RECEIVER FUNCTION ANALYSIS OF THE SAN FRANCISCO VOLCANIC FIELD

There are many unanswered questions regarding the formation and eruptive processes of monogenetic volcanic fields and intraplate volcanism within the North American Cordillera. The San Francisco volcanic field, which contains over 600 volcanoes ranging in age from around 6 million years old to less than 1,000 years old, is situated well within the margins of the North American tectonic plate along the southern margin of the Colorado Plateau. Given the extensive volcanism associated with the field and the availability of recently collected seismic data, it provides an excellent locale to study intraplate volcanism. Teleseismic P-wave receiver functions are used to investigate lithospheric structure beneath the volcanic field. Measurements of crustal thickness and Vp/Vs ratio were made at five seismic stations deployed in the vicinity of the San Francisco volcanic field and preliminary results depict widespread heterogeneities in crustal structure and thickness throughout the study area. The crust appears thickest in the western and eastern portions of the volcanic field (~50-53km). Towards the center of the field the thickness decreases to ~44-47km, and H-K stacks indicate higher Vp/Vs ratios (around 1.83) within this zone. Receiver functions also indicated a complicated Moho signature beneath the volcanic field, possibly indicative of magmatic underplating at the base of the crust. Beneath Sunset Crater, the youngest volcano in the field, a shallow low-velocity zone is present that may be indicative of a molten zone beneath the crater. Recent fieldwork has located felsic material within the crater, indicative of more crustal interaction than previously thought, which is consistent with the presence of this low-velocity zone. Based on preliminary results, we suggest that the deeper crustal structure is impacted by magmatic addition in the western portion of the volcanic field, which has a diminished effect towards the east. These features correspond with the eastward trend of volcanic activity over the history of the San Francisco volcanic field and suggest that crustal interaction is greater than previously thought in intraplate-monogentic volcanic fields. As more data are collected we will be able to further assess the validity of these observations.