IMPLICATIONS FOR THE GRENVILLE OROGENY AND ASSEMBLY OF RODINIA FROM GRAVITY ANOMALIES ALONG THE MIDCONTINENT RIFT AND GRENVILLE FRONT IN CANADA

Key aspects of the complex history of the assembly of Rodinia are recorded in North America by the Grenville Front (GF) and Midcontinent Rift (MCR). The GF results from collisions between Laurentia and Amazonia ~1300-980 Ma. The MCR records a major rifting event that occurred between orogenic phases of the Grenville Orogeny. Large positive gravity anomalies along the MCR reflect thick high-density volcanics and the combined effects of a sequence of rifting, volcanism, subsidence, sedimentation, compression, inversion, and any later effects. Differences in any of these would cause differences in the final structure and the resulting gravity anomaly. The gravity highs along the west arm are larger and bounded by pronounced lows, whereas those across the east arm are smaller and lack the sharp bounding lows. Possible causes include more magma in the west arm, less inversion in the east arm, or more uplift and erosion of the igneous rocks in the east arm. Gravity highs along the Fort Wayne Rift (FWR) and East Continent Gravity High (ECGH) are similar in dimensions and magnitudes to those elsewhere along the MCR in showing a distinct high, consistent with their being part of the MCRs east arm. In contrast, no similar high occurs across the Grenville Front in Canada, where differences in crustal accretion and deformation give rise to two very different gravity anomalies. The southern GF shows no gravity signature, where seismic data show primarily stacked thrusts, from which we would not expect a significant gravity anomaly. The northern GF produces a moderately negative gravity anomaly, interpreted as reflecting progressive crustal thickening of the northwestern Laurentian block with continued orogenic thrusting. It is difficult to see how a northern GF-type negative anomaly could exist along the east arm of the MCR, but we cannot exclude southern GF-type structures which would not produce anomalies along the MCR or elsewhere. Understanding the differences between the west and east arms, and along the GF, will provide additional insight into the tectonic evolution of the MCR and the complex interactions between Laurentia and Amazonia during the assembly of Rodinia.