AN UPDATED MODEL FOR THE TECTONIC EVOLUTION OF THE SOUTHEASTERN NEW ENGLAND APPALACHIANS ALONG THE LATITUDE OF MASSACHUSETTS

The Appalachians record the history of a series of terranes that were rifted from Gondwana and accreted to Laurentia, and the subsequent formation and eventual breakup of Pangea. Along-strike variations are present in terms of differing terranes, terrane geometries, and degrees and ages of metamorphism and deformation. We compiled new and previous structural, geochronology and metamorphic petrology data from the Nashoba-Putnam terrane (NPT) and adjacent terranes and present them in the context of geophysical data from New England.

Cambrian-Ordovician sedimentary and volcanic rocks in the NPT of eastern MA and eastern CT formed in an arc/back-arc setting. These rocks were metamorphosed to upper amphibolite facies and locally partially melted. Ages of metamorphism young progressively from N to S in the Nashoba terrane (NT) between ~420 and ~340 Ma, based on SHRIMP U-Pb zircon ages. Partial melt and/or metamorphism in the Putnam terrane (PT) occurred between ~420 and ~355 Ma based on SHRIMP U-Pb zircon ages of one sample. Of the metamorphic zircon analyzed, no grains record late Paleozoic high-grade metamorphism within the NPT, suggesting that the NPT was relatively unaffected by the Alleghanian orogeny. Terranes adjacent to the NPT preserve late Paleozoic Ar/Ar cooling ages, which also young to the S. The NPT rocks were intruded by Silurian to earliest Carboniferous plutons. The NPT shows multiple generations of folds and a generally steeply NW-dipping foliation in the north that progressively shallows to the S.

These geological trends correlate with geophysical anomalies in New England. Bouguer gravity maps indicate a gravity high over the NT, and an anomalous gravity low over the southern-most NT and PT. Both a magnetic and a depth-to-Moho survey record anomalies in this same region. The magnetic survey depicts the Avalon terrane pinching out, possibly over top of the NPT. The depth-to-Moho survey records a greater depth to Moho in the southern-most part of the NT and for the entirety of the PT. These geophysical observations may at least in part be linked with the Paleozoic geological data, but may have been modified more recently.