EVIDENCE FOR THE EVOLVING PLATE GEOMETRY IN THE TACONIC HINTERLAND OF THE NORTHERN APPALACHIANS AND IMPLICATIONS FOR FORELAND BASIN DEPOSITS

The Ordovician to Silurian clastic wedge in NY records protracted tectonic activity in the hinterland in western New England, which is collectively thought of as the Taconic orogeny. Evidence in western New England reveals major changes in plate geometry during this interval. By Early Ordovician, the Laurentian margin entered an east dipping subduction zone under the Gondwanan Moretown terrane (MT). This suture is located between the Rowe Schist and the Moretown Fm, and contains numerous lenses of mafic and ultramafic rocks. The Shelburne Falls arc (SFA) formed above the subduction zone on the MT, and the high volume of 475 Ma magmas in the arc may reflect subduction of Laurentian crust and/or slab-breakoff following collision. Detrital zircon grains from five samples in the Hawley Fm in the SFA contain (1) early Ordovician grains derived from arc volcanism, (2) numerous grains between 550 and 650 Ma consistent with a Gondwanan source, and (3) a major component of 960 to 1200 Ma Laurentian derived detritus indicating that by 475 Ma, the MT and SFA were proximal to Laurentia. Ordovician magmatism in the hinterland and foreland are linked by the 466 Ma Barnard Volcanic Member in VT and coeval ashes in the Indian River Fm in the Giddings Brook thrust sheet (GBTS) in NY. A west-dipping subduction zone under the Laurentian margin may have been established by 466 Ma, but was certainly active by 456 Ma when arc-related magmas intruded rocks already deformed during the earliest phase of the Taconic orogeny in western CT. If the Moretown, Cobble Mountain, and Albee Fms are all part of one Gondwanan terrane, as suggested by detrital zircon populations, the younger 454 to 442 Ma magmatic rocks in the Bronson Hill arc (BHA) formed above the same west-dipping subduction zone after accretion of the MT. Thus, magmatic rocks in the SFA and BHA probably formed on the same Gondwanan terrane, but above two oppositely dipping subduction zones, resulting in spatial overlap of older and younger arc rocks. The Taconic clastic wedge began as a pro-arc foreland basin, which is preserved in the Poultney through Pawlet Fms of the GBTS. By ca. 465-460 Ma, a superimposed retro-arc foreland basin developed in which the classic Late Ordovician Taconic clastic wedge was deposited. Late Ordovician to Silurian accretion of Ganderia further modified the plate geometry.