Postglacial rebound at the northern Cascadia subduction zone

Abstract

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 ( 14 C yr ago) and rapid sea level fall (land uplift) at 12–12.5 ka. Modelled mantle viscosity values range from 5×10 18 to 5×10 19 Pa s, and are consistent with previous viscosity inferences from observations of crustal deformation following subduction zone earthquakes (10 18–10 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 20 Pa s 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 0.1 mm/yr). As the ICE-3G adjustments were substantial, this indicates the need for re-evaluation of the geodetic data.