Collaborative Research: Dynamics at the Base of a Pseudotachylyte-bearing Fault System

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A concerted effort through EarthScope is currently underway to better understand seismicity along the San Andreas Fault, which poses serious threats to society from large-magnitude earthquakes. The San Andreas Fault Observatory at Depth features an instrumented drill hole, allowing characterization of the rocks, fluids, kinematics, and stress orientations and magnitudes within the fault zone to depths of approximately 3 km. These studies are designed partly to investigate the strength of the San Andreas Fault, which is critical for seismic forecasting. In conjunction with these studies it is important that we locate and study appropriate exhumed examples of similar faults at the surface today. One of the very few surface occurrences of an analog for the San Andreas Fault is the Norumbega Fault System (NFS), which cuts across the entire State of Maine and is the field site for this study. Through the NFS study we hope to clarify the value of near-surface stress measurements on the San Andreas Fault as indicators for the strength of the fault. This work is intended to support the dissertation of a female PhD student, and several undergraduate student theses. Our close cooperation with the Maine Geological Survey, and our involvement with K-12 Earth Science teachers in Maine, lends additional societal relevance to the work. Due to the large elastic moduli of solid Earth materials elastic stresses are transmitted over great distances and depths within the Earth. For this reason, the stress orientations and magnitudes measured near Earths surface provide only a partial picture of the state of stress on active faults. For a more complete picture, we must understand the state of stress on faults at greater depths, particularly near the frictional-to-viscous transition where the strongest part of the crust occurs. For this reason we are focusing on the deeply exhumed NFS. To provide relatively tight constraints on the stress tensor the field locality should contain irrefutable evidence for coseismic rupture (pseudotachylyte), and be characterized by: (a) tightly constrained kinematic boundary conditions, (b) microstructures amenable to estimates of differential stress and mean kinematic vorticity, and (c) lithological (rheological) heterogeneity providing natural variability in the stress and vorticity estimates. In such a system, 3D numerical experiments can provide best-fit solutions for the field-derived stress and vorticity estimates, and in doing so solve for the principal and Cartesian stresses. Our work in the NFS involves field mapping, microstructural analysis using optical and electron-beam techniques, and 3D numerical modeling that is tightly constrained by the field-derived data. Our primary aims are to: (a) systematically evaluate the microstructural variation across the strain gradient in the fault zone mylonites, (b) use microstructural measurements to calculate temperatures, differential stresses and mean kinematic vorticity numbers for numerous samples across the field area, (c) use these calculated values and 3D numerical experiments to help constrain the stress tensor at the time when this mylonite zone was overlain by a seismically active fault, and (d) extend our numerical modeling to the surface to explore how kinematic boundary conditions and stresses at depth influence active faults above.