Kinematic Vorticity Gauges and the Rheology of Mylonitic Shear Zones

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Many hazards encountered by humans, including earthquakes, volcanic eruptions and tsunamis, result from plate tectonics. How tectonic plates move and interact with one another, and how deformation that occurs at their interacting boundaries is localized into structures like the San Andreas fault in California, are first-order questions in the Earth Sciences. At active tectonic plate boundaries, GPS data have allowed a much clearer understanding of plate interactions, localization of deformation, and relations to seismic and volcanic hazards. However, such data provide little information about plate interaction and deformation at great depth. The most direct way to study these deeper processes is to work in ancient plate boundary zones that have been exhumed by uplift and erosion. Geologists use a range of tools to evaluate the histories of deformation in these exhumed rocks, but the validity of some of these tools still needs to be tested. The primary goal of this project is to test some microstructural tools used to extract from exhumed, deformed rocks an important quantity known as kinematic vorticity. Through this study the investigators hope to clarify under what conditions these tools can be reliably used to measure kinematic vorticity. The work will be conducted in the Norumbega Fault System, which cuts across the entire State of Maine, and is one of the very few ancient analogs for the San Andreas Fault in California. Thus, our results will have applicability to a well-known seismically active fault. More specifically, these researchers will investigate the oblique quartz shape preferred orientation and rigid clast rotation methods for determining kinematic vorticity in a well-characterized mylonitic shear zone with approximately monoclinic strain symmetry. They will provide a detailed analysis of the microstructural factors that may compromise clast methods of kinematic vorticity analysis, and determine if one of the other methods gives consistent results even where clast methods are compromised. In addition, they will develop criteria for estimating the degree of strain localization at clast/matrix boundaries and conduct numerical sensitivity analyses to better understand the effects of clast lubrication on the bulk shear strength of mylonitic shear zones that form the roots of seismically active faults. Detailed microstructural investigations will utilize optical, scanning electron microscopy and electron backscatter diffraction techniques. 2D and 3D parametric numerical sensitivity analyses will be used to investigate a range of parameters that may affect clast kinematics, and assess how the evolution of clast lubrication with increasing strain may contribute to long-lived weakening of shear zones. The preliminary results are novel, and suggest caution when using clast rotation methods for determining the kinematic vorticity number, but they open exciting new possibilities for investigating strain-dependent changes in rock strength that arise from feedbacks among chemical and mechanical processes during deformation.