CONTROVERSIES ABOUT THE MIDCONTINENT RIFT'S HISTORY AND GRENVILLE-AGE DEFORMATION DURING RODINIA'S FORMATION

How the Midcontinent Rift (MCR) formed during the Grenville orogeny, a collisional and hence compressive series of events resulting in the formation of the supercontinent Rodinia, is a long-standing question. Much of the question involves the MCR's relation to the Grenville Front (GF), the continentward (western) extent of deformation of the fold and thrust belt from the Grenville orogeny. The Front is observed in SE Canada from surface geology and reflection seismic data, and has traditionally been assumed to extend into the central U.S. along the lineated gravity and magnetic anomalies of the East Coast Gravity High (ECGH). However, recent work has indicated that these anomalies appear to be the southward continuation of the MCR's east arm and that the MCR formed before the GF. Hence the GF in the central U.S. may be obscured by the MCR arm or younger tectonic events. Alternatively, a distinct Front may have never formed there. The only exposed Grenville deformational features in the area are the Grenville-Age Appalachian Inliers (GAAIs). In Canada, deformed rocks occur immediately east of the GF, whereas in the central U.S., the GAAIs are far outboard from the traditionally presumed GF. Petrologic data suggest that the northern GAAIs have Laurentian affinities and that the southern GAAIs (from about Pennsylvania to Georgia) have Amazonian affinities. We explore whether the southern GAAIs were located approximately in their current locations during the Grenville orogeny or alternatively if they are part of terranes, originally deformed nearer Canada and then moved to their current location by younger plate tectonic events. If the latter is the case, it would have major implications for geological interpretations. Two of Warren Hamilton’s earlier papers (Hamilton, 1957; King et al., 1958) involved studying parts of the southern GAAIs.