THE PERI-LAURENTIAN MARGIN OF SOUTHERN NEW ENGLAND: A TRANSPRESSIVE BOUNDARY FOR 200 MILLION YEARS

New mapping and U/Pb and ⁴⁰Ar/³⁹Ar geochronology show that the mid-Paleozoic margin of Peri-Laurentia between Bridgeport and Seymour, CT, was actively deforming for more than 200 million years. The history of deformation at this margin begins with the ~450 Ma intrusion of the Beardsley granodiorite (Ohb) into the Collinsville Formation, followed immediately by the intrusion of the Pumpkin Ground granite (Ohp) into the Ohb (both newly dated). Syntectonic fabrics restricted to the Ohb demonstrate that this was already an actively deforming transpressive margin before the intrusion of the Ohp. Subsequent structures and fabrics crosscutting both bodies show ductile transpression persisted into the Early Devonian and stretched these units into ~NNE striking lenses > 30 km long.

By the Middle Devonian these feldspathic rocks had cooled to <600°C and became rheologically strong units. The ductility contrast between these units and weaker schists to the east led to strain localization at their interface and to the formation of the NNE trending East Derby Shear Zone. Continued and protracted regional transpression from the Neo-Acadian through the Alleghanian deformed the Wepawaug and Maltby Lakes schists to the east, and also localized strain in the East Derby Shear Zone (EDSZ) without significantly affecting rocks in the adjacent Ohp and Ohb. New and existing ⁴⁰Ar/³⁹Ar growth ages from muscovite in the rocks of the EDSZ record the times of recrystallization in these mylonites (~360, ~305, and ~270 Ma) and demonstrate that deformation persisted throughout the Carboniferous and into the Permian. Thus, these orthogneisses intruded into New England near the end of Ordovician but became a buttress for deformation for much of the later Paleozoic.