THE PROTEROZOIC EVOLUTION OF SOUTHWESTERN NORTH AMERICA: NEW CONSTRAINTS AND on-GOING CHALLENGES

Proterozoic rocks of SW North America record the multi-stage southward growth of Laurentia that culminated in the assembly of Rodinia and stabilization of lithosphere under much of the present-day USA. Paleoproterozoic rocks locally record the Yavapai orogeny (1720-1680 Ma), Mazatzal orogeny (1680-1650 Ma), 1.48-1.35 Ga intracratonic metamorphism and deformation, and continent-continent collision during the 1.1-1.0 Grenville orogenic cycle. New data bear on a number of first-order tectonic questions including: (1) the degree to which the additions to Laurentia reflect juvenile materials or materials built on older basement; (2) the tectonic significance of the thick (ca. 1.7 Ga) Mazatzal-type quartzite-rhyolite sequences; (3) the degree of Yavapai-age burial and exhumation before deposition of Mazatzal sediments; (4) the degree of tectonic burial during the Mazatzal orogeny (ca. 1.65 Ga); (5) the significance of the 1.5-1.6 Ga tectonic quiet period; and (6) the cause of the 1.48-1.35 intracratonic tectonism. New Hf isotope data suggest that older basement may be more common than previously thought. Archean crust was apparently a source for 1.89 Ga basement in the Mojave region, for detritus in the Vishnu Schist, and may underlie parts of the Mojave province. Yavapai and Mazatzal province basements are dominantly juvenile, but locally contain older (1.84 Ga) basement. Thick 1.7 Ga Mazatzal orthoquartzite-rhyolite sequences were probably deposited in back-arc-style extensional basins that may have inverted soon after deposition. Adjacent crustal blocks (Grand Canyon, Bradshaw Mtns., etc.) underwent mid-crustal metamorphism during quartzite deposition such that the modern basement includes different crustal levels of the long-lived convergent orogeny. Crustal recycling in successive orogenic basins offers an important tool for tectonic analysis. The lack of zircon dates in the range 1.6- 1.5 Ga suggests a Laurentian tectonic gap for the otherwise progressive tectonism. Dates as young as 1.6 Ga (Manzano Mtns.) and as old as 1.48 Ga (Picuris Mtns.) are narrowing the “tectonic gap”, but the lack of dates in the 1.5-1.6 range is a fingerprint for SW Laurentia. Finally, the extent and intensity of the long 1.48-1.35 event is increasingly recognized but the tectonic cause and significance remains enigmatic.