Dissolution-precipitation creep in polymineralic granitoid shear zones in experiments II: Rheological parameters

Abstract. The transition from strong to weak mechanical behaviour in the Earth's continental middle crust is always caused by an initiation of viscous deformation. Microstructural evidence from field examples indicates that viscously deforming polymineralic shear zones represent the weakest zones in the crust and may dominate mid-crustal rheology. The results of recent experiments (as in companion paper 1) demonstrate that the observed weak behaviour is due to the activation of dissolution-precipitation creep (DPC). Formation of fine-grained material and efficient pinning of grain growth are important prerequisites for the formation of a stable deforming microstructure. However, available rheological parameters for fine-grained polymineralic rocks deforming by DPC are insufficient. A series of three types of experiments was conducted on a granitoid fine-grained ultramylonite to different strains at 650 °C–725 °C, 1.2 GPa with strain rates varying from 10^-3 s^-1 to 10^-6 s^-1. Type I and II experiments are solid natural samples, providing key microstructural evidence for DPC. Type III are general shear experiments performed on coarse- and fine-grained ultramylonite powder. All experiments were combined to estimate rheological parameters for such polymineralic shear zones. A stress exponent n≈1.5 and grain size exponent m≈-1.66, with uncertainties, were estimated and coupled with microstructural observations. Extrapolations indicate that at slow natural strain rates, DPC in polymineralic granitoid fault rocks can occur at lower temperatures than monomineralic quartz. A deformation mechanism map is proposed, indicating a transition in the deformation mechanism from dislocation creep in monomineralic quartz to DPC in weaker polymineralic fine-grained granitoid, based on strain rate and grain size. Most importantly, the polymineralic composition is the determining factor in achieving the fine grain sizes necessary for DPC to become activated. This is due to the presence of additional chemical driving potentials and phase mixing, both of which are absent in monomineralic systems.