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Across- and along-strike structural and geochronological variations of the Nashoba-Putnam and Avalon terranes, eastern Massachusetts, Connecticut, and Rhode Island, southeastern New England Appalachians
Severson, Allison R.
Severson, Allison R.
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2020
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Abstract
A comprehensive structural and geochronologic investigation was combined with existing structural, geochronologic and geophysical data to characterize and interpret across- and along-strike variations associated with the last two major orogenic events that formed the southeastern New England Appalachians of eastern North America. These orogenies are the mid-Paleozoic Acadian orogeny and the late Paleozoic Alleghanian orogeny. The northeast-trending Nashoba-Putnam terrane, located in eastern Massachusetts and Connecticut is part of the Gondwanan-derived microcontinent Ganderia. It is in fault contact with greenschist facies rocks of the Merrimack belt to the west, and with generally low grade rocks of the Avalon terrane to the east. Structural data in the Nashoba-Putnam terrane show five generations of deformation. Structural and metamorphic data are generally consistent with a channel flow model, where a hot, viscous mid- to lower crustal layer flows between relatively cool and more rigid layers, to remove material laterally in an overthickened orogenic belt. The occurrence of contemporaneous localized sinistral movement in schist and distributed dip-slip movement in migmatitic gneiss may represent strain partitioning of lateral movement and ductile extrusion of the channel in a sinistral transpressional regime. Based on new and existing U-Pb geochronology data, ductile deformation and possibly channel flow occurred during the latest Silurian to earliest Carboniferous Acadian orogeny, which resulted from the accretion of the Avalon terrane. Foliation orientations in the northwest-dipping Nashoba-Putnam terrane show a decrease in dip angle from subvertical and moderate dips in the north to moderate and shallow dips in the south, changing in particular at the latitude of Boston, Massachusetts. Furthermore, localized zones of west-trending foliation and west-trending terrane boundary between the Nashoba-Putnam and Avalon terranes occur along the generally northeast-trending foliations and boundary. One of these zones occurs at the latitude on Boston, at the same latitude as the transition from steeper to shallower dipping foliations. The Avalon terrane in southeastern New England is a composite terrane that rifted from Gondwana in the Ordovician and accreted to Laurentia during the Acadian orogeny. U-Pb and Lu-Hf isotopic analyses of zircon in three sedimentary rocks in the western-most Avalon terrane in southeastern New England was carried out to compare the detrital zircon signature with those of other metasedimentary rocks of the southeastern New England Avalon terrane and with Avalonia in eastern Canada. Analyzed samples are isotopically similar to others in the New England Avalon terrane, and to Avalonia in Canada, especially the Gamble Brook Formation in the northern part of Nova Scotia. Finally, detrital zircon of the western Avalon terrane has a mixed source between Baltica and Amazonia. U-Pb analyses of zircon from migmatitic rocks in the southeastern Nashoba terrane, combined with existing 40Ar/39Ar and U-Pb analyses of high temperature minerals suggest a younging of leucosome and high-grade metamorphism from north to south in the Nashoba-Putnam and Avalon terranes. The major transition in ages of leucosome occurs at the approximate latitude of Boston, Massachusetts. Furthermore, west-trending lineaments and a gravity anomaly are visible in aeromagnetic and Bouguer gravity datasets, respectively. The most notable of these west-trending lineaments is located along the latitude of Boston, Massachusetts. This lineament coincides with a change in foliation orientations as outlined above. Furthermore, U-Pb monazite ages along this lineament are late Paleozoic in the Nashoba terrane. In the Avalon terrane, a transition occurs from predominantly middle Paleozoic 40Ar/39Ar hornblende cooling ages relating to the Acadian orogeny to the north and mostly late Paleozoic ages relating to the Alleghanian orogeny to the south. This lineament is therefore interpreted as the ‘Alleghanian Front’, where rocks to the south were affected by the Alleghanian orogeny, while rocks to the north show only minor effects.
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