Abstract

In addition to traditional in-line wide-angle seismic profiles, the 1989 Pacific to Arizona Crustal Experiment (PACE-1989) included some off-line shots and seismic recorders, which, along with the data from the main profiles, provide tomographic coverage of the late Cenozoic San Francisco volcanic field. This major volcanic center is interesting because of its location on the margin of the Colorado plateau and because of the diversity of the composition of the volcanics. We employed arrival times for the Pg phase to undertake a tomographic analysis of the structure of the upper crust. The tomographic image indicates high-velocity zones at upper crustal depths under the San Francisco Mountain region. The P-wave velocities within these zones are 6% higher than the surrounding basement rocks. Although other interpretations are possible, we believe that these high-velocity regions most likely represent igneous intrusions associated with the volcanic centers that formed 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-velocity regions are coincident with high-density bodies at upper crustal depths.

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