STRAIN WITHIN FOLDED LAYERS AT THE WHALEBACK ANTICLINE, SHAMOKIN, PA, USA

The abandoned Bear Valley coal mine presents an outstanding opportunity to examine buckle fold growth in three dimensions. The Alleghanian Whaleback anticline and adjacent folds were uncovered during mining of the Pennsylvanian Mammoth coal seam, exposing expansive underlying bedding planes containing distinguishable pre- and syn-folding microstructures and mesoscale faults with mm- to m-scale slip. Resolution of the spatial and temporal patterns of these structures permits a detailed examination of the mechanisms and partitioning of deformation during fold initiation and growth. Kinematic analyses of ~ 900 faults on these folds reveal the transition from early layer-parallel shortening (LPS) before/during fold initiation into layer-parallel extension (LPE) during fold amplification and tightening. During LPS, extension occurred both normal to bedding (layer thickening by thrust faulting) and within the bedding plane normal to the direction of LPS by strike-slip faulting. During fold amplification and tightening, some faults that originated as LPS strike-slip faults underwent continued slip, transitioning to LPE faults. This transition is evidenced by curved slip lineation patterns preserved on minor fault planes. Most LPE faults, however, initiated during fold growth. LPE occurred both along and across the fold axes during fold growth and was coincident with bed-normal shortening. The Whaleback anticline exposes a 4-m-thick sandstone layer over much of its surface, whereas the neighboring north anticline is capped by a 30-cm-thick shale that directly overlies the Whaleback sandstone. LPE faults have smaller dihedral angles, higher displacements and wider spacing in the Whaleback sandstone compared to the overlying shale on the north anticline, suggesting a rheologic control on brittle deformation accommodation; however, both lithologies appear to accommodate similar patterns and magnitudes of LPE during fold growth.