NEW OBSERVATIONS ON THE CHOPAWAMSIC FAULT, AN EARLY PALEOZOIC TERRANE BOUNDARY IN THE WESTERN PIEDMONT OF VIRGINIA

The c. 200 km long Chopawamsic fault is the most significant structure within the western Piedmont of northern Virginia, separating the pre-Early Ordovician Potomac terrane accretionary complex from the Middle-Late Ordovician Chopawamsic terrane volcanic arc. It has been hypothesized that the fault could represent the main Iapetan suture in the southern Appalachians because the Potomac terrane appears to be affiliated with Laurentia, whereas the affinity of the Chopawamsic terrane is unknown, but possibly Gondwanan. Previous field, geophysical, and regional interpretations suggest that the Chopawamsic fault is an east-dipping, west-vergent thrust that is overlain by two small successor basins to the west of Fredericksburg, VA. In order to assess the significance, kinematics and timing of this prominent structure, we have initiated a multidisciplinary investigation of the fault and surrounding rocks.

In the area south of Lake Anna, limited observations of winged sigma feldspar grains indicate a sinistral component of shear along the Chopawamsic fault. Nearby, the Ellisville granodiorite intrudes across the fault, providing a timing constraint on fault motion. New TIMS U-Pb zircon ages indicate that the latest movement on the fault was pre- c. 437 Ma.

Field studies to date confirm the existence of a successor basin atop the fault in the area of Storck, VA; we intend to use detrital zircon analysis of metasediments in the basin in order to further constrain the timing of fault motion. In the Wilderness, VA area, our mapping indicates that the other successor basin previously mapped to overlie the fault, does not exist and that rocks previously mapped as the basin are most likely metasedimentary rocks within the Chopawamsic terrane.

Our ‘in progress’ investigation will involve additional field and laboratory research along the Chopawamsic fault. We plan to assess the cratonic source of the Chopawamsic terrane with the use of Nd and Pb isotopic analyses as well as detrital zircon populations from each side of the Chopawamsic fault. The detrital zircon study should also provide supplementary constraints on the timing and nature of fault motion. High precision U-Pb TIMS data from previously undated intrusive bodies will also augment our understanding of the early Paleozoic evolution of the western Piedmont.