A billion years of tectonic history makes southeastern Canada and the northeastern United States an exciting area to investigate the evolution of continental lithosphere. Our study area comprises terranes with either Laurentian or Gondwanan provenance that accreted to eastern North America at different times. With the aim of resolving the isotropic velocity variations across the northern Phanerozoic Appalachians and the southeastern Proterozoic Grenville Province, we adopted a Rayleigh wave tomography technique that takes multipathing, scattering, and finite frequency effects into account. Our data sets include records of teleseismic earthquakes recorded by 71 broadband seismic stations over a 2‐year period. Our high‐resolution tomography models indicate significant (±3.5%) variations in shear wave velocity across different lithospheric domains, enabling us to discuss tectonic implications. In contrast to the Peri‐Laurentian zones, seismic signatures in the Peri‐Gondwanan domains are more complex and variable. Although systematic variations of seismic velocities across different tectonic zones are observed, we find no simple relation between the lithospheric thicknesses of different tectonic zones and their age. We interpret the lithosphere‐asthenosphere boundary in our study area, located at 70–120 km depth, with the thickest and fastest lithosphere beneath New Brunswick. We suggest that this relatively thicker lithosphere is due to a slab stacking process that occurred after flat subduction of a younger domain in the Late Silurian. Occurrence of flat subduction in the Late Silurian in the northern Appalachians is also supported by geochemical and paleomagnetic evidence.

Southeastern Canada and the northeastern USA include terranes that were tectonized since the Archean, making this region an excellent place to investigate the evolution of continental crust. Our study area covers the Archean southeastern Superior Province, the Proterozoic eastern Grenville, and the Phanerozoic northern Appalachians comprising terranes with either Peri‐Laurentian or Peri‐Gondwanan heritage. Adopting a Rayleigh wave ambient noise tomography method, we used noise data recorded between 2013 and 2015, and obtained high resolution anisotropic tomographic images of the crust enabling us to discuss tectonic implications. The azimuthal anisotropy orientations follow a dominant NE‐SW trend across the study area, but some localized changes of anisotropy direction in the Bay of Fundy and across the Appalachian front are observed. The crust beneath the older Superior and Grenville provinces is generally fast, whereas the Appalachians include strong slow anomalies, especially at upper crustal depths, where they represent thick sedimentary basins beneath the St. Lawrence valley, the Gulf of St. Lawrence, and the Bay of Fundy. We suggest that the boundary between the Peri‐Laurentian and the Peri‐Gondwanan terranes at depth is marked by a Moho‐offset feature observable in our models. A generally similar crustal seismic signature for the two youngest easternmost tectonic domains suggest that they were never separated by a wide ocean basin. Our results provide important evidence for evolution of the continental crust during and after accretionary/collisional episodes in the study area.

Summary Ambient seismic noise is mainly generated in oceans through the interactions between the atmosphere, ocean waves, and the solid Earth. Study areas located near the edges of continents are thus subject to receiving an inhomogeneous noise field which could cause bias in ambient noise wave attenuation measurements and tomography studies. Ambient seismic noise characteristics across SE Canada and the NE USA are studied in detail at a regional scale for the first time, due to the availability of over two years of data (2013-2015) recorded at 69 broadband seismographs. This large, dense data set allowed us to use a back-projection technique to investigate both the azimuthal and temporal variations of the ambient noise. This method is based on a statistical analysis of signal-to-noise ratios (SNR) of the waveforms in the calculated empirical Green’s functions for pairs of stations. We propose a new method of analyzing the SNR by modifying the already existing concept of fan diagrams to include both causal and acausal components of the noise cross-correlograms in the analysis. We investigate directional and seasonal variations of the recorded noise data across the study area at the three main passbands of the seismic noise spectrum including the secondary microseisms (SM; 3-10 s), the primary microseisms (PM; 10-30 s), and the seismic hum (Hum; 30-300 s). We observe that the strongest and weakest signals are received at the SM and Hum bands respectively. Considering the results of this study along with those from previous studies, we conclude that the strongest seismic noise arrivals at the three passbands investigated in this study (i.e., SM, PM, Hum) are generated at different locations in the Atlantic, Pacific, and Arctic oceans.

<p>Long-duration stacks of ambient seismic noise cross-correlation can be used to generate high-resolution images of the lithosphere. In this study, we investigate the crustal structure beneath southeastern Canada and the northeastern USA, using an ambient noise tomography technique. Our study area covers the Phanerozoic northern Appalachians and the Proterozoic eastern Grenville Province, recording a complex tectonic history since ~1 Ga. Our datasets include continuous records of vertical component time series, recorded by 69 stations belonging to 7 seismograph networks over a more than two-year period. The ambient seismic noise directionality and seasonality variations of our datasets are analyzed in detail, and possible noise source locations are proposed in the Atlantic and Pacific oceans. Our analysis suggests strong variations of dominant seismic noise sources at both Primary (11-20 s) and Secondary (5-10 s) bands in various months, with different observed patterns at these passband periods. Our tomographic models indicate complex and strong variations of Rayleigh wave phase velocities across the study area, providing us evidence to discuss tectonic implications. The resulting Rayleigh wave phase velocity maps suggest generally slower velocities beneath the Appalachians than the Grenville province. A sharp velocity contrast is observed across the Grenville Province-Appalachian domain boundary at periods sensitive to the lower crust, suggesting a step-like geometry of the Moho interface beneath this area.</p>

No abstract