Deformation and metamorphism on the east side of the Green Mountain massif in southern Vermont

An early deformation in the Jamaica, Vermont, area on the east side of the Green Mountain massif produced kilometre-scale, north-plunging F₁ folds overturned to the west with an axial-planar foliation, S₁. Motion on two major thrust faults juxtaposed three similar but distinctive cover-rock sequences after or late in F₁ folding. Smaller-scale F₂ folds deformed F₁ fold limbs and the thrust faults and have an axial-planar crenulation cleavage, S₂. In the structurally higher central and eastern cover-rock sequences, an early stage of garnet growth began late in S₁ development or after it. A retrogression partially resorbed first-stage garnet and was followed by a second prograde stage of garnet growth late in (or after) S₂ development. This metamorphic history resulted in distinctive textural unconformities and zoning anomalies in garnet. The western cover-rock sequence lacks evidence for a major retrogression. F₁ folding and F₂ folding approximately bracket both the motion on the thrust faults and the retrogression. The second prograde metamorphism and the younger deformation are Acadian. Tentative correlation of the first garnet growth stage with Ordovician metamorphism recognized in western New England and of motion on the thrust faults with Taconian synmetamorphic thrusts in the region suggests that these early thermal and tectonic events occurred in the Ordovician. The retrogression recorded in the central and eastern cover sequences furthermore may reflect thrusting of these rocks to a structurally higher, colder environment, followed by reburial and later prograde metamorphism.