LATE PALEOZOIC EXTRUSION AND ELONGATION AS IMPORTANT MECHANISMS IN THE TECTONIC EVOLUTION OF THE SOUTHERN BRONSON HILL TERRANE

Deformation and high-grade metamorphism in the Bronson Hill terrane (BHT) of CT and MA have long been attributed to Acadian (~420-360 Ma) orogenesis; nappe development, backfolding, and doming have dominated most tectonic models. However, increasingly numerous reports of St to Ky grade, late Paleozoic metamorphism and deformation, and terrane modification as far west as the BHT, makes this insufficiently understood phase of Appalachian orogenesis difficult to ignore in tectonic reconstructions. Reconnaissance fieldwork, and published geochronologic, structural, and petrologic studies in south-central New England indicate northward, orogen-parallel extrusion and elongation in the BHT of CT and MA is a tectonically viable mechanism to explain some structural features. Southward, orogen-perpendicular narrowing and northward widening of the exposed Glastonbury and Monson orthogneisses suggest mid- to deep crustal ductile flow to the north. The Bonemill Brook-Conant Brook shear zone bounds the eastern contact of the Monson gneiss and suggests progressive movement from west-side-up to dextral. S-C fabrics within the Partridge Fm. between the northwestern contact of the Belchertown complex and the western contact of the Monson indicate west-side-down movement; near Bolton, CT, sinistral shearing has been reported. High strain fabrics were not recognized in surrounding orthogneisses implying strain was accommodated in cover units. Bounding high strain zones for the Glastonbury have not yet been identified, but structural relationships at the Belchertown complex-Glastonbury contact are consistent with northward elongation of the Glastonbury gneiss. This is consistent with a change in deformation style of the Monson from orogen-parallel, northward extrusion in CT and southern MA to vertical extrusion in central/northern MA. Although metamorphic grade is reported to decrease in grade to the north from Rhode Island and southern CT where high-grade, late Paleozoic mineral assemblages are prevalent, the presence of late St- to Ky-grade fabrics are recognized throughout the BHT (e.g., Bolton Notch, CT) and in and around the Pelham window, indicating that rocks were at sufficient temperatures for ductile flow; geochronology constrains deformation to the late Paleozoic.