Resolving complexities associated with the timing of macroscopic folds in multiply deformed terrains: The Spring Hill synform, Vermont

Determining the timing of macroscopic folds of bedding in multiply deformed terrains is difficult, especially for rocks that have undergone a successi on of overprinting near-orthogonal deformations. The Spring Hill synform in southeast Vermont is an example of such a fold. The origin and timing of t his structure has been the subject of several previous studies; understandi ng its development is crucial to unraveling orogenesis in the Vermont Appal achians. The fold formed during a deformation path that involved a successi on of overprinting near-orthogonal deformations that produced matrix fabric s S-3, S-4, and S-5. These foliations developed with subvertical, subhorizo ntal, and subvertical orientations, respectively, before being rotated by t he effects of younger deformations. The Spring Hill synform is generally th ought to have formed as a recumbent structure during regional nappe develop ment, S-4 developing as an axial planar foliation. However, we demonstrate that the Spring Hill synform developed as a fold with a steeply dipping axi al plane that was overprinted by S-4 and S-5. Although this geometry and ov erprinting history are consistent with a D-3 time of formation, we can find no change in the asymmetry of pre-S-3 foliations across the fold. We sugge st that the synform may have formed at a much earlier stage in the orogen's history and was subsequently modified and rotated to its present geometry by the long history of west-to-east shortening that dominated the later sta ges of Acadian orogenesis in southeast Vermont.