| Abstract: | Total strain patterns estimated across the Pulaski thrust sheet of the southwest Virginia Appalachians show an approximately homogeneous, plane strain deformation associated with folding and distortion above a subsurface décollement. Estimated strains are low (1.2 <
< 2.0) with a subvertical extension. Chlorite fibers in pressure fringes on framboidal pyrite indicate that non-rotational deformation produced weak cleavage and pencil structure in mudrock. Variations in shape of pencils and fiber lengths in pressure fringes define highest strains in fold hinges and adjacent to contraction faults. Fabric transitions, delineated by distribution and intensity of cleavage, pencil structure and bedding fissility across the thrust sheet are strain dependent. Balanced cross-sections suggest 35% horizontal shortening due to regional folding and faulting within the Pulaski sheet. Strain integration techniques give 17–35% horizontal shortening associated with cleavage formation. Removal of this strain indicates that cleavage was superposed on open to tight, class-3 folds. Pre-existing thickness variations and anomalous low strains in tight folds require early folding accomodated by intergranular deformation (perhaps controlled grainboundary sliding). Suppression of cleavage formation and penetrative strain was possibly due to higher pore fluid pressure in the early stages of thrust sheet deformation. Observed variations in bedding-cleavage angle and low cleavage fans are compatible with this deformation sequence. |
