Compositional dependence of lower crustal viscosity

Abstract

We calculate the viscosity structure of the lower continental crust as a function of its bulk composition using multiphase mixing theory. We use the Gibbs free-energy minimization routine Perple-X to calculate mineral assemblages for different crustal compositions under pressure and temperature conditions appropriate for the lower continental crust. The effective aggregate viscosities are then calculated using a rheologic mixing model and flow laws for the major crust-forming minerals. We investigate the viscosity of two lower crustal compositions: (i) basaltic (53 wt % SiO2) and (ii) andesitic (64 wt % SiO2). The andesitic model predicts aggregate viscosities similar to feldspar and approximately 1 order of magnitude greater than that of wet quartz. The viscosity range calculated for the andesitic crustal composition (particularly when hydrous phases are stable) is most similar to independent estimates of lower crust viscosity in actively deforming regions based on postglacial isostatic rebound, postseismic relaxation, and paleolake shoreline deflection. Key Points Crustal viscosity estimates are most similar to predictions for an andesitic lower continental crust The andesitic model predicts aggregate viscosities similar to feldspar An andesitic crust is ~1 order of magnitude less viscous than basaltic crust