High-stress crystal-plasticity versus creep of rock-forming minerals – the importance of stress-loading rates indicated by deformation microfabrics

There is ample evidence of transient high stresses of several hundred MPa at the base of the seismogenic zone in the continental crust, i.e. at greenschist-facies conditions. The microstructural evidence from these depths includes twinning and kinking of jadeite and amphibole, as well as quasi-instantaneous cataclastic deformation of garnet and quartz. At interseismic strain rates, known flow laws for dislocation creep of the rheological dominant mineral, quartz, and/or dissolution precipitation creep of crustal rocks predict lower stresses at the given pressure-temperature conditions. Thus, fast stress-loading rates are required to explain the inferred high-stress crystal-plasticity at greenschist-facies conditions, i.e. loading rates from few tens of MPa to several hundred of MPa within minutes, corresponding to the rupture times for major earthquakes in the seismogenic zone. Although high-stress crystal-plasticity is not allowing to accumulate a high amount of strain, as high stresses prevail only transiently, it provides a driving force for accelerated but rapidly decaying creep, where higher amounts of strain can be accumulated. The strength of both, fault rocks and their host rocks, is strongly depending on the stress conditions that control whether they behave by high-stress crystal-plasticity or creep at given pressure-temperature conditions. Thus, the rheology of crustal rocks is dependent on the stress-loading rates during the seismic cycle controlled by the distance to the tip of the seismic active fault.