LATE OLIGOCENE TO EARLY MIOCENE EXTENSIONAL REACTIVATION OF THE INDEPENDENCE MINE SHEAR ZONE IN THE SANGRE DE CRISTO RANGE, SOUTHERN COLORADO

The Sangre de Cristo Range in southern Colorado may be the only location in the Rocky Mountain region that exposes mylonitic shear zones associated with both the Laramide orogeny and Rio Grande rift. We investigated the relation between Rio Grande rift extension and pre-rift contraction near the brittle-plastic transition through detailed geologic mapping, kinematic analysis, and geochronometry in a 50 km² area centered on the Independence Mine shear zone (IMSZ). We identify the IMSZ as the thickest (15–100 m-thick) and most laterally extensive member in a system of shallowly-to-moderately (~45±20°) WSW-dipping brittle-plastic shear zones which crop out near and along the topographic base of the western flank of the range. These shear zones display microstructural evidence for initiation as top-NE contractional mylonite zones consistent with Laramide strain but are pervasively overprinted by shear fabrics indicating top-SW extensional reactivation. Both top-NE and top-SW shear fabrics involve cataclasis and quartz dislocation creep, though top-SW shear is more commonly localized along mica-lined shear bands. Shear zones are hosted predominately within Proterozoic gneiss and contain abundant chlorite and white mica derived from alteration of hornblende and feldspar, suggesting fluid-driven reaction weakening played an important role in localizing strain. Extensional overprinting appears to be most pervasive where secondary phyllosilicates form an interconnected weak phase. We interpret extensional reactivation as being at least partially controlled by rheological contrasts inherited from host rock mineralogy. Shear zone thicknesses and estimates of shear strain from angles between foliation and shear zone boundaries suggest total normal-sense displacement across the IMSZ and adjacent shear zones is ≤0.5 km. One top-SW shear zone adjacent to the IMSZ cuts a 26 ± 1 Ma gabbro stock, and presumed synkinematic monazite grains in two samples from the IMSZ yield LA-ICP-MS U-Pb ages of 25 ± 3 Ma and 22 ± 1 Ma. These dates are consistent with Late Oligocene to Early Miocene extensional reactivation. Reactivation of weak reverse faults may represent an important structural control on early stages of extension in the rift.