GRAIN-SCALE STRAIN ANALYSIS OF THE WHALEBACK ANTICLINE, BEAR VALLEY STRIP MINE SHAMOKIN, PENNSYLVANIA: PRELIMINARY RESULTS

Three-dimensional strain analyses were conducted to investigate grain-scale strain patterns on an iconic fold, providing insights into folding mechanisms. The Whaleback anticline is a tight, north verging, third order fold within the central Appalachian Valley and Ridge province. Superb exposures on the Whaleback anticline allowed for sampling within a single sandstone layer in the Pennsylvanian Llewellyn Formation. As part of a preliminary study of grain-scale strain on the whaleback anticline, thirty-nine mutually perpendicular thin sections were prepared from 13 oriented hand samples along three axis-normal transects. Bulk rock finite strain was examined using the normalized Fry method and the finite grain strain was calculated from sand grains using the shape matrix eigenvector method. From the results of each of these methods, 3D finite strain ellipsoids were calculated for each sample site.

Both methods yield finite strain ellipses in the plane of bedding that are generally elongate sub-parallel to the fold axis. In the bedding plane, bulk rock and grain strain ratios range from 1.05 ± 0.02 to 1.24 ± 0.03 and 1.05 ± 0.03 to 1.38 ± 0.06, respectively. Normal to the fold axis, bulk rock and grain strain ratios range from 1.04 ± 0.02 to 1.21 ± 0.03 and 1.08 ± 0.03 to 1.47 ± 0.06, respectively. At the Whaleback anticline, the shape matrix eigenvector method produced more systematic strain patterns than the normalized Fry method. This could be partly due to differences in deformation mechanisms in the sand grains and matrix, matrix heterogeneity, and differences in the precision of sand grain shape approximation. In fold-axis normal profiles, finite grain strain ellipses appear to be sheared towards the hinge of the fold. Our preliminary study of strain patterns on the Whaleback anticline suggests that flexural flow occurred during folding.