A STRUCTURAL, GEOCHRONOLOGICAL AND GEOPHYSICAL BOUNDARY IN THE SOUTHEASTERN NEW ENGLAND APPALACHIANS: AN ALLEGHANIAN FRONT?

The southeastern New England Appalachians preserve a transition in structural, geochronological, and geophysical data that may explain some of the differences in metamorphic and deformation histories between the northern and southern Appalachians at large. The youngest events in the southeastern New England Appalachians resulted from the Acadian and Alleghanian orogenies. The middle Paleozoic Acadian orogeny formed as a result of collision between North America and Avalonia, a microcontinent that rifted from supercontinent Gondwana. The late Paleozoic Alleghanian orogeny occurred as a result of collision between North America and West Africa during formation of supercontinent Pangea. Metamorphism and deformation associated with the Acadian orogeny are pervasive in the northern Appalachians throughout eastern Canada and south into northeastern MA. Conversely, widespread high-grade metamorphism and deformation related to the Alleghanian orogeny are minor north of the MA-CT border, but prominent in and south of CT and RI.

An east-trending lineament runs approximately 50 km west from Boston to Westborough, MA and represents a localized zone of shallowly to moderately north- and south-dipping foliations within a domain of generally moderately to steeply northwest-dipping foliations that makes up eastern MA, eastern CT and RI. North of this lineament, U-Pb zircon, monazite, and titanite ages and Ar-Ar hornblende cooling ages are predominantly middle Paleozoic (Acadian) in eastern MA. South of it, Ar-Ar hornblende cooling ages are late Paleozoic (Alleghanian) in the Avalon terrane (part of the Avalonian microcontinent) in eastern-most CT and RI. The east-trending lineament is also preserved as an aeromagnetic anomaly, and gravity anomalies are high to the north and low to the south. We interpret this lineament as a previously unrecognized east-trending tectonic boundary that may represent an Alleghanian front. This boundary separates a zone in eastern MA, eastern CT, and RI dominated by middle Paleozoic (Acadian) tectonism to the north from a zone dominated by late Paleozoic (Alleghanian) tectonism to the south. Other minor east-trending lineaments are visible in the datasets in the southeastern New England Appalachians and may be subsidiary structures related to the proximal Alleghanian Front.