Strain Localization Processes in Polymineralic Rocks: Insights from the Santa Rosa Mylonite Zone (CA)

Abstract

Strain localization in ductile shear zones is a fundamental process governing the mechanical behavior of the lithosphere, yet the relative roles of mineralogical heterogeneity and microstructural evolution remain incompletely understood. This study investigates strain localization processes in the Santa Rosa mylonite zone (SRMZ), southern California, where deformation is heterogeneously distributed over centimeter scales, ranging from weakly deformed protoliths to ultramylonites. We integrate field observations with optical petrography, scanning electron microscopy (SEM-EDS), and electron backscatter diffraction (EBSD) to characterize microstructures, mineral chemistry, and crystallographic preferred orientations across proto-, meso-, and ultra- mylonitic rocks derived from a granodioritic protolith. Quartz deformation is dominated by grain boundary migration recrystallization, with minor subgrain rotation, and exhibits c-axis fabrics indicative of prism <a> and subordinate rhomb <a> slip, consistent with deformation temperatures exceeding ~500 °C and a significant coaxial strain component. Feldspar records mixed brittle–ductile behavior, including fracturing, core-and-mantle structures, and limited recrystallization, while biotite defines the mylonitic foliation and develops S–C fabrics, locally forming mica fish and undergoing partial chloritization. Our results demonstrate that strain is preferentially localized in quartz- and biotite-rich domains, where reduced aggregate strength and enhanced phase connectivity promote distributed deformation. In contrast, feldspar-rich domains tend to preserve a load-bearing framework, inhibiting strain localization. These observations indicate that primary mineralogical heterogeneity exerts a first-order control on strain partitioning, even under broadly uniform temperature and stress conditions. This study highlights the importance of mineralogy and phase distribution in governing shear zone initiation and development and provides new insights into the rheological evolution of mid-crustal deformation zones.