Gravity and magnetic study of the crustal structure of the San Francisco volcanic field, Arizona, USA

Abstract

Bouguer gravity, residual aeromagnetic, high- and strike-passed filtered gravity and magnetic anomaly maps are analyzed to model the general crustal structure of the San Francisco volcanic field in northern Arizona. Analysis of these maps indicate that the sources of the gravity and magnetic anomalies reflect predominately Precambrian tectonic, Cenozoic volcanic and possibly Laramide-age (Cretaceous/Tertiary) tectonic events. Northeast- and north-trending gravity and magnetic maxima and minima are thought to be caused by Precambrian metavolcanics and granites intruded along regional fault systems although the gravity minima may be caused by Laramide-age igneous intrusions known to exist southwest of the study area. A strike-pass filtered gravity anomaly map and seismic tomographic models roughly delineate the lateral extent of a high-density/velocity zone which may represent the crystallized source material of the San Francisco volcanic field. Two gravity and magnetic models constrained by magnetotelluric models, seismic tomographic models, petrologic studies and geologic mapping indicate that surficial volcanic material of up to 1.5 km in thickness overlie a silicic/intermediate body which may be the part of the San Francisco volcanic field source. This body is approximately 3 km below the Earth's surface and extends to 8 km with a width of 15 km. Below this now crystallized body is a low-density zone extending to 34 km which is interpreted as a partially melted zone and may ultimately be the source of the volcanic field.