IS THE EAST CONTINENT RIFT BASIN REALLY A RIFT BASIN, OR IS IT A FORELAND BASIN? AND DID THE GRENVILLE OROGENY ‘TAKE A BREATHER’ DURING MIDCONTINENT RIFTING? NEW EVIDENCE FROM ZIRCON U-PB GEOCHRONOLOGY OF THE MIDDLE RUN FM. AND BASEMENT ORTHOGNEISS IN DRILL CORE FROM KENTUCKY

The relationship between spatially overlapping and nearly contemporaneous Grenvillian collisional orogenesis and late Mesoproterozoic continental rifting in the eastern U.S. mid-continent is a long-standing conundrum. The East Continent Rift system (ECRS) and its presumed syn-rift basin fill (Middle Run Fm.: MRF) relied on geophysical evidence and scattered drill cores and cuttings to delineate the subsurface distribution of inferred syn-rift mafic magmatism and sedimentation. Although modern analogs of spatially associated rift-orogen pairs exist, an alternative hypothesis is that the Middle Run Fm. is a foreland basin fill, partly resolving another conundrum – where is the enormous foreland basin clastic sequence that should have been generated by one of Earth’s great collisional orogens? New detrital zircon (DZ) U-Pb geochronology and examination of published zircon ages on Grenville basement rocks from eastern Laurentia provide a new perspective on these broadly contemporaneous events. Based on DZ, the MRF in the subsurface of KY-OH can be no older than ca. 1030 Ma and is therefore NOT a temporal equivalent of Midcontinent Rift (MCR) basin fill – in fact the MRF is at least 70 m.y. younger than the MCR. The MRF also does not have an eastern Laurentia Grenville provenance: its dominant age peak is at ~1120 Ma and it lacks the “Grenville doublet”, i.e., DZ age modes corresponding to Ottawan (1040-1080) and Shawinigan (1150-1180 Ma) phases of the protracted Grenville orogeny. Basalts that overlie MRF-like volcaniclastics in KY also cannot be correlative to ca. 1100 Ma MCR basalts. The dominant 1120 Ma DZ age mode matches that of Grenville foreland basin sediments west of the Llano Grenville terrane in TX where there exist similar aged Grenville magmatic rocks. Basement orthogneisses along the buried Grenville Front have crystallization ages of ca. 1450 Ma, are of likely Eastern Granite Rhyolite affinity, and were metamorphosed to high grade at ca. 1030 Ma. These rocks are at the same crustal level as the MRF and require tens of km of exhumation during late Grenville thrusting. If anything, the MCR event (1105-1095 Ma) most closely overlaps with the youngest Llano magmatic event, and apparently occurred when the Grenville of eastern Laurentia “took a breather” from high-temperature collision-related magmagenesis.