History and isostatic effects of the last ice sheet in southern British Columbia

The Late Wisconsinan Cordilleran ice sheet covered British Columbia, southern Yukon Territory, and parts of Alaska, Washington, Idaho, and Montana. Its major source areas were the high mountain ranges of the Canadian Cordillera, and flow was strongly controlled by topography. The last ice sheet began to develop about 30,000-25,000 (14)C yr ago, but it did not achieve its maximum extent until 15,000-14,000 (14)C yr BP. Ice sheet growth was interrupted locally by stillstands and retreat. Ice sheet decay was rapid and was characterized by complex frontal retreat at the periphery, accompanied locally by brief readvances, and by downwasting and stagnation. By 10,000 (14)C yr BP, < 5000 yr after the Last Glacial Maximum, ice cover in British Columbia was similar to that of today. The Cordilleran ice sheet depressed the crust over which it formed. Rapid isostatic rebound during deglaciation caused the sea to fall relative to the land along the southern British Columbia coast. A postglacial rebound model explains crustal tilting and rapid uplift in this region during retreat of the ice sheet in the late Pleistocene. Small values of modelled viscosity for the upper few hundred kilometers of the mantle deliver small present-day crustal tilt rates, consistent with mid- and late-Holocene sea-level observations. Rapid sea-level fall (land uplift) on eastern Vancouver Island during deglaciation requires, in addition to a low mantle viscosity, rapid unloading of the crust due to accelerated wastage of coastal portions of the southern Cordilleran ice sheet about 12,000 (14)C yr ago. (C) 2001 Elsevier Science Ltd. All rights reserved.