STRUCTURAL GEOLOGY OF SUCCESSOR BASINS IN THE CHOPAWAMSIC TERRANE, CENTRAL VIRGINIA PIEDMONT: A CASE FOR TRANSPRESSION AND OROGEN-PARALLEL ELONGATION

Three narrow northeast/southwest trending belts of predominantly metasedimentary rocks overlie Ordovician rocks of the Chopawamsic terrane in the central Virginia Piedmont. The Columbia-Quantico, Arvonia, and Long Island belts have long been interpreted as synclinal infolds of younger successor basin rocks that unconformably overlie Taconic metavolcanic and plutonic rocks. Regional metamorphism and ductile deformation in the Chopawamsic terrane occurred under greenschist to amphibolite facies conditions during the Alleghanian orogeny.

The easternmost Columbia/Quantico belt is a complex structure characterized by multiple gently plunging map-scale anticlines and synclines. The belt of Quantico schist and quartzite forms the core of an early (F₁) fold. Later open to tight, asymmetric, northeast plunging antiformal and synformal F₂ folds overprint the F₁ fold producing a Type 3 interference pattern. At its northeastern termination, the Arvonia belt forms a simple, gently southwest plunging syncline, but along strike to the southwest the geometry is more complex. The westernmost Long Island belt is characterized by moderately to steeply plunging reclined folds consistent with dextral transpression along the Brookneal/Shores high-strain zone to the west. The eastern contact of the Long Island belt is cut by a steeply dipping brittle reverse fault. Strain and vorticity analyses of metaconglomerates and quartzites in the Arvonia, Buffards, and Quantico formations indicate XZ ratios of 2:1 to >20:1, zones of both strongly prolate and oblate strain, and general shear. Regardless of the three-dimensional strain geometry, the cumulative deformation records significant (20 to >100%) orogen-parallel material elongation.