GRAVITY AND MAGNETIC ANALYSIS OF THE MIDCONTINENT RIFT SYSTEM IN IOWA

The Proterozoic Midcontinent Rift System (MCRS) is considered one of the most important tectonic features in North America. The MCRS is hidden beneath Phanerozoic sedimentary rocks except where it is exposed in the Lake Superior region, and its geological characteristics are primarily inferred from extrapolations from the outcrop area and a few drill holes. Consequently, the MCRS is mainly known from a variety of geophysical investigations including gravity, magnetic and seismic reflection profiles. The MCRS is notable by prominent positive gravity and magnetic anomalies because it contains a large volume of basaltic rocks. The 2000 km long MCRS was formed during a continental breakup event 1.1 Ga, and trends from northwest Michigan to northern Oklahoma. The MCRS has two major arms meeting in the Lake Superior region. One extends southwestward at least as far as northern Oklahoma, and the other extends southeastward at least through Ohio.

A large segment of the MCRS transverses Iowa, which to date has only been generally investigated geophysically with little information regarding the lower crustal and upper mantle structure under the rift. In order to understand the crustal and upper mantle structure, we will analyze the available gravity and magnetic data in Iowa. Residual gravity and magnetic anomaly maps clearly outline the main rift system with maxima anomalies over the basalt and minima anomalies over the flanking rift basins. To further the investigation, four profiles, perpendicular to the rift were constructed using gravity and magnetic data. These 2D models will be constrained by previous gravity and magnetic models performed elsewhere on the MCRS, seismic reflection data, basement penetrating drillholes and broadband seismic results from the recent Earthscope experiment. The models will be integrated with 2D inverse models for the Earthscope long period magnetotelluric experiment in order to determine the crustal and upper mantle structure of the Iowa segment of the MCRS. These models will aid us in determining if mantle anomalies still persist from the extrusion of large amounts of mafic material from the upper mantle.