Large volume loss during cleavage formation, Hamburg sequence, Pennsylvania

Green reduction spots in red slate of the Hamburg sequence exposed near Shartlesville, Pennsylvania, have axial ratios of 1.42:1.0:0.28 on the limbs of near-isoclinal folds and 1.0:0.79:0.41 in fold hinge zones. Conodont cusps and denticles within the reduction spots have been brittlely pulled apart and give independent measures of extension in various directions. Comparison of conodont extensions with reduction spot shapes on limbs and hinges indicates that sedimentary compaction of 44% preceded the tectonic strain associated with cleavage formation. This strain, having identical maximum extensions but greater shortening in fold hinges as compared to limbs, was characterized by 41% extension in X, no change in Y, 50% to 59% shortening in Z, and 29% to 42% tectonic volume loss. The general lack of directed overgrowths on grains reflects the large volume loss and contrasts with other slates, where deformation was an almost constant volume process and extension in X compensated for shortening in Z.