Core Complexes Are Tectonic Windows of Opportunity: A Microstructural Evaluation of Shear Zone Rheology and Strain Localization in the South Mountains Core Complex, Arizona

Core complexes are a known manifestation of regional or local tectonic extension, but the strain localization processes associated with the development of a discrete detachment fault surface remain controversial. We investigate the process of strain localization within a mylonitic shear zone associated with core complex development in the South Mountains, central Arizona, in order to evaluate the rheology of crustal rocks during tectonic extension. The footwall rocks of the South Mountains core complex are dominated by a large composite middle Tertiary (22-25 Ma) granitoid pluton that is synkinematic with respect to the ductile mylonitic deformation associated with middle Tertiary core complex development, and are therefore ideal for this study. Our preliminary microstructural observations from South Mountains Granodiorite mylonites suggest that strain is preferentially localized within quartz, and that quartz deforms under dislocation creep. Quartz is extensively dynamically recrystallized, forming quartz ribbons with high aspect ratios. We observe dynamically recrystallized quartz showing both Regime 2 and Regime 3 microstructures indicative of subgrain rotation and grain boundary migration recrystallization, respectively. In contrast, plagioclase and K-feldspar form porphyroclasts that are commonly brittlely fractured. The plastic behavior of quartz and the brittle behavior of the feldspar porphyroclasts together suggests that these mylonites developed at temperatures above the threshold for quartz plasticity (~300°C), but not above the threshold for feldspar plasticity, interpreted to be around ~500°C. Our results suggest that the rheology of the footwall rocks is largely controlled by quartz, and that quartzite flow laws may best describe the rheologic behavior of these crustal rocks during core complex development.