TECTONICS OF THE CHOPAWAMSIC TERRANE AND THE ARVONIA SUCCESSOR BASINS IN VIRGINIA

The Chopawamsic Terrane is a complex and composite arc terrane in the central and northern Virginia Piedmont. The suprastructure of the Chopawamsic Terrane consists of mid-Ordovician arc-related metavolcanics/metasedimentary rocks intruded by late Ordovician to early Silurian granodioritic to dioritic plutons. The Chopawamsic volcanic arc is built upon a foundation of older (Cambrian to Ediacaran?) arc complexes with a peri-Gondwanan provenance. The Chopawamsic Terrane is unconformably overlain by a suite of predominately meta-sedimentary rocks (Arvonia, Quantico, and Buffards formations), long assumed to be of latest Ordovician to earliest Silurian age (455 to 440 Ma) based on paleontological evidence. However, over the past 15 years, multiple detrital zircon studies demonstrate that the maximum depositional age for the post-Chopawamsic meta-sedimentary sequence is latest Silurian to Devonian (400 to 380 Ma). As such, the post-Chopawamsic sequence forms a set of successor basins, herein referred to the Arvonia successor basins, that are 40 to 75 million years younger than previously understood.

During the Taconian orogeny (455 to 440 Ma) the Chopawamsic Terrane formed the overriding plate and was ultimately accreted to the Laurentian margin. Partial subduction of buoyant Laurentian continental crust (Grenvillian felsic rocks) effectively ended accretion. Foundering of the accreted Chopawamsic Terrane occurred by ~400 Ma, this was driven by the accretion and loading of other peri-Gondwanan terranes to the east (outboard from Laurentia) during the Acadian orogeny. Arvonia successor basins record vertical tectonism that produced a significant marine transgression across the beveled remnants of the Chopawamsic Terrane. The significance of Devonian magmatism and tectonism in the eastern Piedmont has only recently been recognized. The Chopawamsic terrane and its overlying successor basins experienced regional metamorphism and ductile deformation, under greenschist to amphibolite facies conditions, during the Alleghanian orogeny (310 to 280 Ma). Alleghanian deformation was transpressional characterized by NW-SE directed shortening, orogen-parallel elongation, and dextral shearing.