GEOPHYSICAL ANALYSIS OF YOUNG MONOGENETIC VOLCANOES

Seismic refraction and magnetic surveys were completed at two locations in the San Francisco volcanic field (SFVF), Arizona, USA and one location in the Serdán-Oriental volcanic field (SOVF), Puebla, Mexico. The SFVF is characterized primarily by basaltic cones, which often occur in clusters that appear to have a general vent alignment and elongate vents. The SOVF has an abundance of maar volcanoes and scoria cones. The vents are often elongate and some areas display vent clustering. These features suggest structural influences on eruption mechanisms. Quaternary volcanoes and deposits in both volcanic fields are generally well preserved. In an attempt to better understand the plumbing systems associated with monogenetic volcanism, seismic refraction and magnetic surveys were used to image the subsurface geology beneath volcanoes in both fields.

Results suggest planar feeder dikes are the main method of magma transport in both fields and directly influence the formation of elongate vents and alignments in vent clusters. High-velocity structures imaged using seismic refraction have widths between ~20 and ~100 meters and are roughly perpendicular to vent alignment in the SOVF. Magnetic data show positive magnetic anomalies that correlate well with locations of high-velocity features imaged in the seismic refraction data. These data are interpreted as indicative of the presence of a dike. One location within the SFVF has similar ~80 to ~100-meter-wide high-velocity structures roughly perpendicular to vent alignment that are also interpreted as planar dikes. The other location revealed a ~100 meter-wide low-velocity feature that is interpreted as the expression of an evacuated feeder dike in-filled by scoria following eruption. Magnetic data in the SFVF prove to be less conclusive with higher levels of interference from a thick blanket of younger basaltic material from nearby eruptions. The seismic refraction data with support from the magnetic surveys suggest that eruptions in the study locations are fed by feeder dikes that influence vent alignment and elongation trends in both volcanic fields.