GEOPHYSICAL STUDIES OF THE MIDCONTINENT RIFT SYSTEM IN THE LAKE SUPERIOR REGION: A REVIEW AND A LOOK TOWARDS THE FUTURE

Geophysical studies are an integral part of geologic investigations of the Midcontinent Rift System in the Lake Superior region; they provide a means of inferring bedrock geology in areas of poor exposure, and they provide the principal means of investigating geologic structure at depth. High-quality gravity and aeromagnetic data sets are routinely used in bedrock mapping studies, both at the regional and the prospect scale, and model studies based on these data are effectively used to infer geology to depths of 10 km or more. Rock density and magnetization data have provided helpful constraints on gravity and magnetic interpretation, and paleomagnetic studies have provided important time-stratigraphic markers for the rift sequence. Conventional seismic refraction and reflection studies have been essential for investigating the entire crust beneath the rift, which in the Lake Superior region can exceed 50 km thickness.

Geophysical studies have greatly enhanced our knowledge of the Midcontinent Rift System, but much work remains to be done, especially in the third dimension of depth. Recent initiatives at the Minnesota Geological Survey and the U. S. Geological Survey are focusing on three-dimensional geologic mapping of the upper crust, to which the rift basins, their underlying basement and the overlying glacial sequence pose significant geophysical challenges. Mapping in the depth dimension currently relies on widely scattered data from deep drillholes, existing seismic reflection and refraction soundings, and modeling of gravity and magnetic data. However, to realize the full potential of three-dimensional geologic mapping, new geophysical data are needed, including data from both passive and active-source seismic soundings, as well as data from deeply-focused EM methods. If achieved, such a three-dimensional visualization would serve a variety of geologic applications, and would help in reconciling upper crustal geology with the lower crustal and lithospheric interpretations of the EarthScope experiments.