Velocity structure of the upper crust under the San Francisco volcanic field, Arizona

In addition to traditional in-line wide-angle seismic profiles, the 1989 Pa cific to Arizona Crustal Experiment (PACE-1989) included some off-line shot s and seismic recorders, which, along with the data from the main profiles, provide tomographic coverage of the late Cenozoic San Francisco volcanic f ield. This major volcanic center is interesting because of its location on the margin of the Colorado plateau and because of the diversity of the comp osition of the volcanics. We employed arrival times for the Pg phase to und ertake a tomographic analysis of the structure of the upper crust. The tomo graphic image indicates high-velocity zones at upper crustal depths under t he San Francisco Mountain region. The P-wave velocities within these zones are 6% higher than the surrounding basement rocks. Although other interpret ations are possible, we believe that these high-velocity regions most likel y represent igneous intrusions associated with the volcanic centers that fo rmed the San Francisco volcanic field. Geologic studies indicate that these intrusions would be of intermediate to mafic composition. Gravity data in the San Francisco volcanic field area also indicate that these high-velocit y regions are coincident with high-density bodies at upper crustal depths.