12-5 The nature and age of deformation along the Nashoba-Avalon terrane boundary in Eastern Massachusetts

Session: New advances in geological and geophysical research on the Appalachian orogen. (II)

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Abstract:

The New England Avalon terrane (AT) accreted to the southeastern margin of the Nashoba terrane (NT), the trailing edge of Ganderia, at the onset of the Acadian orogeny (~421–400 Ma). The presence of high-grade rocks of the northwest-dipping NT between lower-grade rocks of adjacent domains, a progression from northwest-side-down folds at high structural levels towards symmetric folds at lower levels in the NT, and widespread evidence of partial melting suggest that NT rocks may have undergone channel flow and ductile extrusion over the AT to the southeast. However, northwest-side up folds, expected towards the bottom of the extrusion zone, have not been found in the NT. To test whether the bottom of the zone may instead lie in the adjacent AT, structural mapping was undertaken in a ~26 by ~11 km area. Based on structural styles, the area was divided into northeastern and southwestern domains (NED and SWD, respectively). In the NED, foliations exhibit steep-moderate northwestward dips, and lineations plunge northeast and southwest. Shear senses along northwest-dipping mylonite zones vary. In the SWD, foliations and lineations are more randomly orientated, and shear senses along mylonite zones are not consistent. Rocks in the SWD are faulted to a greater degree than those in the NED. In the NED, most fault planes and slickenlines are oriented parallel to foliations and lineations. In the SWD, fault orientations are random. In both domains, faults are predominantly normal. U-Pb LA-ICP-MS zircon ages of three migmatite samples are ~590 Ma, ~593 Ma, and ~602 Ma, indicating that high-grade metamorphism and partial melting occurred in the Ediacaran, not during the Acadian orogeny. Zircons from a granite crosscutting a mylonite yielded a ~356 Ma crystallization age, suggesting that mylonitization may have occurred as late as during the Acadian orogeny. Receiver function analysis of data from the GENESIS array of broadband seismometers revealed a northwest-dipping transition in crustal structure across the NT-AT boundary. This may either represent the bottom of the mid-crustal flow zone or a fault that removed the base of the flow zone in the NT. The lack of evidence for crustal flow during the Acadian orogeny in the AT adjacent to the NT, and the strong evidence of faulting, may suggest the latter.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2, 2026

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