ABSTRACT: New relative sea-level observations from the Northern Cascadia Subduction Zone and implications for Cordilleran Ice Sheet history and mantle rheology.

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Abstract

New data from Barkley Sound on the west coast of Vancouver Island, British Columbia, and from the mainland coast north of Vancouver add to the inventory of constraints on relative sea level during and after the collapse of the Cordilleran Ice Sheet (CIS). Previously published observations lie on a northwest-southeast profile along the east coast of Vancouver Island. The new sea-level observations extend existing data further back in time and create a southwest-northeast profile across Vancouver Island and the Strait of Georgia that is approximately perpendicular to the strike of the Cascadia Subduction Zone. The two profiles intersect in the central Strait of Georgia, where the sea level history was described by Hutchinson et al. (2004). At the western end of the study area, sea level in Barkley Sound dropped from a high-stand above 30 m elevation just before 15 000 cal BP to below the present shoreline around 14 500 cal BP, consistent with the timing of a previously documented, well-constrained low stand. Relative sea level in the central Strait of Georgia dropped from well over 150 m to about -15 m from 14 000 cal BP to 11 500 cal BP. Near Sechelt Inlet, on the mainland coast, sea level closely followed the mid-strait trend, dropping from a high-stand of at least 150 m just prior to 14 000 cal BP. Sometime between 12 500 and 11 000 cal BP, sea level near Sechelt fell past present levels to a poorly constrained low-stand between about 11 000 and 10 000 cal BP. The data are consistent with a previously noted trend of relatively early and deep low-stands on the periphery of the former ice sheet and later and shallower low-stands towards the interior. The mean rate of glacial isostatic rebound near Sechelt was similar to that of the mid-strait (about 11 cm/yr), while the rate was somewhat lower in Barkley Sound. The new data will refine existing models of the ice-sheet history during the rapid CIS deglaciation and may reveal spatial variations in shallow mantle viscosity related to the structure of the Cascadia Subduction Zone.

Bibliography of Canadian Geomorphology