This grant supports acquisition of an Electron Backscatter Diffraction (EBSD) detector that would be mated to an extant scanning electron microscope in the Keck Microanalysis Laboratory at Sonoma State University (SSU). EBSD allows for rapid quantitative determination of crystallographic preferred orientation in deformed rocks to support structural investigations of crustal deformation processes. The PIs research on crustal deformation associated with transpression will immediately benefit from EBSD. The EBSD will support undergraduate student and faculty research at SSU, promote collaborations with regional scientists in the Cal State University system and promote partnerships with local aerospace businesses with interests in alloy microstructural characterization.
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Shear zones are often found in association with tectonically active areas, such as the rifting in the Basin and Range (e.g., Death Valley), plates sliding past each other (e.g., the San Andreas Fault), and magma bodies intruding at a collisional boundary (e.g., the Cascade Subduction Zone, WA, OR, and northern CA). As a structural geologist, I am interested in the movement, the mechanics, and the evolution of these shear zones. One aspect of understanding the development of a given shear zone is to determine how its shape changes, that is, whether it becomes narrower or wider with time. Until relatively recently, it has been difficult, if not impossible, to determine the relative amount of thinning versus shearing that takes place within a given shear zone. Fortunately, with new technology (Electron Backscatter Diffraction [EBSD]) and a technique known as a vorticity analysis, these questions can start to be addressed. The Rosy Finch Shear Zone (RFSZ) is the product of the collision of an oceanic plate with the North American Plate (NAP) and is known as a transpressional shear zone because it has undergone compression as well as simple shearing (i.e., like shearing a deck of cards). The oceanic plate, being denser, slides under the NAP in a process known as subduction. The oblique nature of this collision gives rise to simple shear parallel to the zone and compression perpendicular to the zone. This compression gives rise to shortening/thinning of the shear zone. The Bitterroots Detachment Fault (BDF), is located near Hamilton, Montana and is known as a metamorphic core complex (MCC). A MCC forms due to rifting of the continental crust. As the upper portion of the crust gets stripped away, the lower crustal shear zone becomes exposed to the surface. There are two competing, end-member, mechanical models for how continental rifting take place (the "pure shear" model and the "simple shear" model). A vorticity analysis can differentiate between these two models by quantifing the relative propositions of pure shear versus simple shear. The simple shear model predicts no shortening of the shear zone (i.e., no thinning) where the pure shear model predicts a significant amount of thinning.
