Experimental Constraints on Crustal Rheology

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The rheology of the lower continental crust controls a wide range of processes that are important for understanding the development and evolution of plate boundary shear zones, seismic hazards, the geochemical evolution of the crust and mantle, the coupling of mantle flow and crustal dynamics and the long-term support of mountain belts. While existing laws that describe the flow behavior of lower crustal rocks can explain many geologic observations, the extrapolation of these laws to conditions appropriate for lower continental crust indicates that the interpretation of geophysically- and geologically-derived data can be improved with new experimental data. The goal of this project is to provide new experimental data on the rheology of rocks at pressures appropriate for the lower continental crust. The project will involve three lines of investigation involving a set of experiments conducted in a Griggs and Paterspm deformation apparatuses at temperatures from 900 to 1100 degrees C and strain at confining pressures from 500 MPa to 1.5 GPa. These experiments will address the following: (1) The role of water on rheology/viscosity of lower crustal rocks. Initial work demonstrates dramatic changes in viscosity of diabase with increasing water content. These results will be extended to higher temperature/lower stress to increase the resolution of extrapolation to natural conditions; (2) Dynamic recrystallization and rock fabric (i.e., lattice preferred orientations) development in plagioclase. The results of initial experimental work challenge previous interpretations based on analyses of naturally deformed rocks. However, at face value, interpretation of the natural rocks remains consistent with other geological data. Resolution of this issue requires better understanding of grain size evolution during deformation, and better understanding of lattice preferred orientation development in plagioclase. (3) Rheology of amphibole. There are very few experiments on the rheology of amphibole, despite its importance for crustal rheology and ubiquitous presence in mid- to lower-crustal shear zones. The lack of data partly reflects difficulty in deforming amphibole in the lab owing to its limited thermal stability. Motivated by microstructures in natural rocks, the researchers will use very fine-grained aggregates to explore the role of low stress deformation processes in amphibole, which is a new approach. Understanding the rheological properties of the lower crust is important for a broad range of geodynamic problems. Probably the most important reason to understand crustal rheology is for the accurate assessment of earthquake hazards produced by time-dependent loading of seismogenic faults - as may be quantified by geodetic studies. The techniques used in this research effort involve state of the art application of electron microscopy and mechanical testing equipment which bridge the gap between materials science and the geological sciences. An additional component of the project will be the creation of a searchable archive of results from previous deformation experiments in the Brown University Rock Mechanics laboratory. The archive will contain information from more 2000 experiments covering more than 40 years of research. Access to such a database will facilitate new ideas and encourage scientists outside of the immediate experimental rock deformation community to explore rheological data and attendant microstructures pertinent to their own research.