SUBDUCTION ZONE TECTONICS COMES TO NEW ENGLAND: FOLLOWING THE ROLFE STANLEY APPROACH TO THE HINTERLAND

Several workers related features of the Taconian foreland of New England to subduction processes, but Rolfe Stanley was the first to carry these ideas deep into the hinterland even into the magmatic arc. Influenced by experiences in a subduction system in Taiwan and his mentor, John Rodgers, he used a comparative approach to explain New England tectonics. Here we apply this approach to understand the nature of subduction and collision between Laurentia and Baltica during middle Paleozoic in Scandinavia, and between Laurentia and Avalonia in middle and late Paleozoic in New England. Tectonostratigraphic relationships and geochronologic data in Scandinavia indicate that development of NW-directed subduction and related thrust imbrication gradually stepped toward and deeper into the Baltica craton, carrying previously metamorphosed rocks over cooler rocks of the craton margin. Most remarkable were rocks that were subducted as deep as 125 km at ~407Ma and then exhumed at "plate tectonic rates" to 60 km by continued thrusting over about ~6 m.y., synchronous with high-level extensional faulting. The essential shut down of magmatic activity in this orogen ~430Ma, except minor pegmatites, testifies that exhumation was not due to buoyant exhumation following delamination of a lithospheric root. A well developed stratigraphy and igneous geochronology in New England suggests there was a similar system related to SE-directed subduction of a Silurian margin of Laurentia beneath Avalon. Here, however, no exhumed products of deep-seated cool metamorphism are found, but instead voluminous magmatism progressing in a wave from coastal New England in latest Silurian ~423Ma to the Connecticut Valley region in Mid-Devonian ~380Ma, and additional heat and magmatism in Late Devonian - Early Mississippian, Late Pennsylvanian, and Permian. The location of magmatism both in the overriding and subducting plates, and occurrence of mantle- and crust-derived melts speaks for one, or several events of lithospheric delamination in this part of the continental collision zone.