Migration of volcanism in the San Francisco volcanic field, Arizona

The remanent magnetization of volcanic rocks has been determined at 650 sites in this volcanic field in the S part of the Colorado plateau. The polarity of remanent magnetization, combined with K/Ar age determinations, spatial and petrographic association, stratigraphic relations and state of preservation of the cinder cones, provides a basis of assignment to a known magnetic polarity epoch of 610 mafic vents and >100 intermediate to silicic flows, flow sequences and vents. Basaltic volcanism migrated NE before Matoyama time (2.48-5.0 m.y.) at a rate of approx 1.2 cm/yr and eastward over the past 2.5 m.y. at a rate of 2.9 cm/yr. Total magma production and frequency of basaltic eruption accelerated between 5 and 0.25 m.y. and have decreased thereafter; this evolutionary sequence, coupled with the Sr-isotopic composition of the rocks, can be explained by magmatism caused by shear heating at the base of the lithosphere. The eastward drift of volcanism represents the absolute westward motion of the North America plate.