UNRAVELLING THE STRUCTURAL EVOLUTION AND REGIONAL TECTONIC IMPORTANCE OF THE HONEY HILL FAULT ZONE, SE, CONNECTICUT – A KEY TO UNDERSTANDING THE OROGENIC COLLAPSE OF THE SOUTHERN NEW ENGLAND APPALACHIANS

The Honey Hill Fault system (HHF) is one of the most significant fault systems in southern New England because it forms a boundary between two major crustal domains (`Avalonia' and `Ganderia'). It also links with the Lake Char Fault further north which has experienced repeated low-level historical seismicity, including a notable earthquake swarm in Plainfield in 2015. The fault system is poorly understood in terms of its motion history and timing. However, understanding the structural evolution of the HHF is essential for determining: 1) how the crust of southern New England was assembled, 2) how the Appalachian Mountains were created, and 3) how the mountain belt was later disassembled by orogenic collapse. All models of the tectonic evolution of southern New England have remained incomplete until now, because of uncertainty regarding the HHF system's history. In this study, detailed field investigations of the HHF between Bozrah and Norwich, CT were carried out during summer-fall, 2019 to document the rock types that the fault passes through and separates, structural evidence for brittle and ductile deformation, and kinematic indicators. In addition, samples were collected for thin-section analysis of micro-structures that reveal mineral-scale deformation mechanisms and shear sense. Our results indicate that the fault zone cuts through rocks that experienced two phases of top-to-the-S directed contractional deformation (D1/D2). Overprinting the older contractional structures are spectacular mylonites and ultramylonites that reveal high-temperature ductile shearing indicative of top-to-the-NW sinistral transtensional displacement (D3). Ductile D3 transtension was followed progressively by brittle top-to-the-NW transtensional and E-W sinistral faulting that cuts across earlier structures (D4). The full range of ductile-brittle kinematic indicators are preserved in the mylonitic gneisses. Late-stage retrogressed pseudotachylite is also present. Our results provide further evidence that the HHF was a regionally significant tectonic boundary that accommodated deep-seated orogenic collapse in the southeastern New England Appalachians following Pennsylvanian-Permian Pangea assembly.