST SYSTEM, SOUTHERN VANCOUVER-ISLAND, BRITISH-COLUMBIA

The Cowichan fold and thrust system on southern Vancouver Island compr ises a linked system of folds and thrust faults in crystalline and sed imentary rocks of the Wrangellia terrane and its sedimentary cover, th e 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 degrees C) for both the Island Pluton ic Suite from the Wrangellian basement and the Nanaimo Group occurred at similar to 50-40 Ma (Middle Eocene). Thrusting is interpreted to ha ve occurred immediately prior to this time in the Middle Eocene, but a fter post-Cretaceous regional burial of the Turonian to Maastrichtian Nanaimo Group and deposition of Upper Paleocene and Lower Eocene sands tones, which are involved in the deformation. These timing constraints support the hypothesized temporal linkage between contraction and upl ift of Wrangellia during the Middle Eocene and continued margin-normal contraction of the Pacific Rim terrane (+/- the Crescent terrane) at similar to 45 Ma.