Deep Structure of Lithospheric Fault Zones

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This project will investigate how fault zones, of the type and scale that form plate boundaries, change character with depth in the lithosphere below the seismogenic upper crust. This project aims to reconstruct the characteristics of a well-studied example of a major fault zone at the time of motion, with the aim of determining if there is a scaling relationship between important factors that control fault zone behavior: shear zone width, rheology, stress, strain-rate and temperature. The project will focus on the Simplon fault in the Swiss Alps, a fault with a well-constrained rate of displacement, so that strain-rate at different structural levels can be calculated and the shear zone rheology determined. The results of this research will be of value for geodynamic modeling of the lithospheric response to plate motions, the interpretation of geophysical data across plate boundary shear zones, the interpretation of geodetic measurements of post-seismic deformation, and for understanding how seismogenic faults are loaded. The proposed activity will advance desired societal outcomes by full participation of women in STEM, public engagement with STEM through virtual field trips, and development of a diverse, globally competitive STEM workforce through undergraduate and graduate student training plus an undergraduate field course.Faults that accommodate relative plate motion must transect much of the lithosphere, yet their width, internal structure, and mechanical properties as a function of time and depth are poorly understood. This proposal aims to reconstruct the characteristics of a well-studied example of a major fault zone at the time of motion, with the aim of determining if there is a scaling relationship between shear zone width, rheology, stress, strain-rate and temperature. The Simplon fault in the Swiss Alps, will be investigated using field and laboratory techniques to determine its original width, flow stress, strain-rate, and rheology as a function of temperature and depth. The fault is a normal sense shear zone, active between 25 and 3 Ma at a rate of 3-5 mm/year, which exhumed its footwall during motion, exposing highly deformed rocks from depths of up to 30 km. Analytical methods will include: (a) microstructural analysis using optical and SEM methods; (b) measurement of the dynamically recrystallized grain size of quartz for determination of stress during deformation; (c) measurement of the crystallographic preferred orientation of quartz, using electron backscatter diffraction methods, to determine the active slip systems; (d) determination of temperature and pressure during deformation using the Ti content of quartz, laser Raman spectrometry of carbonaceous material, and multi-equilibrium thermobarometry of coexisting minerals; (e) thermochronology using 39Ar/40Ar analysis on hornblende, muscovite and biotite plus U-Th/He analysis on zircon and apatite; and (f) numerical analysis of the thermal evolution of the exhumed shear zone. Strain-rate during deformation will be calculated from the width and displacement rate; together with the stress, this can be used to calibrate the shear zone rheology.