PROTEROZOIC ROCKS OF THE SOUTHWEST: EPISODIC CRUSTAL GROWTH, LONG-LIVED (1.8 TO 1.0 GA) PLATE MARGIN ALONG SOUTHERN LAURENTIA, AND PRESERVATION OF PROTEROZOIC SUBDUCTION SCARS IN THE MODERN-DAY LITHOSPHERE

From 1.8 to ~ 1.0 Ga, Laurentia grew by successive additions of dominantly juvenile oceanic terranes and magmatic arcs to a long-lived compressive/transpressive plate margin. The crust and lithospheric mantle for these accreted juvenile oceanic elements was likely to have been thin, and lithosphere was stabilized by a combination of syn- accretion tectonic thickening and syn- and post- accretion lithospheric differentiation that progressively developed depleted mantle, mafic lower crust, and middle crustal granitoids. Significant juvenile additions in the southwestern U.S. include the 1.78-1.68 Ga Mojave province (which includes reworked Archean and Paleoproterozoic basement), the 1.8-1.7 Ga Yavapai province and the 1.67-1.65 Mazatzal province. Northwest- striking foliation in SW Colorado is interpreted to reflect a complex, Banda Arc-type accretionary complex within the Yavapai Province. Paleoproterozoic high temperature/moderate pressure metamorphism associated with the Yavapai orogeny took place 1741 to 1695 Ma at >600„aC and 2-4 kbars. Granitoid magmatism (e.g. 1,713 Ma Pitts Meadow granodiorite of the Black Canyon region) followed each accretion pulse and progressively helped stitch and stabilize the assembled orogens. An underappreciated element were additions of 1.5-1.3 Ga juvenile crust that underlies much of the mid-continental U.S., extending from Texas through eastern Canada. This was linked to ~1.45-1.35 Ga A-type magmatism and mafic underplating that intruded and helped stabilize all of the older Proterozoic provinces. Accretion of juvenile crust to southern Laurentia culminated with the 1.3-1.0 Ga Grenville orogeny and included voluminous magmatism along the eastern and southern (Llano) continental margins. Combined seismic reflection and mantle tomography of the southwestern U.S. shows preserved suture zones at the Cheyenne belt, Colorado mineral belt, and Jemez lineament, with correlations between crustal shear zones and mantle velocity domain boundaries that are interpreted to be Proterozoic subduction scars preserved in today's lithosphere.