Stress Magnitude, Strain Rate, and Rheology of Extended Middle Continental Crust Inferred from Quartz Grain Sizes in the Whipple Mountains, California

Knowledge of the magnitude of differential stress and strain rate during the formation of mylonitic shear zones in metamorphic ore complexes provides constraints on the mechanical behavior of the middle continental crust during extension. We analyzed the differential flow stress during the mylonitization of quartzofeldspathic rocks in the Whipple Mountains, California, using grain-size piezometers and kinetic laws for grain growth. Mylonitic gneisses collected from two widely separated transects have grain sizes that cluster in a range from 32 to 61 gm. Analysis of grain growth kinetics indicates that mylonitization of the gneisses continued during cooling to temperatures _<500øC, compatible with estimates from two-feldspar thermometry. Quartz grain-size piezometers uggest hat the mylonitization occurred under differential stresses ((•1-(•3) of -40-150 MPa, or maximum shear stresses of 20-75 MPa. Extrapolation of quartzite flow laws to 500øC indicates that the mylonitization occurred at strain rates faster than 10 -14 s -1. These stimates suggest that he mylonitic zone within the Whipple Mountains had an effective viscosity of the order of 1018+4-1020•--4 Pa s. These low viscosities and rapid strain rates, combined with seismic reflection data showing that continental crust is layered, suggest hat more realistic physical models of extension of the continental lithosphere should treat the' lithosphere as a heterogeneous distribution of high-viscosity regions separated by lowviscosity zones.