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

Abstract

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 S3, S4, and S5. 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, S4 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 S4 and S5. Although this geometry and overprinting history are consistent with a D3 time of formation, we can find no change in the asymmetry of pre-S3 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.