KINEMATIC AND STRAIN ANALYSIS OF MYLONITES FROM THE SUIZO MOUNTAINS, SANTA CATALINA METAMORPHIC CORE COMPLEX, ARIZONA, USA

Located ~50 km north of Tucson, the Suizo Mountains form a small, isolated part of the larger Santa Catalina metamorphic core complex to the south. Our research seeks to quantify the kinematics and strain associated with the ductile deformation produced during low-angle normal faulting as related to major crustal extension in the mid-Tertiary. The Suizos are underlain by the Proterozoic Pinal Schist and a younger, two-mica, garnet-bearing leucogranite thought to be part of the early Eocene Wilderness Suite. Rocks are variably mylonitized in an east dipping high-strain zone at least 350 m thick, the base of which is exposed on the western flank of the range. Mylonitic foliation strikes N-S and dips ~20°-40° east, while a well-developed elongation lineation trends ~040° to 090° and plunges 10° to 20°. Stepped porphyroclasts, S-C fabrics, and asymmetric quartz c-axis fabrics indicate to a top-to-the west shear sense. Additional displacement is accommodated by discrete, narrow (~cm thick) high-strain zones with a top-to-the west shear sense, as evidenced by deflected quartz veins and leucogranite pegmatite. The present-day east-dipping foliation, combined with the top-to-the west shear, would normally indicate reverse (thrust) faulting. We suggest, however, that rocks were deformed and mylonitized during extension with the foliation originally dipping west, and that a subsequent deformation allowed for tilting or warping of the mylonites into the current, east-dipping orientation. Sectional strain values were determined through quartz grain shape analysis, R_f/φ_f analysis, and NIH image analysis, with average R_XZ values ranging from 10-15:1. K-values vary from ~0.3-3, but overall 3D strain ellipsoids are consistent with plane strain. Based on the finite strain geometry and vorticity, we estimate the displacement accommodated by ductile deformation to be 2 ± 1 km.