RELATION BETWEEN LARAMIDE CONTRACTION AND RIO GRANDE RIFT EXTENSION IN THE SANGRE DE CRISTO RANGE, COLORADO

The Sangre de Cristo Range in southern Colorado exposes a system of SW-dipping brittle-plastic shear zones that record evidence for both Laramide contraction and Rio Grande rift extension. Detailed mapping and microstructural analysis of the Independence Mine shear zone (IMSZ) and related structures indicate an early phase of top-NE reverse-sense displacement overprinted by top-SW normal-sense shearing. Laramide shortening likely initiated along cataclastic fault zones that evolved into mylonitic shear zones at depths near the brittle-plastic transition. Secondary phyllosilicates generated through fluid-driven reaction weakening primarily of feldspar and hornblende during Laramide deformation later facilitated extensional reactivation.

Geochronological constraints indicate that top-SW extensional shearing initiated in the latest Oligocene, contemporaneous with the onset of regional rifting. A gabbroic stock dated at 25.7 ± 1.0 Ma (zircon U-Pb) is cut by a top-SW shear zone, with zircon fission track data (26.7 + 3.6, -3.2 Ma) suggesting rapid post-emplacement cooling. U-Th-Pb dates from late-synkinematic monazite grains within the IMSZ constrain the end of ductile extensional shearing to ≥21 Ma.

Extensional reactivation was controlled by shear zone geometry and the rheological effects of interconnected phyllosilicates. More steeply-dipping (≥40°) segments of the shear zones and intervals rich in chlorite ± white mica preferentially record top-SW shearing, whereas shallowly-dipping (<30°), phyllosilicate-poor domains more commonly preserve Laramide fabrics. The presence of weak, non-optimally oriented structures that originated during Laramide shortening allowed for accommodation of mid-crustal extension during the earliest stages of Rio Grande rifting, prior to the establishment of the higher-angle, range-bounding fault systems.

Recent thermochronology data from across the range indicate that this early phase of brittle-plastic extensional shearing did not result in significant exhumation-related cooling, which began several million years later. The thermochronology data suggest that extension associated with significant exhumation may have initiated along the eastern range-bounding Alvarado fault in the Early Miocene before shifting westward to the Sangre de Cristo fault system during the Middle Miocene.