GEOLOGIC MAPPING AND KINEMATIC ANALYSIS OF THE BRITTLE-PLASTIC INDEPENDENCE MINE SHEAR ZONE IN THE SANGRE DE CRISTO RANGE, SOUTHERN COLORADO: OLIGOCENE-MIOCENE EXTENSIONAL REACTIVATION OF A LARAMIDE REVERSE FAULT

The Sangre de Cristo Range in southern Colorado records some of the deepest Cenozoic structural levels in the Rocky Mountain region. Exposures of presumed Laramide-age contractional mylonites show evidence for brittle-plastic extensional overprinting associated with the Rio Grande rift. We investigated the relation between Laramide contraction and Rio Grande rift extension by detailed geologic mapping and kinematic analysis in a 50 km² area centered on the Independence Mine shear zone (IMSZ), formerly called the Independence Mine thrust. The IMSZ is a 15- to 100-meter-thick, shallow-to-moderately (23°–62°), WSW-dipping brittle-plastic shear zone near the topographic base of the western flank of the range. It displays microstructural evidence for initiation as a top-NE contractional mylonite zone consistent with Laramide kinematics but is pervasively overprinted by deformation fabrics indicating top-SW extensional reactivation. Top-SW microstructures are characterized by mica-lined C- and C’-shear bands and mixed brittle-plastic deformation of quartz. Our mapping shows that the IMSZ is the thickest member of a system of mylonitic shear zones that dip shallowly to moderately (24°–55°) to the WSW and are hosted primarily within Proterozoic gneiss. Shear zones in amphibole-rich gneiss are commonly dominated by chlorite whereas those in quartzo-feldspathic gneiss have abundant white mica. Many of the thinner shear zones also record top-SW overprinting of top-NE fabrics. Extensional overprinting appears to be mostly restricted to mylonites where secondary micas form an interconnected weak phase. We interpret these relations as fluid-mediated, reaction-weakening gradients where lithologically controlled rheological contrasts were variably sensitive to extensional reactivation. One top-SW shear zone adjacent to the IMSZ cuts a diorite stock that we dated at 26 ± 1 Ma using LA-ICP-MS zircon U-Pb geochronometry. This date is consistent with extensional reactivation occurring during late Oligocene to early Miocene time. The IMSZ and associated reactivated shear zones may represent a mid-crustal extension mechanism that was widespread in the earliest stages of Rio Grande rifting before extension shifted to high-angle brittle-regime normal faults along the range front.