THE PICURIS OROGENY, PART 2: 1.50 TO 1.35 GA METAMORPHISM AND DEFORMATION IN THE SOUTHWESTERN UNITED STATES

Isotopic ages in the southwestern United States document metamorphism and deformation during the 1.53-1.35 Ga Picuris orogeny. The most intense deformation and metamorphism occurs in northern New Mexico and southern Colorado, where regional amphibolite to granulite facies metamorphism occurred between 1510 and 1350 Ma at a regional scale. Garnet Lu/Hf ages, monazite ages, zircon ages of deformed and crosscutting igneous rocks, and depositional ages of supracrustal rocks require major shortening deformation during this time interval.

The distribution of deformation and metamorphism does not match simple models for intracontinental tectonism inboard of a distant convergent plate margin. The presence of the highest grade and most intensely deformed and metamorphosed rocks in northern New Mexico and southern Colorado suggests instead that the plate margin may not have been as distant as previously suggested. Instead, the distribution and timing of high grade metamorphism, intensity of deformation, and resetting of isotopic cooling ages is indicative of an active plate margin near these regions. Thus, the conception of a distant convergent plate margin may require reevaluation in light of our newly compiled zircon age database (Aronoff et al., this session). We present a comprehensive model for the metamorphic and deformation history of the Picuris orogen in the southern Rocky Mountains in the context of the regional age distribution of coeval deformation and metamorphism in the southwestern United States.