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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. 

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. 

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. 

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. 

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. 

Plate-tectonic reconstructions use Euler poles about which plates rotate in small circle patterns. These small circle patterns are outlined by oceanic transform faults and contiguous fracture zones. 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, some fracture zones are shown to be contiguous with pre-existing discontinuities in adjacent continents. Combined with results from published 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 plate movement directions, are controlled by pre-existing continental lithospheric discontinuities. 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 they are inherited from pre-existing continental discontinuities, and control plate movement directions. If so, identifying such discontinuities in ancient continental lithosphere may constrain plate motions in deep geologic time. 

The New England Appalachians provide a fascinating window into a host of fundamental geological problems. These include the modification of crustal and mantle lithospheric structure via orogenesis, terrane accretion, and continental rifting, the evolution of individual terranes through processes such as channel flow and ductile extrusion, and the causes and consequences of the Northern Appalachian Anomaly (NAA), a prominent geophysical anomaly in the upper mantle. Recent and ongoing deployments of dense seismic arrays in New England are providing images of the crust and upper mantle in unprecedented detail, allowing us to address both new and longstanding science questions. These deployments include the Seismic Experiment for Imaging Structure beneath Connecticut (SEISConn, 2015-2019), the New England Seismic Transects (NEST, 2018-present), and the GEology of New England via Seismic Imaging Studies (GENESIS, 2022-present) arrays. Here we present results from these experiments that are shedding new light on the tectonic evolution of New England and the ways in which structures and processes in the upper mantle can affect the structure of the overlying lithosphere. These include detailed new images of crustal architecture beneath central and southern New England, including a sharp transition from thick (~48 km) crust Laurentia terranes to thin (~32 km) crust beneath Appalachian terranes. The character of this offset beneath the SEISConn and NEST arrays suggests an overlap of two Moho boundaries, forming an overthrust-type structure that may have resulted from reactivation of faults during the compression and shortening associated with the formation of the hypothesized Acadian Altiplano. Beneath SEISConn, there is evidence for multiple relict structures preserved in the lithosphere from past episodes of terrane accretion and suturing, as well as anisotropic layering that constrains the kinematics of past lithospheric deformation events. Beneath the NEST line in central New England, we infer a relatively shallow (~80 km) lithosphere-asthenosphere boundary above the NAA upper mantle geophysical anomaly, providing evidence for lithospheric thinning above a presumed asthenospheric upwelling. Finally, preliminary results suggest layered crustal anisotropy beneath the GENESIS array, perhaps corresponding to a past episode of channel flow in the mid-crust. 

Ganderia and the Southeastern New England Avalon terrane are both terranes that rifted from Gondwana and accreted to North America in the early to mid-Paleozoic. Accretion of the Avalon terrane was accompanied by plutonism, deformation, and metamorphism including partial melting within the Nashoba terrane, the trailing edge of Ganderia, and may be interpreted as indicators for mid- to lower-crustal channel flow. Channel flow describes the flow of weak, partially molten material between more competent crust as a result of pressure gradients in the mid- to lower crustal levels. Such flow should typically result in seismic anisotropy due to the crystallographic preferred orientations of minerals and shape preferred orientations at various scales. Here, we present first results for the crustal anisotropic structure beneath the Nashoba terrane that were produced with a newly developed approach from currently collected data in the region. To investigate the hypothesis of crustal flow during the orogenic history of Southeastern New England, we deployed a dense profile of 6 broadband seismic stations crossing the Nashoba terrane. We analyze the harmonic variation of amplitudes in teleseismic P-Receiver Functions (RFs) to identify interfaces of isotropic and anisotropic contrasts within the crust. In the case of particularly prominent anisotropic features that have significantly larger amplitudes than other signals, it is feasible to derive quantitative constraints on the strength and orientation of the anisotropy. However, with growing complexity, a classical forward modelling or grid search approach becomes unfeasible. These difficulties can be mitigated by applying Bayesian inversion, which infers values of model parameters from a probabilistic perspective. Applying a Bayesian inversion to the harmonically decomposed RFs has the potential to infer complex anisotropic seismic structures. We find evidence for two crustal anisotropic layers with confined properties to the geologic units of the Ganderia, Nashoba and Avalon terranes that might be related to episodes of lower crustal flow. In addition, we identify anisotropy at shallow mantle levels beneath the Nashoba and Avalon Terranes possibly indicating the upper interface of a shallow asthenosphere in the region. 

Seismic waves with different propagation and oscillation directions can exhibit different velocities when going through a medium with some directional properties; this phenomenon is called seismic anisotropy. Seismic anisotropy observed beneath eastern North America is often attributed to present-day flow in the upper mantle. The mantle flow causes shear waves oscillating in the direction of flow (e.g., in the direction of North America plate motion) to travel faster than those that travel in other directions. However, this pattern does not hold true for some regions along the Appalachian orogen, suggesting that past tectonic events can result in long-lived, ‘frozen-in’ anisotropy in the lithosphere, which modifies the predicted anisotropic behavior beneath these regions. In this study, we investigate sources of seismic anisotropy beneath southern New England using a method based on directionally dependent variations of P-wave to S-wave conversions at interfaces with contrasts in anisotropy. This method can separate signals caused by different anisotropic features and constrain the depth distribution of anisotropy. 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. 

We provide an overview of aeromagnetic characteristics of the Appalachians. Whether terrane or domain boundaries are clearly defined by aeromagnetic lineaments depends on differences in rock types, and on whether the boundary is intruded by later plutons or overlain by sedimentary rocks. Sedimentary rocks and plutonic rocks of the Meguma terrane in Nova Scotia form magnetic lows, resulting in clearly defined boundaries with higher anomalies in adjacent terranes. The Caledonia terrane, an Avalonian terrane in New Brunswick, is defined by a strong high. The boundary between peri-Laurentian arcs and Carolinia in the southeastern U.S. is well defined, where the arcs form a magnetic high and parts of Carolinia form a low. Other terrane and domain boundaries are less well defined by aeromagnetic imagery. The Goochland terrane of Virginia shows a sharp boundary with the magnetic highs of the peri-Laurentian arcs to the northwest, but a diffuse boundary with the slightly lower magnetic rocks of Carolinia to the southeast. Peri-Laurentian arcs of the northern Appalachians form a low, while in the southern Appalachians they have mixed signatures. The boundary between Avalonia and Ganderia in Newfoundland is cut by plutonic rocks, and difficult to trace based on aeromagnetic images. Late Devonian to Permian basins, including the Maritimes basin of southeastern Canada and Narragansett basin of southeastern New England, and Mesozoic basins, including the Hartford basin of southwestern New England, form magnetic lows within and across terranes. They obliterate underlying rocks and structures. Strong magnetic contrasts also occur within terranes or domains. An example is the magnetite-bearing part of the Nashoba Formation in the Nashoba terrane of eastern Massachusetts, which is part of the trailing edge of Ganderia. Plutonic rocks may form magnetic highs, such as the German Bank Pluton southwest of Nova Scotia, or lows, such as the South Mountain Batholith of Nova Scotia. The New York – Alabama lineament separates a magnetic high to the northwest from a low to the southeast. NE-trending faults, including the Norumbega Fault of eastern New England, form strong lineaments. Two sets of east- and ESE-trending lineaments throughout the Appalachians represent late structures.