37 Tectonics

Wednesday, 11 March 2026
8:30 AM - 10:00 AM
RCC, 106

Strain localization processes in polymineralic rocks: insights from the Santa Rosa mylonite zone (CA)

Paper number: 37-2
Program: Technical Sessions
Time: 9:00 AM – 9:15 AM

Authors

Abstract

Strain localization is a fundamental process that controls the deformation of the lithosphere. Yet, strain localization processes remain poorly understood. The dynamic controls on strain localization such as mineralogical composition, grain size, strain rate, development of mechanical anisotropies such as S-C fabrics and crystallographic preferred orientations, and the presence of fluids—working together and independently of each other, make the process difficult to study. In this project, we focus on the role of mineralogical heterogeneity on the strain localization processes in the Santa Rosa mylonite zone, located in Palm Canyon, California. This area was chosen because the shear zone exhibits various degrees of strain localization, with textures ranging from undeformed to ultramylonitic over a few centimeters, which is challenging to explain. Our hypothesis is that strain is preferentially partitioned according to the relative mineral assemblages of the mylonites. Using petrographic analysis, energy dispersive spectroscopy, and electron backscatter diffraction, preliminary results show that (1) quartz is typically completely recrystallized via grain boundary migration and minor subgrain rotation recrystallization, quartz c-axis pole figures indicate combination of prism <a> and rhomb <a> slip, and high component of coaxial deformation; (2) feldspar, dominantly albite and orthoclase, exhibit both brittle (i.e. bookshelf sliding) and ductile deformation (i.e. δ- and σ-type porphyroclasts and subgrain rotation recrystallization); (3) biotite defines the foliation of the mylonite and forms S-C fabric in the ultramylonites, occasionally forming mica fish, and is replaced locally by chlorite. Our preliminary results suggest that strain is preferentially localized in samples containing higher modal amounts of quartz and biotite, suggesting that mineralogy plays a primary role. These findings will help us improve our 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 Programs. Vol. 58, No. 1

doi: 10.1130/abs/2026SE-411789

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