An official website of the United States government
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.
more »
« less
This content will become publicly available on December 31, 2026
A New Window into Lithospheric Deformation from High-Resolution Receiver Function Imaging and Probabilistic Anisotropic Inversion: Revealing Terrane Accretion Processes in Eastern Massachusetts
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.
more »
« less
- Award ID(s):
- 2220233
- PAR ID:
- 10689017
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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.more » « less
-
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.more » « less
-
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 a new method designed to analyze the crustal anisotropic structure beneath the Nashoba terrane and provide insight into its capabilities in a first application to permanent stations in the area and currently collected data. 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. Here we use a Bayesian framework to invert for the anisotropic model that best fits the observed constant and harmonic terms. Applying a Bayesian inversion to the harmonically decomposed RFs instead of full RF waveforms has the potential to infer the anisotropic seismic model more faithfully, without attempting to fit unrelated signals and artifacts.more » « less
-
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.more » « less
