MICROSTRUCTURAL ANALYSIS AND ELECTRON BACKSCATTER DIFFRACTION OF LATE CRETACEOUS – EARLY PALEOGENE MID-CRUSTAL SHEAR ZONES IN THE PUERTO BLANCO MOUNTAINS, ORGAN PIPE CACTUS NATIONAL MONUMENT, SOUTHERN ARIZONA

Late Cretaceous – Early Paleogene thick-skinned deformation is recorded in the Quitobaquito Hills, Organ Pipe Cactus National Monument, where Precambrian gneissic basement was thrust over Jurassic metavolcanics (quartz+feldspars+white mica) along the Quitobaquito shear zone. Subsidiary shear zones are preserved in the metavolcanics of the Quitobaquito Hills, as well as 10 km to the NE in the Puerto Blanco Mountains, where Precambrian rocks are not exposed. Thin sections from SL tectonites of two shear zones from the NW Puerto Blanco mountains were analyzed to compare shear kinematics and deformation mechanisms and constrain deformation temperatures. Electron backscatter diffraction (EBSD) of recrystallized quartz-rich domains, quartz ribbons, and relict quartz porphyroclasts was used to investigate quartz crystallographic preferred orientations (CPO) in polyphase rocks. Ubiquitous core-and-mantle structures suggest dislocation creep was accommodated dominantly via subgrain rotation recrystallization. Feldspars record brittle deformation and quartz strain shadows, with rare evidence of bulging recrystallization. Sigma-type porphryoclast systems and shear bands record top-to-the-north shearing, although a high proportion of porphyroclast systems are symmetrical. A subsimple shear model is proposed. CPOs of recrystallized quartz grains record different patterns in the two shear zones investigated. One records a statistically significant non-coaxial pattern (M.U.D. > 3), consistent with microstructures, suggesting basal <a> slip in quartz. The other records a weak coaxial, Type I crossed-circle girdle (M.U.D. < 1.5), consistent with combined basal<a> and prism<a> slip. The intensity of the CPO signal decreases along a strain gradient toward the core of each shear zone, where laterally continuous white mica layers, grain sizes <10 µm, and the high percentage of polyphase grain boundaries may be more favorable for grain boundary sliding assisted diffusion creep. Microstructural analysis and EBSD results are consistent with dynamic recrystallization at greenschist facies conditions between ~400 and 500°C.