A LESS-THAN-30 MA TRANSITION FROM LARAMIDE SHORTENING TO RIO GRANDE RIFT EXTENSION IN THE BLANCA PEAK REGION, SOUTHERN COLORADO: CONSTRAINTS FROM FAULT SLIP ANALYSIS, GEOCHRONOLOGY, AND ROCK MAGNETISM

The Blanca Peak region of the Sangre de Cristo Mountains adjacent to the San Luis Valley of southern Colorado records a history of both Paleogene Laramide shortening and Neogene Rio Grande rifting. Fault-slip data collected from Pennsylvanian Minturn Formation in several intramountain thrust plates demonstrate widespread brittle strain via small reverse and strike-slip faults that mostly record ENE-shortening typical for regional Laramide shortening. Sparse slip surfaces within Proterozoic crystalline rocks are also mostly compatible with Laramide shortening via reactivation of preexisting fractures. In contrast, a set of high-angle normal faults that record ENE-WSW rift extension are concentrated in bedrock adjacent to the Sangre de Cristo normal fault zone along the western range front.

The Chokecherry Canyon (CC) granite pluton (30.0 ± 0.5 Ma SHRIMP-RG zircon U-Pb Concordia Age) is exposed along a 2-km length at the base of the southwestern range front. Field magnetic susceptibility (MS) sites in the CC granite demonstrate a MS contrast with adjacent Proterozoic tonalite gneiss of about 1E-2 SI and explain a coincident aeromagnetic anomaly high, constraining the pluton’s lateral extent. Anisotropy of magnetic susceptibility (AMS) for oriented CC granite samples drilled at 16 sites yield consistent, shallowly WSW-dipping magnetic foliations with unidirectional (Dec = 249°, Inc = 19°, a95 = 4°) maximum MS (K1) axes and abnormally high degrees of anisotropy (29% mean) whose persistence into areas of granite lacking macroscopic fabric are interpreted to track magmatic flow. Paleomagnetic analysis is in progress to evaluate tilt of the CC granite pluton. The CC granite is deformed by fault sets with near-conjugate geometry and chloritic alteration whose contrasting attitudes and slip senses are (1) subhorizontal, top-to-WSW and (2) moderately SW-dipping, top-to-ENE. Fault-slip inversion yields a maximum principal stress axis (σ1) whose orientation (Dec = 251°, Inc = 21°, a95 = 11°) is consistent with Laramide compression. The coincidence of the σ1 and K1-AMS axes for the CC granite suggests that pluton was laterally injected as a gently dipping sheet and then further deformed by brittle faulting under a Laramide stress field that persisted to at least 30 Ma in the southwestern Blanca Peak region.