Strain Localization Mechanisms: Insights from the Santa Rosa Mylonite Zone in Southern California

Strain localization and partitioning play a critical role in the formation of shear zones and broader tectonic processes. However, our understanding of strain localization is still restricted. Various factors contribute to how strain is distributed in a shear zone, such as differences in lithology or mineralogy, grain size reduction, the presence of weak minerals, and the formation of crystallographic preferred orientations. In this study, we examine how mineral composition and grain size reduction influence strain localization within the Santa Rosa Mylonite Zone. The Santa Rosa and San Jacinto Mountains are the youngest and highest peaks within the Peninsular Ranges, which stretch southward into Baja California. Within this region lies the Santa Rosa Mylonite Zone—the focus of our study due to its intricate geologic history and its mechanical link with the San Andreas Fault system. The zone features a range of rock types, including protoliths such as granodiorite and tonalite, as well as various deformed rocks like mylonites, ultramylonites, marbles, limestones, and other tectonites. The close spatial association of these diverse rocks offers a valuable opportunity to explore how strain is partitioned and localized, and how mineral fabrics evolved from protoliths to their deformed counterparts. This study utilizes a combination of optical microscopy, scanning electron microscopy, electron backscatter diffraction, and X-ray diffraction to investigate microstructural and compositional changes in the rocks, with a particular emphasis on the deformation of plagioclase, biotite, amphibole, and quartz. The results will provide a deeper understanding of how strain is distributed within well-developed shear zones and offer broader insights into the processes driving lithospheric deformation.