Shear Wave Splitting Beneath Eastern North American Continent: Evidence for a Multilayered and Laterally Variable Anisotropic Structure

Abstract

Eastern North America records a tectonic history of over 3 Ga in duration. Much of this record is preserved within the lithosphere and may be unraveled by detailed studies of its interior structure. Past episodes of tectonic activity likely left their imprints in the form of anisotropy‐forming rock fabric presently preserved in the lithosphere of the continent. We perform shear wave splitting measurements using observations of core‐refracted waves collected from a ~1,300‐km‐long array extending from James Bay in Quebec to the Fundy basin in Maine, with lateral spacing of 10–100 km between instruments. Close spacing of instruments helps us associate anisotropic properties with geological boundaries. We find that the fast polarizations concentrate between N60°E and N90°E with an average of N80°E and change systematically with backazimuth. In addition, we observe a lateral increase in delay time from 0.56 ±0.25 s at the NW end of the array to 0.90 ± 0.41 s at the SE end. The location of lateral change in delay time does not match geological boundaries on the surface but seems to match the geophysical boundary at depth of 160 km. We interpret this boundary in splitting values to be the edge of cratonic lithosphere at depth. Our observations suggest that the anisotropic structure beneath our study area is complex and possibly both multilayered and laterally variable.

Localized Anisotropic Domains Beneath Eastern North America

“…Generally, the widespread similarity of averaged fast polarizations and their overall agreement with the APM direction suggest anisotropy due to the ongoing deformation within the upper mantle due to the motion of the North American plate. This conclusion was also reached by Chen et al () study that extends into the craton to the northwest of our region. On the other hand, a number of investigators (e.g., Gilligan et al, ; White‐Gaynor & Nyblade, ) focused on the lateral changes in patterns of averaged fast polarizations, seeking to relate them to the local trends of tectonic units and thus explain them as reflective of anisotropic texture within the continental lithosphere.…”

“…As discussed in section , this discrepancy likely arises from the choices of measurement methodology. Specifically, the largest delay times are observed in northern Maine, where individual splitting parameters as high as 4 s are also reported by previous studies (Chen et al, ; White‐Gaynor & Nyblade, ). Similarly, the reported delay times near the southern end of the study area are smaller, with most published studies showing values of about 1 s. Some results even report values over 2 s. While our delay time values show a similar trend, the range of values reported by our study is significantly smaller, with a maximum delay time under 1.4 s in northern Maine.…”

“…As discussed in section 3, this discrepancy likely arises from the choices of measurement methodology. Specifically, the largest delay times are observed in northern Maine, where individual splitting parameters as high as 4 s are also reported by previous studies (Chen et al, 2018;White-Gaynor & Nyblade, 2017). Similarly, the reported delay times near the southern end of the study area are smaller, With good back azimuthal coverage emphasized by our study, we document clear evidence for directional variation of the splitting parameters at all of our stations.…”

“…In this study, we present a systematic survey of long‐running sites with an aim of combining the ability to see the lateral variations with the directional dependence of splitting that was previously taken to signify layering of anisotropic properties. A similar approach has been adopted by Chen et al () and Levin et al () to discern potential areas of unique anisotropic properties based on the specific behaviors of directional variation within the data set of an individual station. We extend this mode of characterization to a broader region to examine the finer‐scale lateral variations in detail.…”

A model space search approach to finite-frequency SKS splitting intensity tomography in a reduced parameter space

“…Broad-band seismic experiments with dense station spacing within North America that were enabled by the recently completed EarthScope USArray data gathering effort also represent prime targets for future application of our technique. Examples include the SPREE (Ola et al 2016), OIINK (Yang et al 2017), and NELE (Nyamwandha & Powell 2016) experiments in the mid-continent, as well as the SESAME (Hopper et al 2017), MAGIC (Aragon et al 2017) and QM-III (Chen et al 2018) experiments in eastern North America.…”