THE ENDURING IMPACT OF E-AN ZEN ON TACONIC GEOLOGY
The complex geology of the Taconic region reflects the evolution of a continental margin that was continuously active from Early Ordovician to Carboniferous. By 475 Ma, a major suture formed when the Gondwanan Moretown terrane and the superjacent Shelburne Falls arc collided with Laurentia above an east-dipping subduction zone, outboard of rocks now located in the Taconics. Subduction polarity reversed by 466 Ma when ashes, derived from volcanoes above the newly established west-dipping subduction zone, were deposited in the Indian River Fm. in the Taconics. West of the Taconics, the weakly deformed Utica Shale received 453 to 450 Ma ashes coeval with westward thrusting of the Taconic allochthons. Both the Utica ashes and the critical taper that drove thrusting resulted from collision of the east-facing arc along the Laurentian margin and a second Gondwanan terrain with a west-facing arc in the Late Ordovician. The Devonian collision of Avalon with Laurentia reestablished critical taper, and the Green Mountain and Berkshire massifs were thrust westward up crustal-scale ramps. Taconic thrust sheets were folded and eroded above the basement-cored antiforms, thus isolating the Taconic klippen. The resulting map pattern was the inspiration for Zen’s gravity sliding model for the allochthons.