The goal of this proposal is to provide new experimental data on the rheology of mafic crustal rocks at temperatures and pressures appropriate for the lower continental crust. Rheology is the study of deformation and flow of matter under an applied stress. Understanding the rheological properties of the lower crust is important for a broad range of geologic problems. In terms of societal relevance, the most important reason to understand crustal rheology is for the accurate assessment of earthquake hazards produced by time-dependent loading of seismogenic faults. The rheology of the lower continental crust also controls the geochemical evolution of the crust and mantle (the chemical composition of the continental crust remains a controversial subject; one hypothesis that reconciles differences in current models supposes that melt that moves upwards from the mantle to the crust beneath volcanoes begins to crystallize at depths of ~30 km. As these crystals cool they become denser than the material below and thus can sink back into the mantle, but only if they are weak enough to flow at relatively rapid rates), the coupling of mantle flow to crustal dynamics (for example, how are movements at the Earth?s surface related to the long term convective motion in the interior) and the long-term support of mountain belts. Existing flow laws for mafic rocks do explain many geologic observations. However, extrapolation to conditions appropriate for the base of continental crust indicates that the interpretation of rheologic data derived using geophysical techniques could be improved with new experimental data.
We propose to: (1) Conduct experiments on dry, coarse-grained gabbroic and plagioclase-rich rocks to provide creep data in the dislocation creep regime at conditions where we can minimize (a) the potential effects of microcracking and/or cavitation and (b) problems with resolving the relative contributions of diffusion creep and dislocation creep. (2) Quantify how water influences the rheological properties of plagioclase-rich rocks and evaluate how much water is needed to produce significant differences between dry and wet rheologies. (3) Determine if the phenomenal cavitation microstructures produced in lower pressure torsion experiments are inhibited at higher pressure. We will obtain rheological data at conditions beyond the conventional range used in gas medium machines ? and/or at temperature/strain rate conditions where brittle processes initiate at lower pressure. The experimental program will provide results that bridge the gap between different deformation apparatus, improving confidence in the application of laboratory mechanical data.
