A reversible rheology for water-weakened quartz
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The overall goals of this project are to use experimental techniques to investigate : 1) the effect of chemical environment (water fugacity), strain rate, and temperature on the strength of quartz single crystals deformed in an axial compression; 2) the effect of the same parameters on the strength of synthetic quartzites (quartz aggregates) made from the same starting material as the single crystals; and 3) the fundamental relationship between the strengths of single crystals and polycrystals. These goals will be accomplished by performing experiments on annealed synthetic quartz single crystals and synthetic quartzites over a wide range of temperatures, water fugacities, and strain rates in a Griggs-type piston-cylinder rock deformation apparatus using a molten salt cell for precise determination of mechanical data. Specifically, the researchers will: 1) perform temperature-,strain rate-, and pressure-stepping experiments on single crystal cores in of synthetic quartz crystals, which have been heat-treated to establish a constant density and uniform distribution of freezable fluid inclusions, to investigate basal , prism and prism slip systems; 2) reverse the sequence of experimental temperature, strain rate, and water fugacity conditions to test whether these rate laws are reversible; 3) hot-press polycrystalline quartz aggregates made of powders fabricated from the same heat-treated synthetic quartz materials, and subject these polycrystalline quartz samples to temperature-, strain rate-, and pressure-stepping experiments, testing for reversibility, and comparing the results with T, strain rate, and water fugacity sensitivities of basal and prism slip. The mechanical data from these experiments will be used to develop predictive models of deformation of natural quartzites at tectonic rates.Displacements of middle to lower crustal thrust faults and low-angle normal detachments associated with collisional tectonics and channel flow are commonly localized within quartz-rich lithologies. Postseismic visco-elastic relaxations in quartzo-feldspathic rocks below plate-scale continental fault zones are therefore thought to be governed by quartz strength. Motivated by the importance of quartz deformation to tectonics and rheology of the continental lithosphere, many experimental studies have been performed to investigate the deformation and recovery mechanisms of quartz, and quantify mechanical relations that can be applied to tectonic loading of the crust. Yet, the mechanism(s) of water weakening of quartz continues to be unresolved and little work has been performed to determine the effects of water fugacity, strain rate and temperature on the strength of the common slip systems in quartz. This project uses new experimental methods to address these problems. Simple models can be constructed that relate the strengths of the individual slip systems to the strengths of the quartzites over the range of experimental conditions and extrapolate the results to naturally deformed quartzites. Data from these experiments can be used in field-based research and numerical simulations of the viscosity of the continental crust at plate boundaries and may aid in the understanding of the processes in the continental crust that cause the deformations related to earthquakes.