Structural analysis of the Silurian-Devonian rocks of the Royalton area, Vermont

摘要

Structural analysis of Silurian-Devonian schists of the Waits River and Gile Mountain Formations in the Royalton area, Vermont, provides clear evidence of three Acadian deformational phases and polymetamorphism. Structurally, the Royalton area lies within the Connecticut Valley–Gaspé synclinorium. The Gile Mountain Rocks are part of the western band of the formation, which has its southern termination in the study area and extends northward to join the eastern band around the northern end of the calcareous Waits River Formation of the Strafford-Willoughby arch. The earliest deformation (D₁), is recognized by a pervasive schistosity (S₁) generally parallel to bedding (S₀) and by rarely identified folds (F₁). The second deformation (D₂) has left the most widespread imprint, with an S₂ cleavage or schistosity and open to moderately flattened folds (F₂) in schistosity, parallel to bedding (S₀/S₁). Least easily recognized is the third deformation (D₃), with sparse open folds in S₂. The western band of the Gile Mountain Formation is interpreted as the westward- and downward-facing nose — the Royalton synform — of a large D₁ recumbent fold — the eastern Vermont nappe. D₂ structures have generally been regarded as synchronous with and genetically related to formation of domes and the Strafford-Willoughby arch. However, the D₂ structures are interpreted here as pre-doming, with D₃ structures related to the doming deforming them. Compressive buckling and variable flattening imposed during D₂ upon a variably oriented S₀/S₁ may have been the result of emplacement of higher nappes (now removed by erosion), which increased the depth of burial promoting prograde metamorphism.