Deformation in the hinge region of a chevron fold,Valley and Ridge Province,central Pennsylvania

The hinge region of an asymmetrical chevron fold in sandstone, taken from the Tuscarora Formation of central Pennsylvania, U.S.A., was studied in detail in an attempt to account for the strain that produced the fold shape. The fold hinge consists of a medium-grained quartz arenite and was deformed predominantly by brittle fracturing and minor amounts of pressure solution and intracrystalline strain. These fractures include: (1) faults, either minor offsets or major limb thrusts, (2) solitary well-healed quartz veins and (3) fibrous quartz veins which are the result of repeated fracturing and healing of grains. The fractures formed during folding as they are observed to cross-cut the authigenic cement. Deformation lamellae and in a few cases, pressure solution, occurred contemporaneously with folding. The fibrous veins appear to have formed as a result of stretching of one limb: they cross-cut all other structures. Based upon the spatial relationships between the deformation features, we believe that a neutral surface was present during folding, separating zones of compression and extension along the inner and outer arcs, respectively. Using the strain data from the major faults, the fold can be restored back to an interlimb angle of 157°; however, the extension required for such an angle along the outer arc is much more than was actually measured. This disparity between observed and required deformation suggests that the rest of the folding strain may be attributed to minor faulting, isolated severe pressure solution and to slight grain movements; we were not able to recognize the latter. We propose that a single episode of deformation produced the chevron fold causing the brittle deformation after the sandstone had been lithified. This brittle deformation was accomplished by faulting together with the translation of individual sandstone blocks which do not contain significant internal deformation.