ABSTRACT: Glacio-isostatic adjustment modeling of new relative sea-level observations from the Northern Cascadia Subduction Zone, British Columbia, Canada.

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Abstract

Late-glacial sea-level curves located above the Cascadia Subduction Zone (CSZ) in southwestern British Columbia show that glacio-isostatic adjustment (GIA) was rapid when the Cordilleran ice sheet collapsed in the late Pleistocene. GIA models developed to explain the sea-level observations employ an ice sheet model modified from previous studies. The Earth models vary radially and feature an elastic lithosphere and a linear Maxwell viscoelastic mantle with the VM2 viscosity structure in the deeper parts of the mantle. The thickness and viscosity of a laterally homogeneous asthenosphere are systematically varied to find the combinations that best explain the sea-level observations. The observations can be equally well fit across a wide range of asthenospheric thicknesses, provided that the asthenospheric viscosity is varied from 3 x 1018 Pa s for a thin (140 km) asthenosphere to 1019 Pa s at 220 km thickness to 4 x 1019 Pa s for a thick (380 km) asthenosphere. The sea-level observations are located in the CSZ forearc above the stagnant mantle wedge. Thus, the model viscosity values probably pertain largely to the viscosity of the oceanic mantle beneath the subducting Juan de Fuca plate, although a contribution from the hot, low-viscosity arc and backarc continental mantle is also likely. Effective viscosities for the upper mantle due to tectonics (subduction) were computed using the strain-rates and temperatures of a geodynamic model of the CSZ and a wet-olivine power-law rheology. The effective viscosities agree well with GIA model viscosities of 1019 Pa s or less, corresponding to an asthenosphere of one or two hundred kilometers thickness. Models of the megathrust earthquake cycle at young subduction zones that feature oceanic mantle asthenosphere viscosities larger than about 1019 Pa s need to be modified to incorporate the new constraints provided by the GIA modeling. An implication for megathrust earthquake models of a reduction in oceanic asthenospheric viscosities is that it may require significant deep afterslip on the subduction interface to explain observations of rapid postseismic uplift following great earthquakes. Present-day vertical crustal motion predicted by the GIA models shows rates of a few tenths of a millimeter per year, consistent with previous analyses.

Bibliography of Canadian Geomorphology