NEOPROTEROZOIC STATIC TO DYNAMIC METAMORPHISM OF THE ITHACA BASIN: EVIDENCE FOR A PREVIOUSLY UNKNOWN TECTONIC EVENT IN THE APPALACHIANS

The Ithaca Basin (IB) is a Neoproterozoic rift basin discovered at the bottom of the ESH-1 geothermal well, Cornell Univ, 2022. A sequence of metasiliciclastic and mafic-felsic metavolcanics comprise the Cayuta Fm (CF). Magnetic anomaly mapping shows the basin to be fault-bounded and 4500 km² in extent. CF resides below the Cambrian nonconformity capped by the 510 Ma Potsdam sandstone. The metamorphism had two phases: 1) moderate to high-T static conditions that produced hornfels including remnants of cordierite; 2) formation of greenschist cleavage and microstructures. Basic heat conduction modeling of underplated magma in the Scranton rift (~250,000 km³ at ~80 km from the IB) may explain the initial thermal pulse (~600^oC) that formed hornfels texture. The burial of IB to a depth adequate to produce dynamic greenschist metamorphism requires an additional explanation. Detrital zircons recovered from cuttings of Potsdam sandstone yield concordant ages between ca. 540-630 Ma, corresponding to igneous activity in the St. Lawrence Rift, hydrothermal resetting of zircon xenocrysts in nearby kimberlite dikes, and bimodal NNE-trending dikes in the Adirondacks. Tourmaline-bearing granite veins crosscutting basement phyllites yield xenocrystic zircon derived nearly exclusively from 1000-1200 Ma sources and one tentative age of 471±5 Ma. Although Neoproterozoic eastern Laurentia is characterized as a product of simple continental rifting, the CF and metamorphism add to a growing body of evidence for a dynamic and complex tectonic evolution. Grenvillian rocks of the southernmost Adirondacks and from Mohawk Valley boreholes have greenschist facies overprint that does not occur in the overlying Cambrian strata. Foliated metamorphic xenoliths on Neoproterozoic rocks (ca. 534 Ma) occur in ca. 490 and 516 Ma plutonics in the Laurentian VA-PA Piedmont, and a reported thermal disturbance in the Honey Brook basement massif at ca. 567 Ma. Further, ca. 570 Ma Lynchburg and Catoctin Fm rift facies show metamorphism that is not apparent in overlying Cambrian strata. These argue for a widespread late Neoproterozoic to early Cambrian tectonic event in the Appalachians, possibly a back collision following late rifting, a short-lived subduction margin or continental collision obscured by extensive Paleozoic tectonism.