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Compositional dependence of lower crustal viscosity
Author(s) -
Shinevar William J.,
Behn Mark D.,
Hirth Greg
Publication year - 2015
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1002/2015gl065459
Subject(s) - geology , crust , andesite , viscosity , continental crust , feldspar , basalt , mineralogy , petrology , geochemistry , quartz , volcanic rock , thermodynamics , volcano , physics , paleontology
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 % SiO 2 ) and (ii) andesitic (64 wt % SiO 2 ). 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.