GEOPHYSICAL CHARACTERISTICS OF THE MIDCONTINENT RIFT SYSTEM IN THE LAKE SUPERIOR REGION: A REVIEW

Geophysical studies have been an integral part of geologic investigations of the Midcontinent Rift System in the Lake Superior region. Geophysical data provide a means of inferring geology in areas of poor exposure, and they provide the principal means of investigating geologic structure at depth.

Gravity and magnetic data have been particularly useful. Early studies 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. Gravity and magnetic model studies have been useful for investigating the upper crustal structure of the rift. In addition, paleomagnetic studies have provided important time-stratigraphic markers for the rift sequence. In recent years, high-quality gravity and magnetic data sets have been used to supplement geologic mapping, especially in regions that are largely covered by Pleistocene glacial deposits.

Seismic data have been indispensable for investigating the crustal structure beneath the Lake Superior region. Seismic reflection data acquired by GLIMPCE (Great Lakes International Multidisciplinary Program on Crustal Evolution) and other projects have been very effective in imaging the stratigraphy and structure of the rift sequence, which is interpreted to be over 25 kilometers thick beneath some parts of the lake. Seismic reflection data have also confirmed the existence of basement ridges beneath western Lake Superior, across which the rift sequence thins appreciably. Analysis of seismic reflection and refraction data revealed a high-velocity crust that is over 55 kilometers thick beneath central Lake Superior.

Geophysical studies have greatly enhanced our knowledge of the Midcontinent Rift System, but much more work remains to be done. Gravity and magnetic studies should continue to be valuable for geologic mapping, crustal studies, and mineral exploration for many years to come. New paleomagnetic studies that take full advantage of recent advances in rock-magnetism and geochronology may also be a promising avenue of research. Finally, many aspects of lithospheric structure beneath the Lake Superior region are still very poorly understood, and they should provide worthwhile research targets for the EarthScope initiative.