High Temperature Deformation-Mechanism Maps for Synthetic Plagioclase Rocks

Estimation of crustal stresses and viscosities are based on extrapolations of laboratory experiments to natural strain rates, temperatures, and pressures. We present deformation-mechanism maps for feldspathic rocks in stress-grain size space. The data are from creep tests on fine-grained synthetic aggregates of anorthite, labradorite, and albite, deformed at 300 MPa confining pressure and temperatures between ≈ 0.6-0.9 T_m. Samples were nominally dry (0.005-0.01 wt% H₂O) or contained 0.1-0.3 wt% H₂O (wet). Some specimens contained <5 wt% Si-rich melt. The deformation mechanism maps show a transition from grain boundary diffusion-controlled creep to dislocation creep as a function of grain size, temperature and water content. The critical grain size at the transition between regimes increases with increasing water content. When extrapolated to strain rates of 10^-14s^-1 our data indicates that dislocation creep of wet plagioclase rocks depends on composition and requires minimum temperatures of ≈ 350-450°C at stresses of ≈ 100-300 MPa. This is in good agreement with field estimates. At similar conditions in the diffusion creep regime stresses are between ≈ 1-10 MPa for ultramylonite-type rocks with ≈ 10-30 μm grain size. However, nominally dry plagioclase rocks are drastically stronger and require temperatures ≈ 200°C higher to initiate dislocation creep. At temperatures between 400-600°C and depending on trace content of H₂O and deformation mechanism the viscosity of feldspar-dominated crustal rocks is estimated to range from ≈10²² Paṡs - 10¹⁹ Paṡs. Significantly lower viscosities of rocks containing > 0.1 wt% water require a fine grain size <50 μm at higher temperatures.