223-2 Strain Localization Mechanisms in Shear Zones: Example from the Santa Rosa Mylonite Zone in Palm Canyon, California

Session: Rock Deformation and the Dynamics of Mountain Building: A Session Honoring the Scientific Contributions of John P. Platt (Posters)

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Strain localization and partitioning is a key process in the development of shear zones and tectonic processes in general. Yet, our understanding of strain localization remains limited. Several mechanisms drive strain localization, including lithological/mineralogical variations, grain size reduction, presence of weak minerals, and development of crystallographic preferred orientation. Here, we focus on the role of mineralogy and grain size reduction on strain localization in the Santa Rosa Mylonite Zone. The Santa Rosa and San Jacinto Mountains are the youngest and tallest mountains of the Peninsular Ranges Mountains, which extend all the way down into Baja California. In this area resides the Santa Rosa Mylonite Zone which is of interest to our investigation because of its complex geologic history and its proximity to the San Andreas Fault system. Some of the rocks found here include protoliths: granodiorite and tonalite, as well as a suite of deformed rocks: mylonites, ultramylonites, marbles, and other tectonites. With such textural variety in such a close proximity to each other, we are provided the opportunity to investigate how strain is partitioned and localized as well as how the mineral fabric developed and changed from protoliths to the deformed rocks. Through a combination of optical microscopy, scanning electron microscopy, electron backscatter diffraction, and X-ray diffraction, this research aims to analyze the microstructural and compositional changes in these rocks, particularly focusing on the deformation of plagioclase, biotite, amphibole, and quartz. The findings will enhance the understanding of strain distribution within mature shear zones and contribute to broader insights into the mechanisms of lithospheric deformation.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6984

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