FOLD-RELATED FAULTING AT THE BEAR VALLEY STRIP MINE, PA

Strip mining at Bear Valley coal mine in the central PA Appalachian fold-thrust belt exposed in 3-dimensions, a series of intensely faulted folds in a stratigraphic interval that includes a 30-cm-thick carbonaceous silty shale and an underlying ~4-m-thick (“Whaleback”) sandstone. We examined ~900 faults with mm- to m-scale displacements in an effort to determine the relative timing of fault formation w.r.t. fold development, and to gain insight into folding mechanisms. For each fault, we measured the fault plane orientation, slip lineations, shear sense, net slip, and bedding orientation adjacent to the fault. Scanline surveys and pavement maps were prepared to delineate temporal and spatial relationships between faults, and to calculate linear strain due to faulting. Faults were grouped according to their relative timing, and kinematic analyses were done to determine the maximum principal shortening and extension directions for each set. Nickelsen’s 1979 study of progressive deformation at this locality established that some faults formed prior to folding to accommodate layer-parallel shortening (LPS). Sets that pre-date large-scale folding include low-angle thrust faults, strike-slip faults, and thrust faults along rheological boundaries between iron concretions and surrounding sandstone. LPS faults resulted from contraction sub-parallel to the bedding plane and are overprinted by younger fault sets that caused layer-parallel extension (LPE) regardless of position on the fold. The presence of progressively overprinted or curved slickenfibers on some LPE faults on the limbs of the folds, and a consistent relationship w.r.t. the folds and bedding, indicate that LPE faults formed during folding. LPE faults include a conjugate set that extends bedding sub-parallel to fold axes and a conjugate set that extends bedding sub-perpendicular to the fold axes. Linear strain measurements from scanline surveys indicate the LPE faults produce 2-4% LPE parallel and normal to the fold axes. LPE faults in the carbonaceous silty shale have higher density, smaller displacements, and larger dihedral angles than those in the Whaleback sandstone. Existing fold models do not readily explain the syn-folding flattening observed perpendicular to the plane of bedding on the limbs and hinge zone of Bear Valley folds.