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During the early Paleozoic the terranes of Ganderia and Avalonia both rifted from Gondwana. They accreted to North America in the middle Paleozoic. The late Silurian-Devonian Acadian orogeny, as a result of accretion of Avalonia, originated folding, high-grade metamorphism and northwest-dipping shear zones within the Nashoba-Putnam terrane, the trailing edge of Ganderia. In addition, partial melting produced plutonic rocks in and to the northwest of the Nashoba terrane. These characteristics have previously been interpreted as a result of channel flow and ductile extrusion towards the southeast. In this study, we apply geologically informed seismic imaging to test the hypothesis of the potential occurrence of crustal flow in the tectonic history of the Appalachian orogeny. Such crustal flow is suggested to produce significant seismic anisotropy due to the alignment of minerals within the weakened crust of the flow zone. This anisotropy would result in a characteristic set of effects to the seismic wavefield, such as the splitting of shear-waves, directionally dependent travel-times of seismic phases and directionally varying conversions at boundaries of anisotropic domains. Such effects yield a harmonic pattern that can be best observed in receiver function imaging. We systematically analyze the coherent harmonic patterns in receiver functions along a new dense (~5 km spacing) seismic profile, known as the GENESIS array, that complements existing stations across the Nashoba terrane in Eastern Massachusetts. We identify harmonic signals in the upper and mid-crust and within the lithospheric mantle, suggesting differing mid-crustal anisotropy between two lateral blocks, which correlate well with Avalonia and Ganderia. While we don’t directly identify the contact zone of the two terranes in our imaging, the changes of structural and anisotropic patterns may be consistent with a northwest-dipping suture zone, which is based on geologic observations. 

The Acadian orogeny resulted from the accretion of the southeastern New England Avalon Terrane (AT) to the Nashoba Terrane (NT) - the trailing edge of Ganderia - to its northwest, in eastern Massachusetts. Ganderia and the AT are mostly Gondwana-derived. Previously, rocks of the NT were interpreted to have been extruded to the southeast over the AT as part of a channel flow zone. Only the top and center of this zone are exposed in the NT. Bedrock and structural mapping were carried out in the AT adjacent to the NT to locate the bottom of the channel flow zone. The main rock types are migmatitic biotite gneiss and mafic rock, quartzite, and igneous rocks, exposed in 10s of m to km scale blocks and lenses. Some of these rocks have been sheared and show evidence of mylonitization. Furthermore, they occur near, and in two areas are crosscut by, igneous plutons of unknown age. The foliations of migmatitic rocks, quartzites, and mylonites predominately dip NW, but the orientations of the mylonites vary, especially away from the terrane boundary. Lineations plunge NE and SW in migmatites, NE in quartzites, and NW in mylonites. Migmatitic rocks show abundant isoclinal folds. Predominantly NW to SW dipping normal faults with various slickenline orientations were observed in all rock types. The migmatitic biotite gneiss and its structures resemble those of the NT. However, U-Pb zircon data yielded a detrital zircon signature typical for Avalonia, with predominantly Mesoproterozoic and minor Paleoproterozoic and Tonian populations. Furthermore, zircon overgrowths are ~585 Ma, which suggests that the high-grade metamorphism and partial melting were Ediacaran and did not result from the Acadian orogeny and channel flow at that time. Based on the (1) blocky/lensoid outcrop pattern of rock types, (2) varied orientations of structures, and (3) abundance of faults, the area may represent a brittle fault zone that cut off the interpreted channel flow zone of the Nashoba terrane. Our structural analysis is complemented by and provides context for high-resolution seismic imaging of the crust enabled by the ongoing GENESIS deployment of broadband seismometers across the NT. Preliminary results from GENESIS suggest a transition in crustal structure across the boundary between the NT and AT, consistent with geological observations.