EARLY STAGES OF RIO GRANDE RIFT DEFORMATION RECORDED BY THE OLIGOCENE CHOKECHERRY GRANITE, SOUTHERN COLORADO.

Crustal extension associated with the Rio Grande rift is thought to have initiated in the Oligocene, but the timing and nature of early rift deformation are poorly constrained. We present evidence for the earliest structural signature of Rio Grande rift extension in the Sangre de Cristo Range, southern Colorado through new geologic mapping, structural and microstructural analyses, rock magnetic data, and thermochronology. These analyses focus on the 30.0 ± 0.5 Ma Chokecherry granite, which intrudes Proterozoic crystalline rock as a sill along the southwestern range-front. The intrusion hosts discrete, shallowly-SW-dipping mylonitic shear zones with strain localized primarily in quartz veins and quartz-rich domains. Quartz stretching lineations consistently trend NE-SW, and foliation patterns record simple-dominated shear. Incremental stretching axes parallel shallowly WSW-plunging K1 magnetic lineations determined by anisotropy of magnetic susceptibility (AMS) analyses. Distributed protomylonitic foliations in the granite dip gently SW and mirror the AMS foliation. Quartz microstructures and preliminary electron back-scattered diffraction (EBSD) data record heterogenous dynamic recrystallization ranging from grain boundary migration with an average grain size of >100 µm to bulging recrystallization and microfracturing with an average grain size of ~10 µm. This contrast suggests dislocation creep under highly variable stress-temperature conditions during syn-to-post-magmatic cooling of the intrusion. Minor normal and strike-slip faults overprint the ductile fabrics and record sub-horizontal NE-SW extension axes. Multiple diffusion domain modeling of ⁴⁰Ar/³⁹Ar in feldspar indicates that the intrusion cooled rapidly in the Oligocene and was at ~250 ± 25° C by ~20 Ma. Subsequent slow cooling from ~20-13 Ma was followed by renewed, rapid cooling initiating ~13 Ma, which we propose as the onset of extensional exhumation along the range-bounding Sangre de Cristo normal fault system.