TSK 11 Göttingen 2006 Trepmann et al. Quartz microstructures in na- ture and experiment — evi- dence of rapid plastic defor- mation and subsequent an- nealing Poster Claudia A. Trepmann1 Bernhard Stöckhert1 Dorothée Dorner2 Mar- tina Küster1 Klaus Röller1 Quartz microstructures produced in short-term deformation and annealing experiments are compared with those in naturally deformed vein quartz in cores from the Long Valley Exploratory Well (Long Valley Caldera, California). The experiments are designed to simulate 1. co-seismic deformation of quartz in the uppermost plastosphere and 2. annealing during post-seismic stress relaxation. The experiments are performed in a modified Griggs type solid medium ap- paratus. Natural polycrystalline quartz samples (grain size on the order of mil- limetres) are deformed at a temperature of 400°C, a confining pressure of 2GPa, and strain rates of ca. 10−4 s−1. The differential stress reaches 2–4GPa and the irreversible axial shortening is typi- cally a few percent. In some exper- iments the samples have subsequently been annealed for ca. 14–15 h at ele- vated temperatures of 800–1000°C and low stresses (quasi-hydrostatic or non- hydrostatic conditions). The confining pressure has been chosen to keep the sample in the stability field of α-quartz. 1 Institut für Geologie, Mineralogie und Geo- physik, Ruhr-Universität Bochum, Germany, Collaborative Research Center 526 2 Max– Planck-Institut für Eisenforschung, Düsseldorf, Germany The samples, which have not been an- nealed after deformation, show deforma- tion bands that are characterised by a crystallographic misorientation of up to 25° to the host quartz grain. The de- formation bands vary in thickness and can grade into fractures. Transmis- sion electron microscopy reveals a high density of straight dislocations arranged into subparallel arrays in the vicinity of a deformation band. The density of free dislocations decreases with distance from the deformation band. Microstruc- tures in deformed samples annealed at 900–1000C and quasi-hydrostatic con- ditions are characterised by strings of isometric grains with a diameter of ca. 10–50 µm that show no crystallographic preferred orientation. In one experi- ment, in which the sample has been annealed at non-hydrostatic conditions subsequent to deformation, shear zones have developed that are characterised by elongate, small grains with a diam- eter of up to 30 µm. The grains are preferentially oriented with their axes in a plane perpendicular to the shear zone. These shear zones are inter- preted to have formed during deforma- tion at non-hydrostatic annealing with dynamic recrystallisation. In contrast, the strings of isometric grains in the experiments with quasi-hydrostatic an- nealing are suspected to be due to static recrystallisation from a highly damaged zone generated during high-stress defor- mation, as observed in the non-annealed samples. These microstructures com- pare well with those observed in nat- urally deformed vein quartz from the Long Valley Exploration Well, with in- dependent evidence of episodic deforma- tion due to co-seismic loading. Compar- ison of experimental and natural record provides insight into processes, condi- 1 Trepmann et al. TSK 11 Göttingen 2006 tions, length and time scales character- istic for the episodic loading and relax- ation in the middle crust driven by seis- mic activity in the upper crust. Such insight may be crucial for the appropri- ate inversion of geodetic results and the interpretation of aftershocks. 2