Distribution of pre-folding linear indicators of movement direction around the Spring Hill Synform, Vermont: significance for mechanism of folding in this portion of the Appalachians

Abstract

Three distinctly oriented sets of pre-folding, and one set of syn-folding, axes of curved inclusion trails are preserved in garnet porphyroblasts in 50 samples around the doubly plunging Spring Hill Synform in southeast Vermont. Over one third of the samples contain consistent changes in the trend of these axes from the core to rim. Since the core grew before the rim this enabled the relative timing of each set of axes to be determined, from the oldest to the youngest, as NE-SW, E-W, NNW-SSE and NNE-SSW. The youngest trend is parallel to the axial plane of the regional folds. Only those samples with the latter trend have their inclusion trails connected continuously to the matrix foliation. The three pre-folding sets of axes have the same orientation on both limbs. This consistency in orientation has significant implications for the processes operating during folding, and three mechanisms are presented that could potentially explain it. These involve the classic card deck model of shear folding, De Sitter's model of clay bricks shortening as they shear past one another, and the progressive bulk inhomogeneous shortening model. The relative merits of each of these models are discussed.