CAREER: Experimental Investigation of Viscous Anisotropy of Foliated Rocks: Implications to the Strength of the Mid to Lower Continental Crust

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Earthquake aftershocks in the shallow upper crust are thought to be controlled in part by the viscosity of the underlying deep crust or mantle. Models of how the viscosity of the lower crust affects this portion of the seismic cycle are based on rocks with simplistic homogeneous viscosities. However, most natural rocks have heterogeneities due to alignments of minerals called foliations, which will affect the mechanical behavior of these rocks. The principal investigator and students are determining how the viscosity of common foliated rocks is affected by the orientation of these foliations relative to the direction of loading. They are performing laboratory experiments at conditions that simulate deformation in the mid to lower crust on two different foliated rocks with the foliation in a variety of orientations. The results of these experiments will be incorporated into a model that evaluates how foliations affect the viscosity of rocks relative to homogeneous rocks, which will benefit society by: 1) improving our understanding of the response of the lower crust to loading during the seismic cycle, 2) supporting research experiences of several graduate students and several undergraduate students, and 3) supporting Science, Technology, Engineering and Mathematics (STEM) outreach activities to underprivileged students.Understanding the effects of foliation orientation, interconnectivity and preexisting lattice preferred orientations on viscous anisotropy is critical in order to accurately model the rheology of the lower crust and the earthquake cycle. In this project the principal investigator and students will: 1) determine if micaceous foliation orientation causes viscous anisotropy of lower crustal rocks, 2) determine how interconnectivity of the micaceous foliation affects the degree of viscous anisotropy of lower crustal rocks, and 3) determine how a preexisting lattice preferred orientation (LPO) affects viscous anisotropy of these rocks. These researchers will perform both axial compression and shear deformation experiments with a fine-grained biotite + quartz + plagioclase gneiss or a micaceous quartzite with the foliations and preexisting lattice preferred orientations in a range of orientations relative to the compression direction. The samples from these experiments will be analyzed using the petrographic microscope, Scanning Electron Microscopy/ Electron backscatter diffraction (SEM/EBSD) to characterize grain-scale microstructures and lattice preferred orientations, Fourier-transform infrared spectroscopy (FTIR) to determine water contents and Transmission Electron Microscopy (TEM) to determine deformation mechanisms. Portions of this study will be performed in the Advanced Structural Geology course at the University of Akron and thin sections from experimental samples will be used in the laboratory section of the Structural Geology course at the University of Akron. The expected results will provide significant sets of microstructural and mechanical data which can be applied to modeling the viscous anisotropy of the lower crust. The principal investigator and students will also develop new experimental techniques for high pressure experiments on crustal rocks and a new, inexpensive apparatus for STEM outreach activities about earthquake mechanisms.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.