CUMULATIVE DEFORMATION AND OROGEN-PARALLEL ELONGATION IN A CONTRACTIONAL OROGEN: AN EXAMPLE FROM THE VIRGINIA PIEDMONT, SOUTHERN APPALACHIANS

Solid-state deformation fabrics in the Ordovician Columbia pluton from the Virginia Piedmont developed during late Paleozoic collisional tectonics. Granitic to granodioritic rocks of the Columbia pluton were deformed under middle greenschist to amphibolite facies conditions. In spite of experiencing multiple fold episodes these rocks preserve one penetrative foliation and lineation that records the cumulative deformation. Foliation is folded into northwest-verging asymmetric structures that plunge gently northeast. Elongation and mineral lineations consistently plunge sub parallel to fold axes. Quartz grain shape fabrics and biotite crystallographic fabrics indicate significant changes in both deformation intensity and three-dimensional fabrics across the pluton. Fabric intensity is weakest (D-values <0.4) in the northwest and generally increases towards the southeast (D-values >0.8). LS-tectonites occur throughout the pluton, however strong prolate fabrics (n <-0.5) are localized in the hinge zone of a refolded synform and strong oblate fabrics (n >0.5) occur in homoclinal zones along fold limbs. Regardless of the three-dimensional fabric geometry, the long axes of fabric ellipsoids consistently plunge gently to the northeast and record 10 to 70% strike-parallel elongation. The kinematic significance of the cumulative deformation fabric is: 1) localized zones of strongly non-plane strain, 2) modest orogen-normal shortening, and 3) significant orogen-parallel material elongation.