Lateral Variations in SKS Splitting Across the MAGIC Array, Central Appalachians

Abstract

The eastern margin of North America has been shaped by several cycles of supercontinent assembly. These past episodes of orogenesis and continental rifting have likely deformed the lithosphere, but the extent, style, and geometry of this deformation remain poorly known. Measurements of seismic anisotropy in the upper mantle can shed light on past lithospheric deformation, but may also reveal contributions from present‐day mantle flow in the asthenosphere. Here we examine SKS waveforms and measure splitting of SKS phases recorded by the MAGIC experiment, a dense transect of seismic stations across the central Appalachians. Our measurements constrain small‐scale lateral variations in azimuthal anisotropy and reveal distinct regions of upper mantle anisotropy. Stations within the present‐day Appalachian Mountains exhibit fast splitting directions roughly parallel to the strike of the mountains and delay times of about 1.0 s. To the west, transverse component waveforms for individual events reveal lateral variability in anisotropic structure. Stations immediately to the east of the mountains exhibit complicated splitting patterns, more null SKS arrivals, and a distinct clockwise rotation of fast directions. The observed variability in splitting behavior argues for contributions from both the lithosphere and the asthenospheric mantle. We infer that the sharp lateral transition in splitting behavior at the eastern edge of the Appalachians is controlled by a change in anisotropy in the lithospheric mantle. We hypothesize that beneath the Appalachians, SKS splitting reflects lithospheric deformation associated with Appalachian orogenesis, while just to the east this anisotropic signature was modified by Mesozoic rifting.

Localized Anisotropic Domains Beneath Eastern North America

“…For example, an alignment of tectonic boundaries and the orientations of fast polarization of split shear waves may be taken to signify a substantial lithospheric contribution to the apparent splitting. Furthermore, sharp lateral variation observed between two closely spaced stations can suggest changes in anisotropy that are relatively close to the surface (e.g., Aragon et al, ). Here we compare our station‐averaged splitting parameters and the spatial distribution of our station groups with surface geology (Figure a) and with the shear wave velocity distribution at a depth of 90 km in the model of Shen and Ritzwoller (); Figure b).…”

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

“…Another recent innovation in the study of upper mantle anisotropy beneath the eastern U.S. has been the advent of data sets from arrays that feature dense station spacing, allowing for the sampling of the mantle lithosphere over the short length scales that are relevant for geologic structures. For example, Aragon et al (2017) presented SKS splitting observations for the dense MAGIC array in the central Appalachians, and showed a sharp transition in splitting behavior across the Appalachian Mountains, which they attributed to a change in lithospheric anisotropy. Similarly, Chen et al (2018) measured SKS splitting across the dense QMIII array in the northern Appalachians, documenting a lateral change in splitting delay times that corresponds to the edge of cratonic lithosphere at depth.…”

“…For example, Aragon et al. (2017) presented SKS splitting observations for the dense MAGIC array in the central Appalachians, and showed a sharp transition in splitting behavior across the Appalachian Mountains, which they attributed to a change in lithospheric anisotropy. Similarly, Chen et al.…”

An Anisotropic Contrast in the Lithosphere Across the Central San Andreas Fault

“…Dense arrays afford resolution of changes in anisotropic parameters over short horizontal distances and insight into plausible depths of heterogeneous anisotropy due to Fresnel zone expansion with depth (e.g., Aragon et al, ; Barak & Klemperer, ; Rümpker et al, ). The sharpness of the change in splitting signals west and east of the SAF (Figure ) indicates distinct anisotropy in the lithosphere because of the largely overlapping Fresnel zones of the ~10 s period shear waves at depths greater than ~70 km (Figure d).…”