Transient Creep of Quartz and Granulite at High Temperature Under Wet Conditions

Transient creep of crustal rocks is important to explain time-dependent geological processes such as postseismic deformation following a large continental earthquake. While the steady-state creep flow law parameters of quartz and feldspar, major minerals in the upper and lower crust, are well known, the physical mechanism behind transient creep and the corresponding flow law parameters are poorly understood. We quantify the flow law parameters for both quartz and granulite (mixture of plagioclase and pyroxene) under wet conditions with a nonlinear Burgers model using a Markov chain Monte Carlo (MCMC) method. Modeling results yield an activation energy of 70 +/- 20 kJ/mol and a stress exponent of 2.0 +/- 0.1 for transient creep of quartz aggregates. For granulite/feldspar, we find activation energies of 280 +/- 30 and 220 +/- 20 kJ/mol and stress exponents of 1.0 +/- 0.2 and 0.9 +/- 0.1 under mid (1050-1100 degrees C) and high (1125-1150 degrees C), temperature conditions, respectively. The stress exponents and activation energies of transient creep are consistently smaller than those of steady-state creep for both quartz and granulite/feldspar. Combined with results for transient creep of olivine that were previously obtained (Masuti & Barbot, 2021, ), we suggest that the activation energies and stress exponents of transient creep are smaller than those of steady-state creep for volumetrically important silicate minerals of the crust and upper mantle. Extrapolation of the estimated flow law parameters of granulite/feldspar to natural conditions suggests that transient creep may dominate during the postseismic period and lasts longer than previously thought. Earthquakes induce intermittent deformation of the solid Earth at rates that are higher or lower than the interseismic strain rates. When an earthquake occurs, it is accompanied by a sudden slip on the fault and a rapid stress change in the lower crust leading to an evolving rock strength. Lower crustal rocks represent a complex assembly of minerals and several different micro-mechanisms may be activated during deformation. We use experimental data for quartz and feldspar to constrain the mechanical properties of the transient creep (i.e., stress changes with strain under constant strain rate conditions or strain changes nonlinearly with time under constant stress conditions). Based on our results, we conclude that the transient creep following an earthquake may not be as short lived as previously has been thought. Transient creep flow law parameters of quartz and granulite are determined using a Markov chain Monte Carlo methodLow stress exponent of the transient creep in silicates could be due to weak dependency of dislocation density on the stressTransient creep could be dominant during the postseismic phase of the earthquake cycle