UNITING TACONIC HINTERLAND DEFORMATION WITH ORDOVICIAN FORELAND DEPOSITION IN NEW ENGLAND

Geochronology from the hinterland of the Taconic orogen in New England demonstrates that arc-continent collision peaked at ca. 475 Ma, yet, the classic Katian Taconic foreland in the Mohawk Valley formed between ~454 and 448 Ma. Thus obduction of the Moretown terrane and the Shelburne Falls arc (SFA) onto the Laurentian margin was not responsible for Late Ordovician subsidence in the Mohawk Valley, and begs the questions: Where is the record of the Early Ordovician Taconic peripheral foreland? And what drove Late Ordovician subsidence in the Mohawk Valley? Ordovician basin formation on Laurentia is marked by the Early to Middle Ordovician Knox unconformity followed by deposition of the Chazy Group between ~465 and 458 Ma, and deposition of the Black River and Trenton Groups between ~454 and 448 Ma. We present new U/Pb zircon data for an Early Ordovician collisional basin in the Hawley Fm, and constrain the influx of zircon from the hinterland to the Giddings Brook thrust sheet of the Taconic Allochthon by at least 466 Ma. By this time, the Moretown terrane and SFA had already accreted to the Laurentian margin, the east-dipping slab had broken off, and west-dipping subduction had likely begun under the Moretown terrane, leading to the formation of the Bronson Hill arc. Thus, we suggest that the Early Ordovician Taconic peripheral foreland was meager and is manifested predominantly as an unconformity on the Laurentian margin, likely due to the structural control that the New York promontory played on Early Ordovician Taconic NW-vergent convergence. These data suggest that the Chazy, Black River, and Trenton Groups formed in at least two separate basin-forming events as composite back-arc and retro-arc foreland basins. This new model can explain a number of previously confusing observations including: 1) Ordovician subsidence in the mid-continent, which now can be understood as dynamic subsidence above a west dipping slab; 2) ubiquitous ashes with Grenville inheritance; 3) abundant Ordovician normal faulting on the platform previously related to foreland flexure, but is more easily understood as the product of back-arc extension; and 4) prolonged development of irregular depocenters that can now be attributed to a combination of dynamic subsidence and back-arc extension rather than structural loading.