Proterozoic ancestry is increasingly being proposed for both Ancestral Rocky Mountain (Pennsylvanian-Permian) and Laramide-age (Late Cretaceous–early Tertiary) faults in the southwestern United States. We test this hypothesis by using 40Ar/39Ar thermochronology on Proterozoic basement rocks in the southern Sangre de Cristo Range, New Mexico. Hornblende and mica yield ages consistent with basement rocks cooling below ∼500 °C after ca. 1.4 Ga regional metamorphism, followed by protracted basement residence at temperatures between 300 and 400 °C until the Neoproterozoic. K-feldspar age spectra are dominated by Neoproterozoic apparent ages and constrain basement exhumation from mid-crustal depths (∼10 km) to within a few kilometers of the surface during the Neoproterozoic. Samples were also collected across the north-striking, steeply west-dipping Montezuma fault, which bounds the eastern Rocky Mountain range front and has known Ancestral Rocky Mountain and Laramide-age movement. Contrasting 40Ar/39Ar K-feldspar–derived thermal histories from opposing sides of the fault require two episodes of Neoproterozoic fault movement. Overall, rocks to the west record multiple diffusion domain (MDD) thermal histories, which indicate that cooling was caused by 5–7 km of basement exhumation between 1000 and 800 Ma, whereas rocks to the east were exhumed later, between 750 and 600 Ma. Assuming that the present-day fault geometry was established in the Proterozoic, west-side-up throw late in the Grenville orogeny (ca. 1000 Ma) accommodated east-west shortening. West-side-down extensional movement at ca. 750 Ma is inferred to be associated with the incipient rifting of the supercontinent Rodinia.

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