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 succession of overprinting near-orthogonal deformations. The Spring Hill synform in southeast Vermont is an example of such a fold. The origin and timing of this structure has been the subject of several previous studies; understanding its development is crucial to unraveling orogenesis in the Vermont Appalachians. The fold formed during a deformation path that involved a succession of overprinting near-orthogonal deformations that produced matrix fabrics S (sub 3) , S (sub 4) , and S (sub 5) . These foliations developed with subvertical, subhorizontal, and subvertical orientations, respectively, before being rotated by the effects of younger deformations. The Spring Hill synform is generally thought to have formed as a recumbent structure during regional nappe development, S (sub 4) developing as an axial planar foliation. However, we demonstrate that the Spring Hill synform developed as a fold with a steeply dipping axial plane that was overprinted by S (sub 4) and S (sub 5) . Although this geometry and overprinting history are consistent with a D (sub 3) time of formation, we can find no change in the asymmetry of pre-S (sub 3) foliations across the fold. We suggest 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 stages of Acadian orogenesis in southeast Vermont.