INFLUENCE OF LOCALIZED HIGH-STRAIN AND REGIONAL PLUTONISM ON THE TACONIC-ACADIAN INTERFACE IN CENTRAL VERMONT

The interface between rocks of Taconic- and Acadian-affinity in the central Vermont Appalachians is debated in terms of timing and style of deformation and the potential effect of plutonism. Silurian and Devonian metasedimen­tary rocks of the Connecticut Valley-Gaspe synclinorium (CVGS), east, and Cambro-Ordovician metasedimentary and metavolcanic rocks, west, are separated by a feature interpreted as either the Dog River Fault Zone (DRFZ) or as an unconformable surface, the Richardson Memorial Contact (RMC). Devonian granites are much more abundant in the CVGS. Zones of high strain sub-parallel to the DRFZ/RMC preserve upper-greenschist to lower-amphibolite facies that exceed the regional isograde. Kinematic vorticities in shear zones range from 0.4 to 0.9 with the average shear strain estimated at ~2. Integrating shear strain across strands of the high-strain zone suggests minimum displacement of 25.5 +/- 0.5 km dominated by general shear transpression with triclinic symmetry. Crenulation cleavages define the regional latest Acadian fabric and are sequentially overprinted by biotite-rich shear bands and sigma-type spessartine – grossular porphyroblasts. Fold populations have axial plane orientations of ENE or SSE. W/E limb length ratios vs. fold apertures on N-faces have ratios of ~5:1. On E-faces W/E limb ratios are ~2:1. Non-cylindrical folds reflect stress regime evolution independent of the mapped temporal interface. Undeformed granitic dikes intrude parallel to the main foliation (often bedding-parallel) in the CVGS. However, mingling between larger granite bodies and country rock is more common east of the DRFZ/RMC as well as where granites are exposed immediately to the west, suggesting influence of a thermal welt. The youngest regional structures are sub-vertical joints, a NE-striking set cut by a NW-striking set, and sheeting joints associated with Pleistocene-unloading. The presence of granitic plutons modified sub-vertical joints closer to parallelism with shear-zone geometries. This requires 20-30 degrees of principal stress field rotation. The thermal influence of plutonism and localized deformation along late shear-zones modified the interface between rocks of Taconic- and Acadian-affinity and likely lead to the juxtaposition of different structural levels across strike.