REDEFINING THE LATE MESOPROTEROZOIC GRENVILLE FRONT IN OHIO AND IMPLICATIONS OF CRUSTAL EVOLUTION IN THE MIDCONTINENT USA

Re-examination of Precambrian drill core and seismic reflection profiles defines the chronological sequence of the Ohio’s Precambrian Provinces: 1) Eastern Granite-Rhyolite, 2) East Continent Rift Basin (ECRB), and 3) Grenville; and calls for a re-evaluation of the Grenville Front in Ohio. Core and seismic reflection data confirm that the subsurface Grenville Front in Ohio is a structural, metamorphic, and lithologic terrane boundary, but indicate that it differs in concept and tectonic style from the Grenville Front boundary fault in western Ontario. Re-evaluation of petrographic and geochemical data for Ohio Precambrian core east of the Grenville Front indicates a presence of intermediate granitoids, which may represent coeval, less evolved (continental margin?) equivalents to continental granites of the Eastern Granite-Rhyolite Province. Medium- to low-grade metaplutonics and metasupracrustal sequences, including marbles intercalated with amphibolites, in the Grenville of Ohio, resemble allochthonous crustal domains within the Grenville Central Metasedimentary Belt of Canada. Together, the lithologic and metamorphic data for the Ohio Precambrian preclude a southern extension of the Canadian Grenville Central Gneiss Belt into Ohio. Structural restoration of fault displacements along pronounced unconformities on seismic data reveals a geometry suggestive of a tectonic evolution involving formation of the ECRB, and later foreland basin development during the later part of the Grenville orogeny. The ECRB developed during an extensional Grenville phase, followed by a large foreland basin, which was progressively partitioned during late east to west thrusting. An alternative model of post-ECRB, westward-advancing, smaller, sub-basins is equally viable. Taken together, the re-examination and compilation of data warrant a new systematic look at Precambrian well samples and cores in the region.