Three outstanding questions regarding Paleoproterozoic crustal growth and assembly in southwestern Laurentia are (1) the mechanisms of crustal growth (back-arc rifting and associated juvenile magmatism vs. accretion of juvenile arcs), (2) the nature and significance of pre-Yavapai orogeny (>1.70 Ga) deformation, and (3) how the isotopically mixed boundary zone between the Mojave and Yavapai Paleoproterozoic crustal provinces formed. Supracrustal successions associated with the Mojave-Yavapai boundary zone in northwestern Arizona are dominated by > 1730 Ma bimodal metavolcanic sequences and compositionally siliciclastic metasedimentary rocks, with minor ultramafic rocks, chert, and carbonate. This lithologic assemblage requires juvenile mafic volcanism in a basin that also received abundant quartz-rich (i.e., continental) detritus. A pre-D1 (first deformational event), back-arc rift setting for northwestern Arizona best explains the observed supracrustal assemblage. Plutonic rocks that intrude the supracrustal rocks are generally 5–15 m.yr. younger and span the entire compositional range of granitoids; i.e., they are not plutonic equivalents of the metavolcanic rocks. In the model presented here, supracrustal rocks were deposited in a back-arc basin above an east-dipping subduction zone undergoing slab rollback. Subsequent shallowing of the subducting slab resulted in (1) eastwardmigration of 1.76–1.74 Ga arcmagmatism in eastern California to 1.73–1.71 Ga arcmagmatism within the former back-arc basin in northwestern Arizona, (2) consequent closure of the back-arc basin, (3) thrusting of Yavapai province rocks over the basin (D1), resulting in (4) burial of supracrustal rocks to > 120 km, and ultimately, (5) gravitational collapse of the orogen. These events predated the 1.70–1.68 Ga Yavapai orogeny and occurred within a time interval of 5–20 m.yr. Similar time intervals for tectonic switching have been documented from the Lachlan orogen of eastern Australia. Rifting and associated magmatism and arc accretion are both viable—and not mutually exclusive—mechanisms for crustal growth.
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