OPENING OF THE MIDCONTINENT RIFT DURING THE ASSEMBLY OF RODINIA (Invited Presentation)

How the 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, these anomalies appear to be the southward continuation of the MCR's east arm. Gravity highs along the ECGH are similar to those elsewhere along the MCR, in showing a distinct central high, presumably due to the dense igneous rocks filling the rift. In contrast, no similar high occurs across the GF in Canada. These results offer new insights into the formation of the MCR and GF and the assembly of Rodinia. The traditionally assumed GF location near southeast Michigan implies that the MCR's east arm ended there, presumably because propagation of the rift extension and volcanism were stopped by the preexisting GF. However, it now appears that the MCR formed before the GF. Hence the GF in the central U.S. may obscured by the MCR arm or younger tectonic events. Alternatively, a distinct Front never formed there. The only exposed Grenville deformational features 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 the southern GAAIs (from about Pennsylvania to Georgia) have Amazonian affinities. Traditionally the GAAIs are assumed to be essentially "in place" relative to their current positions during the Grenville Orogeny. Also, erosion from the southern GAAIs and its presumed major mountain belt is thought to have supplied the Grenville-age zircons found in the central US sedimentary rocks. However, if the GAAIs are terranes, originally deformed nearer Canada and then moved to their current location by younger plate tectonic events, they would have major implications for the assembly of Rodinia.