THE ENDURING IMPACT OF E-AN ZEN ON TACONIC GEOLOGY

The complex geology of the Taconic Mountains in the northern Appalachians has fueled controversies since 1840. One of these, the existence of thrust faults, was hotly debated until E-an Zen’s commanding synthesis of Taconic stratigraphy and detailed structural mapping in the 1960s convinced most geologists that Taconic rocks were allochthonous. Zen was also an influential advocate of submarine gravity sliding, and he argued that the Taconic allochthons slid from the tops of the Green Mountain and Berkshire massifs. Gravity sliding was consistent with geosynclinal theory, and it circumvented the perceived mechanical paradox of thrusting. Soon after Zen’s seminal publications, plate tectonics was used to explain the Ordovician Taconic orogeny as a collision between Laurentia and an island arc. However, Zen’s influence echoed for decades as geologists investigated problems he identified, including: hard-rock vs. soft-rock thrusting, the relative timing of thrusting and metamorphism, and the root-zone of the allochthons.

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.