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The accretion of Avalonia to eastern North America during the Paleozoic left a complex imprint of deformation in the crust and mantle lithosphere, including northwest-dipping shear zones, metamorphic gradients, and partial melting within the Nashoba-Putnam terrane, which represents the trailing edge of Ganderia. These features have been previously interpreted as evidence of channel flow and ductile extrusion, processes that likely generate significant seismic anisotropy within the crust. In this study, we test this hypothesis using a novel seismic imaging approach that enables high-resolution recovery of anisotropic structure from Ps receiver functions, which provides detailed insights into crustal deformation during the Avalonian accretion. Our method uses the decomposition of the azimuthally varying content in receiver functions into five components using harmonic regression, allowing us to isolate the directional signatures of anisotropy and dipping structure. By incorporating inter-station coherency weighting and residual-based noise suppression into a probabilistic inversion framework, we robustly resolve complex anisotropic layering and quantify uncertainties using a Bayesian strategy with Markov chain Monte Carlo (McMC) sampling. We apply this approach to data from the recently deployed GENESIS seismic profile across the Nashoba terrane in Eastern Massachusetts, which features dense station spacing (~5 km). The resulting images reveal distinct anisotropic domains in the upper and mid-crust and within the lithospheric mantle. Lateral variations in mid-crustal features align with geological boundaries between Avalonia and Ganderia. The anisotropic structure can be related to two past deformation episodes. A west-dipping structure, consistent with the channel flow hypothesis, is apparently overprinted by a younger east-dipping structure within the Avalonian crust. Our observations demonstrate the power of high-resolution, probabilistic receiver function inversion to extract deformation signatures that were previously inaccessible, offering new insights into the deep structure of Appalachian terrane accretion. 

The accretion of Avalonia to eastern North America during the Paleozoic left a complex imprint of deformation in the crust and mantle lithosphere, including northwest-dipping shear zones, metamorphic gradients, and partial melting within the Nashoba-Putnam terrane, which represents the trailing edge of Ganderia. These features have been previously interpreted as evidence of channel flow and ductile extrusion, processes that likely generate significant seismic anisotropy within the crust. In this study, we test this hypothesis using a novel seismic imaging approach that enables high-resolution recovery of anisotropic structure from Ps receiver functions, which provides detailed insights into crustal deformation during the Avalonian accretion. Our method investigates the directionally dependent content in P-to-S receiver function data, allowing us to isolate the directional signatures of anisotropy and dipping structure at depth. With dense seismic data, we can incorporate inter-station coherency weighting and residual-based noise suppression, allowing us to robustly resolve complex anisotropic layering. We apply this approach to data from the recently deployed GENESIS broadband seismic profile across the Nashoba terrane in Eastern Massachusetts, which features dense station spacing (~5 km). The resulting images reveal distinct anisotropic domains in the upper and mid-crust and within the lithospheric mantle. Lateral variations in mid-crustal features align with geological boundaries between Avalonia and Ganderia. The anisotropic structure can be related to two past deformation episodes. A west-dipping structure, consistent with the channel flow hypothesis, is apparently overprinted by a younger east-dipping structure within the Avalonian crust whose origin is enigmatic. Our observations demonstrate the power of high-resolution receiver function imaging to extract crustal deformation signatures that were previously inaccessible, offering new insights into the deep structure of Appalachian terrane accretion. 

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. 

The southeastern New England Avalon Terrane (AT) accreted to the southeastern margin of the Nashoba Terrane (NT) at the onset of the Acadian orogeny (latest Silurian to Devonian). The NT represents the trailing edge of Ganderia. Rocks of the NT have previously been interpreted as having been extruded to the southeast over the AT as part of a channel flow zone (CFZ). Based on fold symmetries, it was inferred that only the top and center of this zone are located in the NT. Bedrock and structural mapping were carried out in the AT adjacent to the NT to test whether the bottom of the CFZ may be located in the AT. Data were collected from migmatitic biotite gneiss, mylonite, foliated quartzite, and gneiss. Structural data were divided into NE and SW domains. In the NE domain, foliations dip predominantly NW, and lineations plunge NE and SW. Migmatitic and gneissic rocks are absent in the SW domain, and orientations of mylonite zones and foliations in quartzite vary. Compared to the NE domain, rocks in the SW domain are strongly faulted and intruded by Ediacaran and late Silurian/Devonian granitic and gabbroic plutons. The presence of migmatite and consistency in structural orientations in the NE domain, and the general resemblance of structures to those in the NT make the NE domain a likely candidate to represent the bottom of the CFZ. U-Pb zircon data of the migmatitic biotite gneiss 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 high-grade metamorphism and partial melting occurred in the Ediacaran, i.e., not during the Acadian orogeny. Hence, the migmatitic biotite gneiss in the AT terrane does not represent the bottom of the CFZ. We believe that the Bloody Bluff Fault along the Nashoba-Avalon terrane boundary may have cut off the bottom of the CFZ. Our 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 NT and AT, consistent with geological observations.