Don't judge an orogen by its cover: Kinematics of the Appalachian décollement from seismic anisotropy

Abstract

As North America collided with Africa to form Pangea during the Alleghanian orogeny, crystalline and sedimentary rocks in the southeastern United States were thrust forelandward along the Appalachian décollement. We examined Ps receiver functions to better constrain the kinematics of this prominent subsurface structure. From Southeastern Suture of the Appalachian Margin Experiment (SESAME) and other EarthScope stations on the Blue Ridge–Piedmont crystalline megathrust, we find large arrivals from a 5–10-km-deep converter. We argue that a strong contrast in dipping anisotropic foliation occurs at the subhorizontal Appalachian décollement, and propose that such a geometry may be typical for décollement structures. Conversion polarity flips can be explained by an east-dipping foliation, but this orientation is at odds with the overlying northeast-trending surface tectonic grain. We suggest that prior to late Alleghanian northwest-directed head-on collision, the Appalachian décollement accommodated early Alleghanian west-vergence, independent of the overlying Blue Ridge–Piedmont structural inheritance. The geophysical expression of dipping anisotropic foliation provides a powerful tool for investigating subsurface kinematics, especially where they are obscured by overlying fabric, to disentangle the tectonic complexities that embody oblique collisional orogens.

Paleozoic Decollement Displaced the Surface Trace of Iapetus Ocean Closure in Northern Appalachians

“…A slow symmetry axis most commonly found in crustal rocks (Brownlee et al., 2017) and a fast axis associated with deformed amphibolites (e.g., Ji et al., 2013) will yield the same observed pattern if they plunge in the opposite directions. Amplitudes and polarities of sin2 θ and cos2 θ terms (Xie et al., 2020) or forward modeling of specific observed waveforms (e.g., Nikulin et al., 2009) could discriminate between fast and slow axes and allow more detailed interpretations of detected signals (e.g., Frothingham et al., 2022). However, determination of the specific location (above or below the interface) and nature (fast or slow symmetry) of anisotropy will not alter the overall findings of symmetry axes being near‐orthogonal to the strike of the major tectonic boundaries in northern Appalachians.…”

Insights From Layered Anisotropy Beneath Southern New England: From Ancient Tectonism to Present‐Day Mantle Flow

“…Anisotropy-aware RF analysis has been applied to both subduction zones (e.g., Bar et al, 2019;Haws et al, 2023;Wirth & Long, 2014) and continental settings (e.g., Ford et al, 2016;Schulte-Pelkum & Mahan, 2014b;Wirth & Long, 2014) to investigate potential multi-layered anisotropy beneath those study regions. This technique has also been applied to eastern North America (e.g., Frothingham et al, 2022;Li et al, 2021;Long et al, 2017;Yuan & Levin, 2014). Yuan and Levin (2014) conducted both SKS splitting and the anisotropic RF analyses on three long-running stations in northeastern US, and found coherent lower lithosphere anisotropy with a fast axis nearly orthogonal to the strike of the Appalachian orogen and APM-consistent asthenosphere anisotropy.…”