Postglacial rebound at the northern Cascadia subduction zone

Postglacial rebound is the response of the Earth to the decay of ice-sheets. A postglacial rebound model explains crustal tilting and rapid uplift at the northern Cascadia subduction zone that occurred during retreat of the Cordilleran ice-sheet. Observations explained by the model include the shoreline tilts of two proglacial lakes that formed at 13.5-14 ka (C-14 yr ago) and rapid sea level fall (land uplift) at 12-12.5 ka. Modelled mantle viscosity values range from 5 x 10(18) to 5 x 10(19) Pa s, and are consistent with previous viscosity inferences from observations of crustal deformation following subduction zone earthquakes (10(18)- 0(19) Pa s). No lower limit to subduction zone mantle viscosity is apparent from our model, but viscosity values equal to or larger than 10"0 Pa a are definitely ruled out. Our modelled subduction zone viscosity values are smaller than most upper-mantle viscosity estimates derived from postglacial rebound studies of tectonically less-active regions (10(20)-10(21) pa s). The rapid observed uplift at 12 ka requires, in addition to a low mantle viscosity, rapid unloading from a sudden collapse of remaining coastal portions of the southern Cordilleran ice-sheet. The sudden collapse provides 0.18 m of global eustatic sea level rise, approximately 0.7% of the sea level rise associated with melt-water pulse IA. Predictions of a global postglacial rebound model (ICE-3G) with a 10(21) Pa s upper-mantle viscosity were previously applied to geodetic data from this region to isolate signals associated with the earthquake cycle. Owing to the low-viscosity values, and resulting rapid recovery of glacial deformation, our model predicts present-day postglacial rebound uplift rates at least 10 times smaller than ICE-3G (less than about 0.1mm/yr). As the ICE-3G adjustments were substantial, this indicates the need for re-evaluation of the geodetic data. (C) 2000 Elsevier Science Ltd. All rights reserved.