GEOCHRONOLOGICAL AND THERMOCHRONOLOGICAL CONSTRAINTS ON PROTEROZOIC LITHOSPHERIC EVOLUTION, SOUTHWESTERN UNITED STATES

The Proterozoic lithosphere exposed in the southwestern U.S. reflects a protracted history from ca 1.8-1.0 Ga which involved lateral accretion of a collage of island arcs from 1.8-1.6 Ga followed by a major period of dominantly granitic magmatism and associated deformation ca 1.45-1.35 Ga. Present day exposures reveal a laterally segmented orogen that has experienced differential exhumation. U-Pb and Ar-Ar geo and thermochronology of exposures and lower crustal xenoliths and an extensive seismic dataset allow a unique view of the orogen.

A striking feature of the orogen is that different crustal domains expose sharply contrasting histories. Thermochronological studies of surface exposures and xenoliths suggest that large areas experienced long residence times in the middle-lower crust with very low cooling rates (<1 °C Ma) from ca.1.7 to ca 1.1 Ga following initial accretion. Some areas show a strong overprint by ca. 1.4 Ga metamorphism and deformation followed by continued slow cooling. Still other locations record early rapid cooling. Thermal modeling suggests that contrasts in HPE concentration and distribution are in part responsible for differences in cooling and exhumation records. Despite major thermal perturbations at ca.1.4 Ga, the data are consistent with the early attainment of a laterally segmented stable lithosphere with reactivation concentrated along discrete boundaries.

A major uncertainty in models for Proterozoic evolution of the southwestern U.S. is the extent of modification by ca. 1.4 Ga by mafic underplating implicated in voluminous granitic magmatism and associated low P – high T metamorphism (0.35-0.6 GPa, 500-700 C) in middle and upper crustal basement rocks. A regionally extensive high-velocity (7xx) lower crust has been interpreted as evidence of underplated magmas. Geochronological studies of xenoliths indicate mafic magmatism/granulite metamorphism ca 1.4 Ga as well as the presence of older garnet granulites developed during assembly. This indicates a complex origin for the high-velocity layer and confirms a link between ca. 1.4 Ga mafic magmatism and metamorphism in the lower crust and tectonothermal events in the middle crust. Integration of EarthScope seismic studies with regional patterns of accretion and reactivation will lead to new understanding of continental accretion, stabilization, and reactivation.