Persistent influence of Proterozoic accretionary boundaries in the tectonic evolution of southwestern North America: Interaction of cratonic grain and mantle modification events

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Abstract
Northeast-striking tectonic provinces and boundaries were established during 1.8–1.6-Ga assembly of juvenile continental lithosphere in the southwestern United States. This continental grain repeatedly has influenced subsequent intracratonic tectonism and magmatism. After 200 m.y. of stability, cratonic lithosphere was affected by regional, ∼1.4-Ga, dominantly granitic magmatism and associated tectonism that reactivated older northeast-striking shear zones in the Proterozoic accreted terranes, but not the Archean lithosphere. In contrast, 1.1-Ga, dominantly mafic magmatism and rifting did not reactivate northeast-striking zones, but occurred along new north–south fracture zones (e.g., Rocky Mountain trend) that reflect cracking of Laurentian lithosphere at a high angle to the Grenville collision. By 500 Ma, rifting had thinned the crust and mantle in the western United States creating the north–south Cordilleran miogeocline. East of the Cordilleran hingeline, isopachs in Lower Paleozoic sedimentary rocks follow northeast-trending structures (Cheyenne belt, Transcontinental arch, and Yavapai–Mazatzal province boundary), suggesting that older boundaries influenced isostatic response of the craton during thermal subsidence of the margin. Ancestral Rockies and Laramide uplifts and basins did not strongly reactivate northeast-striking boundaries. However, Ancestral Rockies structures end at the Archean–Proterozoic boundary, and Laramide magmatism (Colorado mineral belt) and metallogenic provinces follow northeast-striking Proterozoic boundaries, both suggesting deep-seated lithospheric influences on tectonism.

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