Localized Anisotropic Domains Beneath Eastern North America

Abstract

Seismic anisotropy beneath eastern North America likely reflects both the remnant lithospheric fabrics and the present‐day deformation of the asthenosphere. We report new observations of splitting in core‐refracted shear phases observed over 3–5 years at 33 sites in New Jersey, New York, and states in the New England region and also include data from eight previously studied locations. Our data set emphasizes back azimuthal coverage necessary to capture the directional variation of splitting parameters expected from vertically varying anisotropy. We report single‐phase splitting parameters as well as station‐averaged values based on splitting intensity technique that incorporates all observed records regardless of whether they showed evidence of splitting or not. Trends of averaged fast shear wave polarizations appear coherent and are approximately aligned with absolute plate motion direction. The general disparity between the fast axes and the trend of surface tectonic features suggests a dominant asthenosphere contribution for the observed seismic anisotropy. Averaged delay values systematically increase from ~0.5 s in New Jersey to ~1.4 s in Maine. Splitting parameters vary at all sites, and neighboring stations often show similar patterns of directional variation. We developed criteria to group stations based on their splitting patterns and identified four domains with distinct anisotropic properties. Splitting patterns of three domains suggest a layered anisotropic structure that is geographically variable, outlining distinct regions in the continental mantle, for example, the Proterozoic lithosphere of the Adirondack Mountains. A domain coincident with the North Appalachian Anomaly displays virtually no splitting, implying that the lithospheric fabric was locally erased.

SKS Splitting and Upper Mantle Anisotropy Beneath the Southern New England Appalachians: Constraints From the Dense SEISConn Array

“…These are endmember scenarios, and it is possible (perhaps likely) that the actual Earth structure incorporates elements of both ideas. On balance, we find the idea of a transition to upwelling flow in the upper mantle at the eastern end of the SEISConn array (Figure 13a) to be compelling, since Li et al (2019) has persuasively demonstrated that the region of weak splitting in eastern Connecticut is connected geographically with the region of weak splitting above the NAA first documented by Levin et al (2018). While a scenario in which pervasive N-S oriented anisotropy in the mantle lithosphere exhibits lateral variability in its strength, with weak lithospheric anisotropy in the west and strong lithospheric anisotropy in the east (Figure 13b), is also possible, we do not have a ready explanation for why such a variation in lithospheric anisotropy strength would be present.…”

“…While the long-running stations used by Li et al (2019) allowed for a much more detailed characterization of splitting behavior at each station, the comparatively short-lived SEISConn array offers the advantage of a much larger number of stations. The Li et al (2019) study included data from just one station in northern Connecticut (UCCT, located just to the south of SEISConn), while our dataset, with 15 stations, allows us to probe the anisotropic boundary proposed by Li et al (2019) in greater detail and at higher resolution. Figure 12 shows our single-station average splitting measurements (as derived from the multichannel method using the low-frequency filter) plotted on top of the anisotropic domains proposed by Li et al (2019).…”

“…The Li et al (2019) study included data from just one station in northern Connecticut (UCCT, located just to the south of SEISConn), while our dataset, with 15 stations, allows us to probe the anisotropic boundary proposed by Li et al (2019) in greater detail and at higher resolution. Figure 12 shows our single-station average splitting measurements (as derived from the multichannel method using the low-frequency filter) plotted on top of the anisotropic domains proposed by Li et al (2019). Single-station average measurements obtained using the splitting intensity method by Li et al (2019) are also shown for comparison.…”

“…Figure 12 shows our single-station average splitting measurements (as derived from the multichannel method using the low-frequency filter) plotted on top of the anisotropic domains proposed by Li et al (2019). Single-station average measurements obtained using the splitting intensity method by Li et al (2019) are also shown for comparison. We note that measurements obtained using other methods (e.g., Long et al, 2016;Yang et al, 2017) are not shown on this map; they are not directly comparable, and observed delay times tend to be substantially higher.…”

“…Previous measurements of Li et al (2019), which were used to delineate the anisotropic domains shown with yellow and red shading, are also shown (stations indicated with triangles). We show the single-station splitting parameters from Li et al (2019) derived from splitting intensity measurements, so they are directly comparable to the measurements shown at SEISConn stations. SEISConn, Seismic Experiment for Imaging Structure Beneath Connecticut.…”