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

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 decollement. 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 decollement, and propose that such a geometry may be typical for decollement 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 decollement 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.

Automated summary

This study examines the kinematics of the Appalachian decollement in the southeastern United States through the analysis of seismic receiver functions. The research reveals the presence of a subhorizontal decollement structure with dipping anisotropic foliation, suggesting a typical geometry for such structures. The findings also suggest that the Appalachian decollement accommodated westward compression prior to the late Alleghanian collision, independent of the overlying Blue Ridge-Piedmont structural inheritance.