LITHOSPHERIC EVALUATION OF THE MID-CONTINENTAL RIFT SYSTEM IN IOWA USING GRAVITY AND MAGNETIC ANALYSIS

The Proterozoic Midcontinent Rift System (MCRS) considered one of the most important tectonic features in North America was formed during a continental breakup event 1.1 Ga. 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. The MCRS is exposed only in the Superior region, otherwise it is covered by Phanerozoic sedimentary rocks. Consequently, the geological characteristics of the MCRS are primarily inferred from extrapolations from the outcrop area, scattered drill holes, and from a variety of geophysical investigations including gravity, magnetic and seismic reflection profiles. The MCRS is notable by prominent positive gravity and magnetic anomalies due the presence of a large volume of basaltic rocks. There are a lot of questions related to the tectonic origin of the MCRS, the origin of the magma either from a microplate interaction with another one at spreading ridge at west and a transform fault at East or from mantle plumes within a separated plate. Also, the magma evolution is one of the questions that had been answered by Nd isotope study in Minnesota. The Nd isotope study suggests that the interaction of the mantle magma with the older crust forms a less mafic magma. Iowa contains a large segment of the MCRS which to date has only been investigated geophysically in general terms with little information regarding the lower crustal and upper mantle structure under the rift. In order to understand the crustal and upper mantle structure in Iowa, we analyzed gravity and magnetic data. 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 models, perpendicular to the MCRS were constructed using gravity and magnetic data. These 2D models are 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 suggest that mantle underplating materials that might have persisted from the extrusion of large amounts of mafic material from the upper mantle and initial formation of the rift system.