Geophysics of the Midcontinent Rift Region Completed

Science Issue and Relevance

The Midcontinent Rift system and surrounding Precambrian rocks are known to host highly significant mineral resources. However, more than 90% of the Midcontinent Rift system is covered by Phanerozoic sedimentary rocks and sediments, and many regions lack drilling, meaning much of the region’s geology and mineral potential is poorly constrained and that geophysical methods are critical.

The USGS is uniquely qualified to carry out this important geologic framework research on behalf of the American public. No other government agency has our geological and geophysical expertise and capacity, academia does not do this type of work, and potentially relevant research by private exploration companies is not available in the public domain. Moreover, our Federal role allows us the best position from which to conduct this research, because the Midcontinent Rift system extends across multiple state lines.

Our objectives are the following:

• To map the lithology and structure of concealed and poorly understood Precambrian rocks. This part of the rift is almost completely unknown, and work here will help provide a basis for expanding the understanding of the extent and nature of the entire Midcontinent Rift system.
• To develop an integrated 3D geology model of the midcontinent rift in the western Lake Superior region from legacy geophysical data, new EarthScope and magnetotelluric data, borehole information, and geologic mapping.
• To acquire high-resolution ground and/or airborne electromagnetic data in several target regions and to interpret these data together with geophysical, geochemical, and borehole data to define their structural framework.

Methodology to Address Issue

We will be conducting the following activities to help characterize and increase understanding of the mineral resource potential of the region:

• Geologic Characterization of the Northeast Iowa Intrusive Complex
  Objective: to map the lithology and structure of concealed and poorly understood Precambrian rocks through the collection of aeromagnetic data, ground gravity data and borehole data.
   
• 3D Modeling in the Western Lake Superior Region
  Objective: to develop an integrated 3D geology model of the mid-continent rift in the western Lake Superior region from legacy geophysical data, new EarthScope and magnetotelluric data, borehole information, and geologic mapping.
   
• Regional and Local Electromagnetic Modeling
  Objective: to acquire high-resolution ground and/or airborne electromagnetic data in several target regions and to interpret these data together with geophysical, geochemical, and borehole data to define their structural framework.
   
• Mapping Concealed Precambrian Geology, Central Upper Peninsula, Michigan
  Objective: to map the lithology and structure of concealed and poorly understood Precambrian rocks by securing aeromagnetic data along with ground gravity data.
   
• Scientific Borehole in Northeast Iowa
  Objective: to test geophysical interpretations of the Proterozoic geology in northeastern Iowa by: 1) drilling a scientific borehole through Paleozoic cover to Proterzoic basement, 2) recover core from the Proterozoic basement sufficient for petrologic and radiometric analyses, and 3) obtain data on the overlying Paleozoic section to improve understanding of the Proterozoic-Paleozoic unconformity.

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