UNSCRAMBLING THE MIDCONTINENT RIFT AND GRENVILLE FRONT IN THE EASTERN UNITED STATES

North America’s Midcontinent Rift (MCR) and Grenville Front (GF) jointly record key aspects of the complex history of the assembly of Rodinia. However, their interaction in the eastern United States remains unclear. The 1.1 Ga MCR, which records a failed major rifting event, is exposed along Lake Superior and well defined by gravity, magnetic, and seismic data along its west arm. Large positive gravity and magnetic anomalies along the west arm reflect thick high-density volcanics and the combined effects of a sequence of rifting, volcanism, subsidence, sedimentation, compression, and inversion. Gravity modeling shows 20-25 km of volcanics in the west arm which produce gravity highs that are much larger and more pronounced than those along the east arm, which only has 10 to 15 km of volcanics at its thickest. A revised rift length, along with updated thicknesses from modeling, yield new estimates for the volume of volcanic rocks filling the MCR basin. The GF results from collisions between Laurentia and Amazonia ~1300-980 Ma and is exposed in Canada and identified in seismic and potential field data. Differences in crustal accretion and deformation yield two different subsurface structures; the southern GF, where seismic data show stacked thrusts, shows no gravity signature, whereas the northern GF produces a moderately negative gravity anomaly, reflecting progressive crustal thickening of the northwestern Laurentian block during orogenic thrusting. Magnetic data along the GF in Canada exhibit a strong negative anomaly. Although gravity data from the MCR’s east arm look similar to that along the west arm, evidence of a past collision has been inferred geologically. Recent seismic data reveals shallowly dipping crustal deformation consistent with an orogenic collision in the eastern U.S. To explore whether the GF is present along the MCR’s east arm, we examined the GF in Canada – where it has no possible relationship to the MCR – and the MCR in the west arm – where it has no possible relationship to the GF – to study the differences between the arms and ask whether these differences could be produced by GF-related collisions.