Relationships between mid-Miocene volcanism and deformation of the lithosphere in the northern Colorado River extensional corridor

Miocene lavas erupted in the Colorado River extensional corridor during major continental extension were analyzed for major, trace, and rare earth elements along with Pb, Nd and Sr isotopic ratios to understand the interplay between magmatism and extensional tectonism. Based on geochemistry and timing of eruptions lavas can be broken into three magmatic stages.;Parental magmas for the pre-extensional stage 1 (;Extension and magmatism were caused by mantle upwelling due to the northward migration of the Mendocino fracture zone. Upwelling asthenosphere heated the lithosphere which caused stage 1 magmas. The crustal component in stage 2 magmas suggest that the crust was hot at this time which may have triggered extension.;Changes in the Pb isotopic compositions of Miocene lavas from the highly extended Colorado River extensional corridor resulted from synextensional westward migration of Proterozoic Arizona lithosphere from beneath the unextended Colorado Plateau into the Basin and Range province. Tertiary lavas erupted in the extensional corridor prior to 12.2 Ma have Pb isotopic values similar to those of Tertiary to Quaternary lavas erupted within the Proterozoic Mojave crustal province. In contrast, post-12.2 Ma lavas from the same region have Pb isotopic values similar to that for lavas erupted throughout the Arizona crustal province, which underlies most of Arizona, southern Utah, and New Mexico. These isotopic changes probably reflect ductile flow of lithospheric mantle and middle to lower crust from the Colorado Plateau into the extensional corridor during Miocene extension.