REEXAMINING THE NATURE AND LOCATION OF THE GRENVILLE FRONT IN OHIO, KENTUCKY, AND TENNESSEE

The Eastern Granite-Rhyolite basement province and the Grenville Orogen are features that formed in eastern Laurentia during the middle to late Proterozoic. However, the degree to which these features are interconnected in both area and tectonic evolution is poorly defined. In addition, although the term “Grenville Front” is commonly used in this region, the location and definition of it varies dramatically between authors (continental suture vs. high-angle thrust fault vs. metamorphic front vs. potential field lineament?), further confusing the issue.

Aeromagnetic survey data across Ohio, Kentucky, and Tennessee delineate a linear north-northeast to south-southwest boundary between two regions of different magnetic character—subdued, longer-wavelength anomalies to the west, and highly variable, short wavelength anomalies to the east. Well penetrations below the Paleozoic strata across this area encounter undeformed felsic plutonic rocks with occasional mafic bodies, mafic and felsic volcanics, and lithic arenite sandstones to the west of this boundary, and metamorphosed or thermally altered igneous rocks and meta-sediments to the east. The surface trace of this boundary is what we define as the location of the Grenville Front.

Recent analyses of magmatic zircons strongly suggest that a core of basement orthogneiss from Carter County, Ky. (about 80 km east of the Grenville Front), was originally part of the 1.45 Ga Eastern Granite-Rhyolite magmatic suite before undergoing granulite facies metamorphism around 1.02 Ga. This age of metamorphism concurs with previous studies of basement rocks in the region, further enforcing the theory that the metamorphism that produced the linear boundary described above was a result of the Grenville Orogeny. The interpretation that the protolith for that orthogneiss was the Eastern Granite-Rhyolite Province also disputes the interpretation that the Grenville Front is a continental suture formed during the Grenville orogenic collision, with rocks exotic to Laurentia immediately east of the front. Alternatively, these data suggest that the Precambrian basement under Kentucky is tectonically equivalent to a proposed deformed parautochthonous belt exposed in Ontario, Canada, just east of the Grenville Front tectonic zone.