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Stress redistribution during major earthquakes in the upper crust can cause episodic and non-steady state deformation in the middle crust, leaving a characteristic imprint in the affected rocks. We perform experiments that are designed to simulate the natural stress history at such a situation. The experimental microfabrics are compared to the natural record. Three types of experiments are carried out using a modified Griggs-type solid medium deformation apparatus. All three start with high stress deformation at a temperature of 400°C and a constant strain rate of 10-4s-1 ("kick"), some are followed by annealing in the stability field of α-quartz for 14-15 h at zero nominal differential stress and temperatures of 800-1000°C ("kick&cook"), or by annealing for 15 h at 900-1000°C and at a residual stress ("kick&stir"). Semi-brittle damage zones in the otherwise almost unaffected quartz are characteristic features developed in the "kick" experiment. Similar microstructures are expected to develop in crustal rocks near the tip of seismogenic faults during coseismic loading. While these features cannot be studied in natural rocks, because they are modified during their prolonged thermal history, they can be studied in samples experimentally deformed at near-natural conditions, as in our "kick" experiments. Quasi-static annealing of samples previously deformed at high stresses ("kick&cook") results in statically recrystallized grains aligned in strings restricted to the semi-brittle damage zones that developed in the "kick" experiment. The striking similarity between such microstructures and those found in vein quartz from 2-3 km depth in Long Valley Exploratory Well, California, with independent evidence of seismic activity, suggests development by identical processes. Therefore, the strings of recrystallized grains may serve as criterion for coseismic loading to very high stresses, followed by quasi-static annealing during postseismic stress relaxation in the geologic past. To investigate the effect of deformation during stress relaxation, experiments with annealing under residual stress after deformation at high stress ("kick&stir") are performed. The samples show remnant old grains with high dislocation density, deformation lamellae, shear zones and dynamically recrystallized aggregates. These distinct microfabrics represent flow by dislocation creep accompanied by dynamic recrystallization during postseismic stress relaxation, which followed high stress deformation during coseismic loading. A corresponding natural record is identified in exhumed mid-crustal rocks from the Sesia zone, western Alps. The experiments provide a base to interpret the natural microstructural record in terms of episodic, non-steady state deformation driven by earthquakes in the overlying seismogenic zone. This is considered as crucial for the set-up of advanced models of the earthquake cycle, and for the interpretation of geodetic observations after major earthquakes.
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