Ductile and Brittle Rio Grande Rift Deformation in Oligocene Granite Records a Two‐Stage Rift History in Southern Colorado

Abstract The timing and nature of early deformation in the Rio Grande Rift remains poorly constrained. We present evidence for the earliest structural signature of rift extension in the Sangre de Cristo Range, southern Colorado, based on new geologic mapping, structural analysis, rock magnetic data, and thermochronology. These analyses focus on the ∼30.0 Ma granite of Chokecherry Canyon, which hosts discrete low‐angle mylonitic shear zones and a distributed, gently SW‐dipping protomylonitic fabric. Incremental stretching axes, stretching lineations, and Kmax magnetic lineations plunge gently WSW. Quartz microstructures and crystallographic orientations indicate dominantly coaxial strain in the protomylonite and general shear in the discrete shear zones. Quartz c‐axis opening‐angle thermometry suggests deformation at ∼420°–540°C. Thermal modeling of 40 Ar/ 39 Ar K‐feldspar data indicates rapid postmagmatic cooling below the brittle–plastic transition, supporting shear‐zone formation immediately after emplacement. Slow cooling from ∼20 to 13 Ma was followed by renewed rapid cooling at ∼13 Ma, interpreted as the onset of extensional exhumation along the Sangre de Cristo fault system. These results show that extension in the northern Rio Grande Rift was active by ∼30 Ma, earlier than previously recognized. We propose a two‐stage model for northern Rio Grande Rift evolution: Stage I (30–23 Ma) records ENE–WSW extension localized in low‐angle mylonitic shear zones associated with mid‐crustal intrusions; Stage II (≤18 Ma) reflects brittle high‐angle normal faulting, focused exhumation, and rift narrowing. Stage I magmatism and deformation along the western range front likely established crustal weaknesses that guided later fault development.