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Detailed structural mapping, U-Pb chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS), and sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) analyses were carried out on the Nashoba Formation of the northwestern Nashoba terrane, eastern Massachusetts, to better constrain metamorphic conditions and structural timing in the terrane and provide an initial test for channel flow and ductile extrusion hypotheses. The Nashoba terrane is an early Paleozoic arc/back-arc complex on the trailing edge of Gondwana-derived Ganderia. High-grade metamorphism and partial melting occurred during the latest Silurian to Devonian due to accretion of the New England Avalon terrane. The partially migmatitic rocks of the NW-dipping Nashoba terrane are separated by shear/fault zones from lower-grade rocks of the Avalon terrane to the southeast and the Merrimack belt to the northwest. The Nashoba Formation is characterized by NW-dipping isoclinal folds, overprinted by NW-side-down asymmetric folds, shear zones, and faults. The isoclinal folds formed during a ca. 430–410 Ma period of initial partial melting and extensive plutonism in the Nashoba terrane. Crustal thickening occurred between ca. 419 Ma and ca. 367 Ma. High-grade metamorphic conditions in the Nashoba terrane prevailed between ca. 410 Ma and ca. 370 Ma, while plutonic rocks intruded mainly in the overlying Merrimack belt. Between ca. 370 Ma and ca. 360 Ma, NW-side-down folding and subsequent localized shearing occurred in the Nashoba Formation, while the terrane started cooling, based on new ages of fine-grained dikes and previous 40Ar/39Ar hornblende ages. These data are interpreted in a model of channel flow and ductile extrusion similar to the Himalaya–Tibetan Plateau system. 

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. 

Plate motion directions, and the orientations of rift zones and oceanic spreading ridges, and of transform faults and fracture zones that are perpendicular to these ridges, are generally controlled by tectonic forces such as slab pull, mantle convection, and mantle plumes. Here, it is hypothesized that within the confines of these general orientations, the exact orientations of these structures, and therefore plate motion directions, are partially controlled by suitably oriented sets of steep continental lithospheric discontinuities (CLDs), which work in concert with these larger tectonic forces. Previously, the observation has been made that oceanic fracture zones are contiguous with CLDs, such as suture zones and other lithospheric fault zones. Based on high-resolution bathymetry, geological and geophysical data, it is demonstrated here that continents have multiple sets of lineaments parallel to such CLDs, or contiguous with CLDs where they occur farther inland and do not reach the ocean. Published analog experiments suggest that the orientations of transform faults and fracture zones are controlled by these CLDs if the angle between the spreading direction and the CLDs is no more than ∼45°. Spreading ridge segments evolve in an orientation perpendicular to these transform faults and fracture zones, so that the spreading direction becomes parallel to the transform faults and fracture zones. The implication is that the exact plate motion directions are controlled by CLDs, if a set of CLDs is orientated at low angle with the spreading direction. When plate motion directions need to change due to tectonic forces, the new hypothesis predicts that the exact directions may be controlled by a different set of suitably orientated CLDs. During later stages of oceanic spreading, the larger tectonic forces such as slab pull, mantle convection, and mantle plumes become increasingly dominant and plate motion directions may no longer be controlled by the CLDs. While the hypothesis needs further testing, it has potentially far-reaching implications. For example, Euler pole reconstructions are commonly based on small circle patterns formed by fracture zones and transform faults in the oceanic lithosphere. Oceanic crust older than ∼200 Ma is typically destroyed by subduction, and pre-Mesozoic Euler poles can therefore not be reconstructed based on that method. If the hypothesis presented above is correct, the orientations of CLDs and associated lineament sets may be used as proxies for orientations of past transform faults and fracture zones, at least during early oceanic spreading. The locations of past Euler poles may thus be better estimated based on these CLDs and lineaments, and pre-Mesozoic plate tectonic reconstructions may be much improved in deep geologic time. 

Plate-tectonic reconstructions use rotational (Euler) poles about which plates rotate in small circle patterns, producing oceanic fracture zones. Oceanic fracture zones are contiguous with transform faults. Because oceanic lithosphere older than ~200 Ma is preferentially destroyed by subduction, pre-Mesozoic plate-tectonic reconstructions lack such constraints from oceanic fracture zones. Based on high-resolution bathymetry, geological and geophysical data, with particular emphasis on the Red Sea-Gulf of Aden system, some fracture zones are shown to be contiguous with pre-existing discontinuities in adjacent continents, while others develop parallel to those. Combined with results from existing analog and numerical models, continental rift zones and oceanic spreading ridges that are initially oblique to these discontinuities are demonstrated to evolve into orientations perpendicular to them, while fracture zones and transform faults develop parallel to them. Consequently, oceanic spreading directions, or the exact plate movement directions, are controlled by pre-existing continental lithospheric discontinuities, while other factors such as slab pull control the general direction. This hypothesis constitutes a paradigm shift, from the widespread belief that transform fault and fracture zone orientations are controlled by plate motions, to one where some are inherited from pre-existing continental discontinuities and control the exact directions of plate movements. If so, identifying such discontinuities in ancient continental lithosphere may constrain plate motions in deep geologic time. 

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. 

Abstract Seismic anisotropy beneath eastern North America, as expressed in shear wave splitting observations, has been attributed to plate motion‐parallel shear in the asthenosphere, resulting in fast axes aligned with the plate motion. However, deviations of fast axes from plate motion directions are observed near major tectonic boundaries of the Appalachians, indicating contributions from lithospheric anisotropy associated with past tectonic processes. In this study, we conduct anisotropic receiver function (RF) analysis using data from a dense seismic array traversing the New England Appalachians in Connecticut to examine anisotropic layers in the crust and upper mantle and correlate them with past tectonic processes as well as present‐day mantle flow. We use the harmonic decomposition method to separate directionally‐dependent variations of RFs and focus on features with the same harmonic signals observed across multiple stations. Within the crust, there are multiple features that may be correlated with stratification in the Hartford Basin, faults in the Taconic thrust belt, shear zones formed during Salinic/Acadian terrane accretion events, and orogen‐parallel crustal flow in the Acadian orogenic plateau. We apply a Bayesian inversion method to obtain quantitative constraints on the direction and strength of intra‐crustal anisotropy beneath the Hartford Basin. In the upper mantle, we identify a fossil shear zone possibly formed during oblique subduction of Rheic Ocean lithosphere. We also find evidence for a plate motion‐parallel flow zone in the asthenosphere that is likely disturbed by mantle upwelling near the southern margin of the Northern Appalachian Anomaly in the eastern part of the study area. 

Abstract The crust and upper mantle beneath the New England Appalachians exhibit a large offset of the Moho across the boundary between Laurentia and accreted terranes and several dipping discontinuities, which reflect Paleozoic or younger tectonic movements. We apply scattered wavefield migration to the SEISConn array deployed across northern Connecticut and obtain insights not previously available from receiver function studies. We resolve a doubled Moho at a previously imaged Moho offset, which may reflect westward thrusting of rifted Grenville crust. The migration image suggests laterally variable velocity contrasts across the Moho, perhaps reflecting mafic underplating during continental rifting. A west‐dipping feature in the lithospheric mantle is further constrained to have a slab‐like geometry, representing a relict slab subducted during an Appalachian orogenic event. Localized low seismic velocities in the upper mantle beneath the eastern portion of the array may indicate that the Northern Appalachian Anomaly extends relatively far to the south. 

The Avalon terrane of southeastern New England is a composite terrane, in which various crustal blocks may have different origins and/or tectonic histories. The northern part (west and north of Boston, Massachusetts) correlates well with Avalonian terranes in Newfoundland, Nova Scotia and New Brunswick, Canada, based on rock types and ages, U–Pb detrital zircon signatures of metasedimentary rocks, and Sm–Nd isotope geochemistry data. In the south, fewer data exist, in part because of poorer rock exposure, and the origins and histories of the rocks are less well constrained. We conducted U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis on zircon from seven metasedimentary rock samples from multiple previously interpreted subterranes, in order to constrain their origins. Two samples of Neoproterozoic Plainfield Formation quartzite from the previously interpreted Hope Valley subterrane in the southwestern part of the southeastern New England Avalon terrane and two from the Neoproterozoic Blackstone Group quartzite from the adjacent Esmond-Dedham subterrane to the east have Tonian youngest detrital zircon age populations. One sample of Cambrian North Attleboro Formation quartzite of the Esmond-Dedham subterrane yielded an Ediacaran youngest detrital zircon age population. Detrital zircon populations of all five samples include abundant Mesoproterozoic zircon and smaller Paleoproterozoic and Archean populations, and are similar to those of the northern part of the southeastern New England Avalon terrane and the Avalonian terranes in Canada. These are interpreted as having a Baltican/Amazonian affinity based primarily on published U-Pb and Lu-Hf detrital zircon data. Based on U-Pb detrital zircon data, there is no significant difference between the Hope Valley and Esmond-Dedham subterranes. Detrital zircon of two samples of the Price Neck and Newport Neck formations of the Neoproterozoic Newport Group in southern Rhode Island is characterized by large ~647–643 and ~745–733 Ma age populations and minor zircon up to ~3.1 Ga. This signature is most consistent with a northwest African affinity. The Newport Group may thus represent a subterrane, terrane or other crustal block with a different origin and history than the southeastern New England Avalon terrane to the northwest. The boundary of this Newport Block may be restricted to the boundaries of the Newport Group, or it may extend as far north as Weymouth, MA, as far northwest as (but not including) the North Attleboro Formation quartzite and associated rocks in North Attleboro, MA, and as far west as Warwick, RI, where eastern exposures of the Blackstone Group quartzite exist. The Newport Block may have amalgamated with the Amazonian/Baltican part of the Avalon terrane prior to mid-Paleozoic amalgamation with Laurentia, or have arrived as a separate terrane after accretion of the Avalon terrane. Alternatively, it may have arrived during the formation of Pangea and been stranded after the breakup of Pangea, as has been proposed previously for rocks of the Georges Bank in offshore Massachusetts. If the latter is correct, then the boundary between the Newport Block and the southeastern New England Avalon terrane is the Pangean suture zone.