GEOPHYSICAL STUDIES IN THE LAKE SUPERIOR REGION: WHERE ARE WE AND WHAT'S NEXT?

The Lake Superior region presents a 2500 m.y. window on the evolution of the North American lithosphere. Geophysical methods have been an essential element of Precambrian studies and mapping in the Lake Superior region largely because of poor bedrock exposure and limited geologic control in the subsurface. Owing to generally strong contrasts in rock density and magnetization, gravity and magnetic data have been a particularly effective supplement to bedrock geologic mapping and to investigations of structures in the crust. Recently, the NICE (Northern Interior Continental Evolution) working group has used regional compilations of high-quality gravity and aeromagnetic data sets, in conjunction with new geochronological information, to significantly improve the regional geologic framework and timing of significant events.

This emerging geologic framework, combined with the earlier success of seismic initiatives such as GLIMPCE (Great Lakes International Multidisciplinary Program on Crustal Evolution) and COCORP (Consortium on Continental Reflection Profiling), creates a favorable setting for the new seismic and other geophysical experiments of the EarthScope initiative. Of particular interest are the nature and extent of Archean lithosphere and the lithospheric structure associated with the Paleoproterozoic orogens that build southeastwards from this ancient core. The interaction of a Mesoproterozoic rift, the Midcontinent Rift System, with this southwardly younging lithosphere is also a topic that should be investigated. Other geophysical projects that investigate crustal- scale geology are needed to complement the deeper-focused investigations. Among these are combined studies of gravity, magnetotelluric, and seismic reflection profiling data to better image crustal structure along key transects. In addition, improved gravity and aeromagnetic coverage in the areas bridging the Lake Superior region and the northern Great Plains would be beneficial to groups working in both regions.