Plate-tectonics model for Proterozoic continental accretion in the southwestern United States

Proterozoic crustal provinces in the SW US decrease in age S from the Archean Wyoming province. Supracrustal rocks define three major age provinces from N to S: 1.72-1.80, 1.65-1.72, and 1.1-1.2b.y. Successions within each province are similar and define two assemblages: a bimodal volcanic assemblage commonly overlain by a quartzite-shale assemblage. Precambrian granitic plutons intrude all three provinces. Geochemical and Sr-isotope data indicate a variably depleted upper mantle source for basalts and a short-lived (<100Ma) lower-crustal source for granitic and felsic volcanic magmas. These observations can be explained by a plate-tectonics model involving successive marginal basin closures and Andean-type orogenies associated with S-ward-migrating arcs. The net result is to accrete = or <1300km of continental crust to the SW margin of N America between 1.8 and 1.1b.y. ago.-Author