STRAIN DISTRIBUTION DURING TRANSPRESSIONAL SHEARING ACROSS THE TACONIC-ACADIAN INTERFACE IN CENTRAL VERMONT

The interface between units of pre-Silurian (Taconic) and Siluro-Devonian (Acadian) affinity in central Vermont has been interpreted by previous workers as: 1) a fault, 2) an unconformity or 3) a faulted unconformable surface. Recent mapping across the northeast-southwest trending Taconic-Acadian boundary shows discrete shear zones, at middle-to-upper greenschist facies, that are not restricted to the mapped chronologic interface. Instead, strain partitioning generated distinct domains dominated by either strike-slip or dip-slip components. Steep, west-dipping planar fabrics and, where present, relatively steep stretching lineations preserve sense-of-shear indicators suggestive of dominantly reverse-sense sinistral motion. Evidence of transpressional stresses, with west-side-up sense-of-motion, match observations of previous workers along strike to the south. Narrow zones of rheologically strong, quartz-rich domains preserve strong S-C mylonitic fabrics. These zones include evidence of intense grain-refining, potentially associated with fluid-enhanced grain boundary migration recrystallization. Mylonitized quartz veins preserve a sinistral sense in phyllosilicate-rich, weaker, locally garnetiferous schists. Crenulated garnet schists are characterized by strong shape-preferred orientations of phyllosilicates in warped S-domains and crystallographic-preferred orientations of quartz in C-domains. Quartz-dominated inclusion trails as well as pressure shadows associated with σ-type garnet porphyroblasts, parallel to shallowly plunging stretching lineations, preserve a sinistral shear-sense. Rootless assymetric multi-scale folds in mylonitic quartz-rich domains show sinistral offset and suggest the potential for non-coaxial deformation. Localized shear bands in the C’ orientation suggest the possibility of east-west transitioning of highest strain zones. What are interpreted to be a set of discrete zones of high-strain suggest a relatively late stage of transpressional motion is distributed across a wider zone than that previously mapped as the boundary between units of either Taconic or Acadian affinity. Reworking or reactivation of this structural interface may have implications for the waning stages of Acadian orogenesis in central Vermont.