GEOCHRONOLOGICAL AND THERMOCHRONOLOGICAL CONSTRAINTS ON PROTEROZOIC LITHOSPHERIC EVOLUTION OF THE SOUTHWESTERN U.S

Proterozoic lithosphere in the southwestern U.S. records a protracted history from ca 1.8-1.0 Ga involving 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 1.4 Ga and final exhumation >530 Ma. Present day exposures reveal a laterally segmented orogen that has experienced differential exhumation with cooling ages ranging from 1.65 to < 1.0 Ga. U-Pb and Ar-Ar geo- and thermo-chronological data from these exposures and mid-lower crustal xenoliths have been coupled with seismic data to provide a unique view of the orogen in 4-dimensions.

What is most striking about the orogen is that different crustal domains expose sharply contrasting histories depending on the amount of exhumation. In particular, large areas show long residence times in the middle-lower crust with very low cooling rates (<1 °C/Ma) from 1.7 to 1.1 Ga while others show a strong overprint by 1.4 Ga metamorphism and deformation followed by continued slow cooling while others are overlain by ca 1.23 Ga sedimentary rocks. Comparison of a wide variety of areas allows us to piece together both vertical and lateral gradients in metamorphism and deformation and to gain insight into the maturation and re-activation of continental lithosphere.

Seismic studies indicate a distinctive high-velocity (7.xx) layer across the orogen which has been interpreted as ca 1.4 Ga underplated basalt. Support for this model is the absence of a similar layer in the adjacent southern Wyoming craton and the corresponding paucity of 1.4 Ga magmatism. Geochronological studies of xenoliths have not supported this model and raise the question of whether the layer is best modeled as garnet granulite developed during accretion and thickening of the Proterozoic lithosphere.

The Proterozoic of the Southwest is a laterally segmented, variably exhumed, orogen. A comprehensive thermochronological data set allows vertical and lateral gradients in thermal structure to be integrated with seismic data to better understand the assembly and maturation of Proterozoic lithosphere.