The remanent magnetization of volcanic rocks has been determined at 650 sites in the San Francisco volcanic field in the southern part of the Colorado Plateau. The polarity of remanent magnetization—combined with K-Ar age determinations, spatial and petrographic associations, stratigraphic relations, and state of preservation of the cinder cones—provides a basis for assignment to known magnetic polarity epochs of 610 mafic vents and >100 intermediate to silicic flows, flow sequences, and vents. The age assignments for basaltic rocks include 243 Brunhes (<0.73 Ma) vents, 220 Matuyama (0.73 to 2.48 Ma) vents, and 147 pre-Matuyama (2.48 to about 5.0 Ma) vents. Basaltic volcanism migrated northeastward before Matuyama time at a rate of ∼1.2 cm/yr and eastward (S87° ± 5°E) over the past 2.5 m.y. at a rate of 2.9 ± 0.3 cm/yr. Concomitant acceleration in total magma production (from 75 to 1,400 × 10−6 km3/yr) and frequency of basaltic eruptions (from 1 per 17,000 yr to 1 per 3,000 yr) occurred between 5 and 0.25 Ma. For the past 0.25 m.y., magma production (∼180 × 10−6 km3/yr) and perhaps eruption frequency have decreased. This evolutionary sequence, coupled with the lead and strontium-isotopic composition of the rocks, can be explained by magmatism caused by shear heating at the base of the lithosphere. We propose that this eastward drift of volcanic activity represents absolute westward motion of the North American plate. Our model is in agreement with a model in which the African plate is fixed to the deep mantle.
