RAPID ISOSTATIC DEPRESSION OF THE WESTERN MARGIN OF THE NORTH AMERICA PLATE DUE TO GLACIAL LOADING DURING THE LATE PLEISTOCENE

New data from southwestern British Columbia indicate that the crust at the western margin of the North America plate subsided rapidly due to glacial loading during the early phase of the Fraser Glaciation (transition from oxygen isotope stage 3 to stage 2). Brackish and marine diatoms occur up to 17 m above sea level in the lower silty facies of Quadra Sand at Vancouver. Quadra Sand and correlative sediments occur widely in and around the Strait of Georgia and Puget Sound. The unit is proglacial outwash deposited in front of glaciers that later coalesced to form the Cordilleran ice sheet. The silty sediments containing the brackish and marine microfossils have yielded four AMS and conventional radiocarbon ages ranging from 24,400 to 25,000 radiocarbon years BP. Global sea level at that time was about 90 m below the present datum (Cutler et al., 2003), indicating that the crust was depressed over 100 m at Vancouver 25,000 radiocarbon years ago (29,000 calendric years ago). This large amount of depression can be explained only by ice loading. Our data indicate that (1) ice cover in the southern Coast Mountains 29,000 years ago was much greater than today, and (2) the crust subsided rapidly as the Cordilleran ice sheet began to form late during oxygen isotope stage 3. The rapid response of the crust to loading during the early phase of the Fraser Glaciation is similar, although opposite in direction, to the crustal response in the same area during deglaciation 14,000 years later (James et al., 2000).

Cutler, K.B., Edwards, R.L., Taylor, F.W., Chen, H., Adkins, J., Gallup, C.D., Cutler, P.M., Burr, G.S., and Bloom, A.L., 2003. Rapid sea-level fall and deep-ocean temperature change since the last interglacial period. Earth and Planetary Science Letters 206: 253-271.

James, T.S., Clague, J.J., Wang, K., Hutchinson, I., 2000. Postglacial rebound at the northern Cascadia subduction zone. Quaternary Science Reviews 19: 1527-1541.