195-7 Deformation along the Nashoba-Avalon terrane boundary in Eastern Massachusetts

Session: Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism (Posters)

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Abstract:

At the onset of the Acadian orogeny (~421–400 Ma), the New England Avalon terrane (AT) accreted to the southeastern margin of the Nashoba terrane (NT), the trailing edge of Ganderia. In the northwest-dipping NT, widespread evidence for partial melting, the presence of high-grade rocks between lower-grade rocks of adjacent domains, and a progression from northwest-side-down folds at high structural levels towards symmetric folds at lower levels, suggests that NT terrane rocks may have undergone channel flow and ductile extrusion over the AT to the southeast. northwest-side up folds, expected towards the bottom of the extrusion zone, have not been found in the NT. Structural mapping was undertaken in a ~26 by ~11 km area in the AT adjacent to the NT to test whether the bottom of the zone may instead lie in the AT. This involved the documentation and analysis of structures in separate northeast and southwest structural domains. In the northeast domain, foliations dip northwest, while lineations plunge northwest and southeast. However, consistent northwest-side-up folds or shear zones were not recorded. Furthermore, new U-Pb zircon ages of a migmatitic paragneiss and a migmatitic mafic rock are ~585 Ma and ~591 Ma, respectively, indicating that high-grade metamorphism and partial melting occurred in the Ediacaran before the Acadian orogeny. In the southwest domain, foliated host rocks with various orientations are highly faulted and intruded by undeformed plutonic rocks. Zircons from a granitic sample that crosscuts mylonitic fabric yielded a ~355 Ma age of crystallization, indicating that mylonitization may either be related to the Acadian orogeny or occurred during the Ediacaran. Thus, evidence indicative of ductile extrusion during the Acadian orogeny was not found in the two structural domains. However, (1) the juxtaposition of NT rocks deformed by the Acadian orogeny alongside AT rocks unaffected by the event, and (2) the presence of fault rocks along the terrane boundary and within the southwest structural domain of the AT, suggest that the bottom of the ductile extrusion zone was cut off by a fault. Seismic data obtained through the GENESIS array of broadband seismometers (~5 km spacing) across the NT show a west-dipping transition in crustal structure across the NT-AT boundary. This may represent the base of the channel flow zone and/or a fault zone.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6302

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