TRANSPRESSIONAL HIGH STRAIN ALONG THE TACONIC-ACADIAN INTERFACE IN CENTRAL VERMONT

A high strain zone sub-parallel to the Taconic-Acadian interface in central Vermont, known as the Richardson Memorial Contact (RMC), preserves kinematic indicators for determining late-stage deformation. Outcrop-scale folds and microstructural studies suggest subtle rheological changes localize shear on the mm- to cm-scale at varying orientations that manifest unique differential stresses. Quartz ribbons, mica fish, and fractured feldspar indicate primarily middle-to-upper greenschist-grade conditions during deformation. Tight to isoclinal, W-verging western fold limbs are oriented N-S, dip steeply west, and thin extensively. Axial planes strike NNW-SSE and dip steeply west. E-limbs vary in orientation, but strike N-S and dip either ~20°W or ~70°W or are oriented E-W and dip from ~50° to ~90°. Boudinaged pegmatites are concordant with or crosscut all fabrics and were used to estimate minimum sectional strains of 6-18. E-limbs are often dismembered, and separated from hinges with dextral offset. Late crenulation cleavages host garnet porphyroclasts and associated down-dip mineral elongation lineations, which indicate a reverse sense-of-shear. Fold-intersection analyses show two populations of axial plane orientations between ~170° and ~070°. Limbs are aligned 150°-180°. Limb lengths and apertures were used to quantify fold asymmetry. W/E limb length ratios vs. corresponding fold aperture on N-faces show limb ratios of ~5:1. On E-faces most W/E limb ratios are ~2:1 and apertures vary from ~10° to ~110°. Non-cylindrical folds reflect evolving stress regime orientations, normal to earlier phases of regional Acadian deformation. W-limb lengthening and E-limb rotation, orthogonal to W-limbs, coupled with strain and vorticity analysis, suggests general shear transpression with triclinic symmetry. Strain ratios approaching 5 were measured from elongate quartz grains. Kinematic vorticity ranged from 0.4 to 0.9 and was used to estimate an average shear strain of 2. Integrating shear strain across individual strands of this distributed high-strain zone suggests minimum displacement of 25.5 +/- 0.5 km. A broad high-strain zone suggests significant shortening normal to the RMC, as well as extensive material elongation parallel with this interface, occurred relatively late in Acadian orogenisis.