Transient rheology of feldspar

Transient creep of the lower crustal minerals such as feldspar is important to explain postseismic deformation following a large continental earthquake. However, transient creep of feldspar is poorly understood and the flow law parameters are unknown so far. Therefore, we performed constant strain rate deformation experiments on synthetic fine-grained anorthite aggregates under wet conditions using a Paterson-type gas deformation apparatus. We conducted tests at temperatures from 1000 ºC to 1200 ºC and confining pressure of 400 MPa. Typical strain rates in our experiments were 1x10^-4 s^-1, 2.5x10^-4 s^-1, 5x10^-4 s^-1, and 7.5x10^-4 s^-1, including some strain rate stepping experiments. In general, the transient creep accounted for 6-8% of the total strain (~10-15%), which is high compared to 2-3 % transient deformation observed in previous experiments on anorthite, quartz, and olivine aggregates. Inspection of the microstructures of deformed samples using transmission electron microscopy reveal dislocation activity and antiphase domain boundaries. Analysis of steady-state creep data indicates that the samples were deformed at the boundary between diffusion and dislocation creep with a power law stress exponent of ~1.4 and an activation energy of 272 kJ/mol. Because a constitutive equation for transient creep of feldspar is not well established, we estimated transient creep flow law parameters using inter-granular and intra-granular models. In the intergranular model for a polycrystalline aggregate, where grains are randomly oriented,  it is assumed that low strain (i.e., transient creep) is accommodated by individual grains with soft/easy slip orientation and high strain (steady-state creep) is accommodated by grains with hard/strong slip orientation. In contrast, in the intra-granular model, both transient creep and steady-state deformation are dominated by intragranular processes, such as long-range elastic interactions of dislocations. In the intragranular approach, we find that the full stress vs. strain curve (i.e., including transient and steady-state creep) can be modelled using a stress exponent of ~1.5 and an activation energy of ~200 kJ/mol. Applying the intergranular model, we get a stress exponent of ~3 and an activation energy of ~130 kJ/mol for transient creep of anorthite aggregates. Extrapolated to natural strain rates, these two approaches will have different implications in modelling postseismic deformation.