The Northwestern Margin of the New England Avalon Terrane, Eastern Massachusetts: Nature and Ages of Deformation

Nikhil Arolkar, Department of Geology and Geological Engineering, Colorado School of Mines
Yvette Kuiper, Department of Geology and Geological Engineering, Colorado School of Mines
Frederik Link, Institute of Geosciences, Johannes Gutenberg University Mainz
Maureen Long, The Department of Earth and Planetary Sciences, Yale University
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Abstract

In New England, the onset of the Acadian orogeny (~420 Ma) was marked by the accretion of the New England Avalon Terrane (AT) to the southeastern margin of the Nashoba Terrane (NT), the trailing edge of Ganderia. High-grade, partially melted rocks of the NT are juxtaposed against lower-grade rocks of the Merrimack Belt to the northwest and the AT to the southeast. Northwest-side-down folds are present at high structural levels in the NT's northwestern part, and symmetric folds have been documented at low structural levels in the NT's southeastern part. Previously, these characteristics have been interpreted to represent channel flow followed by ductile extrusion of the NT to the southeast over the AT. However, northwest-side-up folds, expected at the bottom of a channel flow zone, are absent in the NT. The northwestern part of the AT was structurally mapped to test whether the base of the channel flow zone was located there. The region covered ~26 km by ~11 km. Based on structural styles and orientations, it was divided into ductile-dominated and brittle-dominated zones (DDZ and BDZ, respectively). In the DDZ, foliations exhibit steep-moderate northwestward dips, and lineations plunge northeast and southwest. Fault planes and slickenlines are oriented parallel to foliations and lineations, respectively. Mylonite zones dip northwest and show various shear directions and senses. The domain exhibits a northwest-dipping fold pattern. Rocks in the BDZ are faulted to a greater degree than those in the DDZ. Furthermore, the orientations of foliations, lineations, mylonite zones, faults, and slickenlines vary, and folds have not been observed. In both zones, faults are predominantly normal. U-Pb LA-ICP-MS zircon ages from three migmatite samples in the DDZ are ~590 Ma, ~593 Ma, and ~602 Ma, suggesting that high-grade metamorphism and partial melting occurred in the Ediacaran, and not during the Acadian orogeny. In the BDZ, zircons from a granite crosscutting a mylonite yielded a ~356 Ma crystallization age, indicating that mylonitization occurred before the earliest Carboniferous. Cross sections produced using receiver function data from the GENESIS array of broadband seismometers show a northwest-dipping transition in crustal structure across the NT-AT boundary. This may either represent the base of the channel flow zone or a fault that removed it. The absence of evidence for channel flow during the Acadian orogeny in the northwestern AT, coupled with evidence of widespread faulting, suggests that a fault cut off the bottom of the channel-flow zone.

Session

Large-scale tectonics