NOXON/LEMHI ARCH AND RIFT: A MAJOR PROTEROZOIC/PALEOZOIC NORTH AMERICAN PLATE BOUNDARY

A thick, three part Lower-Middle-and Upper Revett was first recognized in Idaho by Don Winston in 1974 and provided a basis for considering relationships between an unusually thick Upper Revett in westerly parts of the Coeur d’Alene Mining District (White and Winston, 1977) and a NNW trending, elongate zone of greatly thinned Belt strata along the Idaho-Montana border, named the Noxon Arch by White (2000). The Noxon arch appears to identify the western edge of the classic Cordilleran miogeosyncline. Rapid westward thickening off the arch extends from the most Pb-Zinc-rich parts of the CDA mining district to the rich Sullivan Mine massive sulfide deposit in Canada, apparently identifying these structures as products of major crustal rifting, Pb and Zn as products of the rifting and in creation of ultimate source rocks for CDA District Pb and Zn veins. The “Noxon line” defined by the arch and its adjacent rift apparently define a major crustal rift zone that should extend far to the south.

A likely southward extension of the “Noxon line” is possibly represented by the Lemhi arch of SE Idaho and an adjacent rift that hosts the Idaho Cobalt Belt, Co being another occasional associate of major crustal rifting. In contrast to the Noxon arch, the Lemhi arch is defined by Paleozoic strata that rests unconformably upon Belt, which would require that the N/L line was active for an incredibly long time.

Intermediate between these two regions lies a patch of pre-Belt metamorphic rock alongside an extensive area of apparently incredibly thick Belt strata to the west. The pre-Belt rock possibly identifies the roots of a once continuous Noxon/Lemhi arch.

Yet farther south within Utah, the Cordilleran miogeosyncline curves toward the southwest, possibly diverting the Noxon/Lemhi structures southwesterly across Nevada.