MIGRATION OF DEFORMATION FRONTS DURING PROGRESSIVE DEFORMATION - EVIDENCE FROM DETAILED STRUCTURAL STUDIES IN THE PENNSYLVANIA ANTHRACITE REGION, USA

Detailed study of selected outcrops in the Pennsylvania Anthracite region has resulted in identification of five, at least partly continuous, stages of Alleghanian progressive deformation. These structural stages are, in sequence: A-layer-parallel shortening and top-to-the-foreland, layer-parallel shear; B-a second episode of layer-parallel shortening; C-flexural-slip and flexural-flow folding; D-fold modification and late, thrust faulting; and E-late veining and fracturing. At each outcrop, an average shortening direction was determined for each observed structural stage. The mean regional shortening direction for each structural stage was calculated and maps showing the spatial distribution of the shortening directions for each stage were constructed. The mean regional shortening directions for the five structural stages successively rotated 25-30-degrees clockwise during the evolution of the fold and thrust belt. Shortening directions for structural stages B and C over the region show a systematic clockwise change in orientation from hinterland to foreland, indicating that these structural stages of progressive deformation were time-transgressive on a regional scale.