Two prominent Precambrian geologic features of central North America are the Midcontinent Rift (MCR) and Grenville Front (GF). The MCR, an extensive band of buried igneous and sedimentary rocks outcropping near Lake Superior, records a major rifting event at ca. 1.1 Ga that failed to split North America. In SE Canada, the GF is the continent-ward extent of deformation of the fold-and-thrust belt from the Grenville orogeny, the sequence of events from ca. 1.3-0.98 Ga culminating in the assembly of the supercontinent of Rodinia. In the central U.S., lineated gravity anomalies extending southward along the trend of the front in Canada have been interpreted as a buried Grenville Front. However, we use recent tectonic concepts and data analyses to argue that these anomalies delineate the eastern arm of the MCR extending from Michigan to Alabama, for multiple reasons: (1) These anomalies are similar to those along the remainder of the MCR and quite different from those across the front in Canada; (2) the Precambrian deformation observed on seismic reflection profiles across the presumed “front” appears quite different from that across the front in Canada, cannot confidently be assigned to the Grenville orogeny, and is recorded at least 100 km west of the “front”; and (3) during the Grenville orogeny, deformational events from Texas to Canada were not caused by the same plate interactions and were not necessarily synchronous. Hence the commonly inferred position of the “Grenville Front” in the central U.S. is part of the MCR, and should not be mapped as a separate entity.
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