Strain factorization and partitioning in the North Mountain thrust sheet, central Appalachians, U.S.A.

By examining finite strain distribution and partitioning strain among deformation mechanisms, a model of thrust sheet deformation is established. The model takes into account kinematic history and environmental conditions. In the North Mountain thrust sheet of northern Virginia, factorizing finite strain into pure and simple shear components shows that the thrust sheet experienced 6-13% pure shear shortening parallel to the transport direction. Thrust-parallel simple shear strains increase slightly from the top of the thrust sheet toward the base. Within 500 m of the floor thrust, however, simple shear values increase markedly toward the basal thrust. This strain pattern occurs throughout the thrust sheet, overprinting earlier imbricate structures. Therefore, the finite strain in the thrust sheet may be modelled as a transport-parallel pure and simple shear applied during the major transport episode, and after thrust sheet imbrication. This transport-related strain overprints a weak layer-parallel shortening that is probably related to earlier passage of the thrust tip. Rocks in the thrust sheet with X/ Z strain ratios greater than 1.35 have a slight crystallographic preferred orientation and are therefore tectonites in a strict sense. The distribution of tectonites within the thrust sheet defines a 'tectonite front' that is inclined toward the foreland. In the North Mountain thrust sheet the 'tectonite front' also generally coincides with the transition to high-temperature deformation mechanisms, and ultimately may parallel a paleo-isotherm within the thrust wedge. Strain partitioning indicates that approximately 70% of the finite strain results from intragranular mechanisms (i.e. dislocation glide, dislocation creep and diffusion mechanisms); 25% results from calcite twinning; and less than 5% resulted from transgranular mechanisms such as pressure solution. In non-tectonites, approximately 50% of the finite strain results from pressure solution and intragranular mechanisms, with the remaining strain due to twinning in calcite.