Vitrinite in coal is optically anisotropic in reflected light, owing to a preferred orientation of its component molecular structure. Such reflectance fabrics can, under some circumstances, provide a record of stress and strain events occurring during coal metamorphism. Based upon observed differences in the orientation and shape of vitrinite reflectance indicatrices (VRIs), differing tectonic histories are suggested for three regions within the bituminous and anthracite coal fields of the northern Appalachian (coal) basin, Pennsylvania. VRIs of coals from the North-Central fields on the Allegheny Plateau have oblate geometries similar to those of most coals worldwide, indicative of only mild tectonic deformation. Maximum reflectance orientations for North-Central field coals are polymodal and aligned with polymodal Alleghanian fold axes. Low-volatile bituminous coals from the Broad Top field, south-central Pennsylvania, exhibit much stronger tectonic reflectance fabrics and bear evidence that low-volatile bituminous rank was attained only after folding was completed in this part of the fold-and-thrust belt. VRIs of coals from the Western Middle anthracite field reflect a more complex tectonic history and indicate that most coalification here preceded the final phase of Alleghanian folding.
VRls of coals from an underground mine in the Western Middle anthracite field provide additional details on strain directions and timing of coalification. Coal rank increases very slightly down section, but Rmin decreases down section, opposite to the expected trend. These coals probably attained anthracite rank prior to the principal phase of Alleghanian folding, but at least some coalification continued after folding, as suggested by reconstructed coalification pathways on the axial ratio diagram. During this final coalification phase, the stratigraphically upper beds "caught up" in rank with the stratigraphically lower beds through elongation of Rmax parallel to the fold axes and elongation of Rmin in a subvertical direction. VRIs of the stratigraphically upper beds were transformed to a prolate geometry, whereas those of the lower beds retained their original oblate form.