Glacio-isostatic Adjustment Modelling of Improved Postglacial Sea-level Constraints from Vancouver Island, British Columbia

The Cordilleran ice sheet reached a maximum at about 14,000 14C years BP (14 ka), and shrank to remnant mountain glaciers by about 9.5 ka. The weight of the ice sheets caused crustal subsidence in southwestern British Columbia, and in some places the postglacial marine limit reaches 250 m elevation. Two recently published relative sea-level curves in the northern and central Strait of Georgia, as well as a new curve for southern Vancouver Island, constrain the magnitude and timing of postglacial rebound. After the ice sheet retreated, sea-level fall was rapid, at a rate that initially exceeded 10 cm yr-1. Initial glacio- isostatic adjustment modeling uses a simple layered Earth model with an elastic lithosphere and Maxwell viscoelastic asthenosphere and lower mantle. A robust early result is that an ice-sheet model previously developed for the region (James et al., 2000) needs to be thickened by about 30%. With the thickened ice model, relatively good agreement is obtained with the observations. The magnitude of the mid-strait sea-level curve is, however, systematically under-predicted compared to the northern strait and suggests the need for additional modifications to the ice-sheet retreat model. Inferred viscosity of the asthenosphere is consistent with previous modeling suggesting a shallow mantle viscosity less than about 1020 Pa s. For an asthenospheric layer thinner than 280 km, the best-fitting viscosity values are less than about 3× 1019 Pa s. Higher viscosity values are found for a thicker asthenosphere, indicating a trade-off between asthenospheric thickness and viscosity.