IMAGING SUBSURFACE VOLCANIC FEATURES IN THE SAN FRANCISCO VOLCANIC FIELD, ARIZONA

The San Francisco volcanic field (SFVF) covers approximately 4700 km² north and east of Flagstaff, Arizona. The majority of volcanic features within this region are monogenetic, basaltic scoria cones. Crater 120, a scoria cone that lies within the eastern region of the SFVF, is a series of vents in a NW-SE alignment that appear to originate from a migrating source. Seismic refraction surveys were conducted at one location off the southeast flank of the largest vent in the Crater 120 vent complex in an attempt to determine if a dike or fissure system drove the apparent vent migration. The data collected from a 575-m seismic line with a 5-m geophone spacing and a 345-m seismic line with a 2-m geophone spacing were processed in order to generate tomographic images of the subsurface. The refraction survey with a 2-m geophone interval was conducted to produce a tomographic image with a higher resolution in an area where a low-velocity feature was observed using the 5-m geophone interval survey. Changes in the seismic wave velocities are interpreted to represent differences in composition between igneous material or separate rock units. Both images produced from the surveys show a region of low seismic velocity in general alignment between vents, however, the data from the survey conducted with the 2-m geophone interval show that the shorter interval did not produce a better image. A low-velocity feature in both tomographic images may be due to a drop in system pressure and subsequent drain-back of magma at the end stages of eruption. This would have produced a hollow vent that may have later been filled in with scoria from an eruption of a nearby vent. Additional data-collection techniques such as new refraction survey locations, and the potential use of seismic reflection surveys may be necessary to fully constrain the processes that lead to the production of monogenetic scoria cones with these migratory vent features.