U-Pb dates on magmatic and detrital zircon from samples in the hinterland of the Taconic orogen place new constraints on the timing and plate tectonic geometry of terrane accretion and magmatic arc activity. The Moretown terrane, a Gondwanan-derived exotic block, extends from the Rowe Schist-Moretown Formation contact in the west to the Bronson Hill arc in the east. Arc-related plutonic and volcanic rocks formed above an east-dipping subduction zone under the western leading edge of the Moretown terrane from approximately 500 to 475 Ma, until collision with hyperextended distal fragments of Laurentia, represented by the Rowe Schist, at 475 Ma. Magmatic arc rocks formed during this interval are primarily located in the Shelburne Falls arc, although some are also located in the Bronson Hill arc to the east. Metasedimentary rocks in the Shelburne Falls arc contain detrital zircon derived from mixing of Gondwanan, Laurentian, and arc sources, suggesting that the Moretown terrane was proximal to Laurentia by 475 Ma. Explosive eruptions at 466 to 464 Ma preserved in the Barnard Volcanic Member of the Missisquoi Formation in Vermont and as ash beds in the Indian River Formation in the Taconic allochthons may record slab-breakoff of subducted lithosphere following collision of the Moretown terrane with distal Laurentian crustal fragments. Between 466 and 455 Ma a reversal in subduction polarity lead to a west-dipping subduction zone under Laurentia and the newly accreted Moretown terrane. Magmatic arc rocks in the Bronson Hill arc formed above this west-dipping subduction zone along the eastern trailing edge of the Moretown terrane at approximately 455 to 440 Ma. The western boundary of Ganderia in New England is east of the Bronson Hill arc, buried beneath Silurian and Devonian rocks deformed during the Acadian orogeny.

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Ordovician strata of the Mohawk Valley and Taconic allochthon of New York and the Humber margin of Newfoundland record multiple magmatic and basin-forming episodes associated with the Taconic orogeny. Here we present new U-Pb zircon geochronology and whole rock geochemistry and neodymium isotopes from Early Paleozoic volcanic ashes and siliciclastic units on the northern Appalachian margin of Laurentia. Volcanic ashes in the Table Point Formation of Newfoundland and the Indian River Formation of the Taconic allochthon in New York yield dates between 466.16 ؎ 0.12 and 464.20 ؎ 0.13 Ma. Red, bioturbated slate of the Indian River Formation record a shift to more juvenile neodymium isotope values suggesting sedimentary contributions from the Taconic arc-system by 466 Ma. Eight ashes within the Trenton Group in the Mohawk Valley were dated between 452.63 ؎ 0.06 and 450.68 ؎ 0.12 Ma. These ashes contain zircon with Late Ordovician magmatic rims and 1.4 to 1.0 Ga xenocrystic cores that were inherited from Grenville basement, suggesting that the parent magmas erupted through the Laurentian margin. The new geochronological and geochemical data are integrated with a subsidence model and data from the hinterland to refine the tectonic model of the Taconic orogeny. Closure of the Iapetus Ocean by 475 Ma via collision of the peri-Gondwanan Moretown terrane with hyperextended distal fragments of the Laurentian margin is not clearly manifested on the autochthon or the Taconic allochthon other than an increase in sediment accumulation. Pro-foreland basins formed during the Middle Ordovician when these terranes were obducted onto the Laurentian margin. 466 to 464 Ma ashes on the Laurentian margin coincide with a late pulse of magmatism in both the Notre Dame arc in Newfoundland and the Shelburne Falls arc of New England that is potentially related to break-off of an east-dipping slab. Following slab reversal, by 455 Ma, the Bronson Hill arc was established on the new composite Laurentian margin. Thus, we conclude that Late Ordovician strata in the Mohawk Valley and Taconic allochthon of New York and on the Humber margin of Newfoundland were deposited in retro-foreland basins.

The crust and upper mantle beneath the New England Appalachians exhibit a large offset of the Moho across the boundary between Laurentia and accreted terranes and several dipping discontinuities, which reflect Paleozoic or younger tectonic movements. We apply scattered wavefield migration to the SEISConn array deployed across northern Connecticut and obtain insights not previously available from receiver function studies. We resolve a doubled Moho at a previously imaged Moho offset, which may reflect westward thrusting of rifted Grenville crust. The migration image suggests laterally variable velocity contrasts across the Moho, perhaps reflecting mafic underplating during continental rifting. A west‐dipping feature in the lithospheric mantle is further constrained to have a slab‐like geometry, representing a relict slab subducted during an Appalachian orogenic event. Localized low seismic velocities in the upper mantle beneath the eastern portion of the array may indicate that the Northern Appalachian Anomaly extends relatively far to the south.