PROGRESSIVE PROTEROZOIC GROWTH OF SOUTHERN LAURENTIA: A LONG-LIVED (1.8 TO 1.0 GA) ACCRETIONARY OROGEN RESULTING IN A DISTINCTIVE HYDRATED, THICK, AND WEAK CONTINENTAL LITHOSPHERE

The southwestern U.S. preserves a rich record of the processes of accretion of juvenile terranes and their stabilization into continental lithosphere. A zone > 1000 km in width was added to Laurentia 1.8 to 1.0 Ga by successive additions of dominantly juvenile oceanic terranes and magmatic arcs to a long-lived compressive/transpressive plate margin. These arcs include bimodal volcanic/plutonic successions suggestive of multiple differentiation events and local involvement of just-older (e.g. 1.84 Ga) crust. Assembly took place by subduction-accretion processes as recorded in subduction scars imaged by combined seismic and geologic studies across several major shear zones and province boundaries. Suture geometries include interwedged crustal terranes, deep crustal bivergent root zones, and abrupt mantle interfaces. Postulated sutures in the Rockies, from N to S, include the Cheyenne belt, Farwell Mountain suture, Crystal shear zone, Colorado mineral belt, and Jemez lineament/Slate Creek shear zone. Sutures in the presently exposed middle crustal (10-20 km) rocks involve distributed deformation within 10- to 100 km-wide accretionary complexes. Cryptic thrusts within these complexes are hypothesized based on exotic rock associations (e.g. lens-shaped ultramafic rocks within turbiditic schists in the Grand Canyon). Shortening and thickening of crust due to progressive accretionary events has transposed initially shallow boundaries into dominant subvertical fabrics. Most of the discrete shear zones, rather than being the sutures themselves, are later reactivations of weak zones from continued accretion. Syn- and post-accretion lithospheric differentiation produced blooms of granitoids that likely were accompanied by progressive depletion of lithospheric mantle and development of a mafic lower crust, further stabilizing the lithosphere. Subduction-related assembly of Proterozoic juvenile terranes in southern Laurentia resulted in anomalously hydrous, fertile, and thick (> 200 km) chemical lithosphere (compared to Archean lithosphere). Heterogeneity preserved in today's mantle at the 10 km-scale in part reflects differing responses of Proterozoic compositional domains to later hydration and heating events, including modern asthenospheric upwelling.