EVIDENCE FOR EXTENSION AT THE WILLIMANTIC DOME, CONNECTICUT - IMPLICATIONS FOR THE LATE PALEOZOIC TECTONIC EVOLUTION OF THE NEW-ENGLAND APPALACHIANS

Structural studies and deformation timing relations at the Willimantic dome, eastern Connecticut, indicate that the dominant structures exposed along the basement-cover boundary and the dome itself formed during Permian northwest-southeast crustal extension. A sharp boundary between Late Proterozoic crystalline rocks and overlying metasedimentary rocks coincides with a transition between discordant structural styles. Strongly foliated gneisses in the core of the dome experienced penetrative, homogeneous deformation at mid- to lower-crustal levels in the Pennsylvanian (approximately 300 Ma). A disrupted zone of heterogeneous deformation extends upward from the basement-cover contact into the overlying metasedimentary rocks and consists of stretched lozenges of high-grade pelitic gneiss metamorphosed in the Devonian (Acadian, approximately 400 Ma) bound by a network of anastomosing normal-sense shear zones that formed in the Permian (approximately 275 Ma). On the west flank of the dome, these structures are overprinted by a 10 to 15 m thick, shallow northwest-dipping, normal-slip shear zone located approximately 15 m above the basement-cover contact and a set of steeply northwest-dipping, planar normal shears. Permian mica cooling ages and a ductile-to-brittle progressive deformation along some high-angle normal shears further link uplift, doming, and cooling with an extensional tectonic regime. These structural and geochronologic observations provide a framework for a new working model for the late Paleozoic tectonic evolution of the central and southern New England Appalachians. A model is proposed where tectonic convergence in the Pennsylvanian (approximately 290-305 Ma) culminated with the deep burial of Avalonian crust beneath a previously metamorphosed Acadian metamorphic belt. The deeply buried Avalonian crust was deformed, metamorphosed, and locally partially melted by this process. The thickened, composite structural section underwent extensional collapse shortly afterward at about 265 to 280 Ma. At the Willimantic dome, extensional thinning caused the extreme attenuation and/or removal of structural section between Avalonian basement and Acadian cover. This brought metasediments from higher structural levels into direct contact with previously deep-seated Avalonian basement gneisses, leading to the different and apparently unrelated metamorphic histories and structural styles characterizing rocks now juxtaposed across the basement-cover contact. The progressive exhumation of these basement rocks led to the development of the regionally warped and domed basement-cover contact.