TERRANE ACCRETION AND FORELAND BASIN FORMATION IN THE NORTHERN APPALACHIANS

Differences in the style and timing of Ordovician foreland subsidence between Newfoundland and New England may reflect first-order variations in the accretion of peri-Laurentian and peri-Gondwanan arcs and terranes during the Taconic orogeny. Detrital zircon in metasediments of the Late Cambrian to Early Ordovician Moretown Formation in western MA and southern VT was derived from a Gondwanan protolith, with prominent peaks between ca. 500 and 600 Ma, similar to those previously reported from the Moretown in northern VT. These data suggest that the Red Indian Line, which separates peri-Laurentian and peri-Gondwanan terranes, lies between the ultramafic-rich Rowe Schist and the Moretown, farther west than previously suspected. The suture predated the Late Ordovician Middlefield granite, which stiched the Rowe and Moretown. Importantly, metasediments in the structurally higher Ordovician Hawley Formation contain distinctly Grenvillian detrital zircon. The Hawley is interpreted to be a forearc to the Early to Middle Ordovician Shelburne Falls arc. If this is correct, the provenance of Hawley metasediments suggests that the Shelburne Falls arc developed on the Moretown after it was proximal to Laurentia. However, the age of the heterogeneous Hawley is poorly constrained; it may post-date the Shelburne Falls arc. The location of the Red Indian Line indicates that the peri-Laurentian Dashwoods terrane, which accreted to the Newfoundland Laurentian margin in the Middle Ordovician, is absent in MA and VT. This is consistent with differences in the subsidence history of the foreland basins on the autochthon. In Newfoundland, the Table Head and Goose Tickle groups record the development of a large Middle Ordovician foreland basin with the obduction of Dashwoods, whereas in New York, Middle Ordovician subsidence and deposition are meager. New CA-ID-TIMS dating of ashes in the Utica shale in New York demonstrates that it formed rapidly in the Katian stage of the Late Ordovician, and is equivalent with the Long Point Group in Newfoundland, both of which formed as foredeeps in the final closure of the Iapetus ocean. Thus, we suggest that differences in the foreland basins record first-order variations in the history of terrane accretion between Newfoundland and New England, in particular the absence of Dashwoods to the south.