Late Cenozoic (<15 Ma) basaltic rocks define two temporally and spatially distinct magmatic belts in southwest Utah that are located in the Basin and Range Province, the transition zone, and into the Colorado Plateau interior. The belts share certain time-space-composition characteristics and are dissimilar in others. The north-south Black Rock–Grand Canyon belt (generally 2.5–0 Ma) exhibits no province-wide time-space patterns, but shows distinct space-composition variations involving both lithospheric and asthenospheric magma sources. Magmatism is inferred to have occurred in response to east-west extension, and the composition of the basalts is compatible with maximum thinning and extension by pure shear of the mantle lithosphere occurs near the transition zone.
In contrast, the Pahranagat–San Rafael belt (15–3.5 Ma) exhibits a northeastward time-space progression of magmatism. The magmatic migration vector is similar to the Yellowstone trend, suggesting the ultimate influence of a fixed heat or magma source in the asthenosphere, although the basalts record apparent lithospheric signatures. Basalts from this belt are progressively more isotopically primitive and more alkaline into the Colorado Plateau. These chemical variations can be interpreted as a change in the mantle source, depth or degree of partial melting, or decreasing crustal contamination to the northeast.