GEOPHYSICAL OVERVIEW OF THE MIDCONTINENT RIFT SYSTEM

The Midcontinent Rift System is one of the most prominent features in gravity and aeromagnetic surveys of North America. This paper aims to review the principal existing geophysical observations on the Midcontinent Rift and to call attention to the fundamental themes that have driven geophysical research for more than half a century.

Early gravity and magnetic data confirmed the extension of rift geology well beyond the Lake Superior region, along the Mid-Continent Gravity High to the southwest, and along the Mid-Michigan Gravity High to the southeast. Inversions of such data have allowed for investigation into the upper crustal structure of the rift. In recent years, high-resolution gravity and magnetic data have been used to supplement geologic mapping, especially in areas that are largely covered by Pleistocene glacial deposits. Geomagnetic reversals during rift development produced correlative polarity zones, which acted as chronostratigraphic markers that were vital prior to the use of high precision U/Pb ages. Plate motion during rifting led to large changes in paleolatitude, providing another means of correlation within detailed stratigraphic datasets (see Swanson-Hysell et al. in this session). Paleomagnetic data generated from the rift system are the backbone for reconstructions of the assembly of the supercontinent Rodinia, and place the onset and termination of rifting in a more complete geodynamic context.

Seismic reflection and refraction data acquired by GLIMPCE and other projects have imaged the rift sequence, which is over 30 km thick beneath parts of Lake Superior. Such data has identified major faults and basement ridges beneath western Lake Superior, where the rift sequence thins appreciably.

Geophysical studies have greatly enhanced our knowledge of the Midcontinent Rift System, but much work remains. Gravity coverage is still poor over much of the rift, and improvement is needed. New paleomagnetic studies that employ modern protocols and leverage advances in rock-magnetism and geochronology are also a promising avenue of research. Finally, many aspects of the lithospheric structure beneath the Midcontinent Rift are still poorly understood, and provide research targets for the EarthScope initiative, such as that being undertaken by the Superior Province Rifting Earthscope Experiment.