GRAVITY AND MAGNETIC ANALYSIS OF PLUTONS, RING PLUTONS AND MAFIC BODIES IN THE ST FRANCOIS MOUNTAINS, SE MISSOURI

The St. Francois Mountains of southeast Missouri are a Proterozoic volcanic complex composed of felsic plutons, ring intrusions, and massifs, and is the largest exposure of Proterozoic lithologies in the south-central US. The development of the St. Francois Mountains is not well understood and there are three main theories that are currently under consideration: 1) hotspot/melting, 2) subduction volcanism, and 3) rift related volcanism. Since the proterozoic volcanic activity, the region has been faulted by forces related to the Reelfoot Rift in the Cambrian and reactivation in the Cretaceous, and more recently by the New Madrid Fault zone. Geologic mapping and regional magnetic anomaly interpretation has proposed that the region consists of a few large felsic calderas. Our research focuses on analyzing gravity and magnetic data to determine the upper crustal structure of the region in order to relate these data to the location of proposed caldera rims, as well as mafic bodies, and better understand their origin.

Using existing gravity and magnetic data as a guideline for the location of new data, we first conducted a gravity survey to add data to regions without data. The merged data, in the form of Bouguer gravity anomaly maps, identified anomalies that agreed with existing structural data of the region. Then additional detailed (1 mile spacing) gravity data were collected along two profiles that cut across the large amplitude anomalies associated with the calderas. These data were be modeled by 2D forward modeling to determine the subsurface geometry of the calderas constrained by surface density measurements and surface features. Additionally, a series of residual, regional and derivative gravity and magnetic anomaly maps have been constructed to aid in the interpretation of the 2D models. Collection of rock samples at the surface has given density data to constrain our models. Core samples will be analyzed for densities and magnetic susceptibilities in order to further constrain the gravity models. Using the final anomaly maps and models we will interpret them in order to determine the nature and origin of the calderas in the St. Francois Mountains region.