COCORP profiling in the eastern midcontinent of North America has (1) traced an extensive sequence of Precambrian layered rocks beneath southern Illinois, Indiana, and western Ohio; (2) detected a broad zone of east-dipping basement reflectors associated with the Grenville front beneath western Ohio; and (3) discovered a wide region of west-dipping reflectors penetrating most of the crust beneath eastern Ohio.

The Precambrian layered assemblage may be as much as 11 km thick beneath southern Illinois, extends at least 170 km in an east-west direction, and contains several strong reflectors that have a lateral continuity of 80 km or more. Industry seismic data indicate that the layering is extensive in a north-south direction as well. Possible explanations for the layering include the silicic igneous rocks of the ca. 1.48 Ga eastern granite-rhyolite province, which are penetrated by basement drill holes throughout the region, perhaps intermixed or underlain by mafic igneous or sedimentary rocks.

The 40-50-km-wide zone of strong, east-dipping (25°-30°) reflectors beneath west-central Ohio corresponds to the position of the Grenville front as determined from potential field and drill-hole data. These dipping reflectors in the upper and middle crust are interpreted to result from ductile deformation zones (mylonites) like those exposed at the Grenville front in Canada and imaged on the GLIMPCE seismic reflection lines in Lake Huron. Both the COCORP and GLIMPCE lines show a remarkably similar reflection geometry, despite the more than 500 km separating the two profiles.

Easternmost Ohio appears to be underlain by pronounced west-dipping (<40°) reflectors in the middle and lower crust, which are also interpreted as marking a region of pervasive ductile deformation 80 km or more in width. Analogy with similar reflection packages elsewhere suggests that these reflections may mark a major collision zone. The west-dipping reflectors may be correlative with similar reflectors imaged on another COCORP survey in northern Alabama. The correlations suggested by these new results, though tentative, imply that the eastern midcontinent is composed of a relatively simple assemblage of crustal blocks.

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