Magnetostratigraphy of the San Francisco Volcanic Field, Arizona

We present remanent magnetizations determined for 657 sites of volcanic rocks in the San Francisco Volcanic Field, an area of about 4800 km² in the southern part of the Colorado Plateau. Comprehensive stratigraphic controls - paleomagnetic polarity data, K-Ar and other absolute-age determinations, superposition relations, and lithologic associations - and state of preservation of the cinder cones provide a basis for age assignments of 503 separate eruptions related to basaltic vents and flows and 80 silicic to intermediate domes, flows, and flow sequences. The eruptions are assigned to three magnetopolarity sequences (polarity chronozones): the pre-Matuyama Chronozone (about 5.0 to 2.48 Ma), the Matuyama Reversed-Polarity Chronozone (2.48 to 0.73 Ma), and the Brunhes Normal-Polarity Chronozone (0.73 Ma to present). This stratigraphy documents a progression of volcanism, first to the northeast and then to the east: volcanic activity was centered along the Mesa Butte fault zone during pre-Matuyama and early Matuyama time and turned eastward through San Francisco Mountain during late Matuyama and Brunhes time. Site averages of magnetic vectors indicate that no appreciable crustal translations or rotations occurred in the field during eruptive activity. -from Authors