The seismic signature of lithospheric deformation beneath eastern North America due to Grenville and Appalachian orogenesis
The eastern margin of North America has been affected by two complete Wilson cycles of supercontinental assembly and breakup over the past ~1.3 b.y. Evidence of these processes is apparent in the surface geology; however, the geometry, strength, and extent of lithospheric deformation associated with these events are poorly known. Observations of seismic anisotropy in the continental lithosphere can shed light on past deformation processes, but information about the depth distribution of
... ribution of anisotropy is needed. Here we investigate the azimuthal dependence of transverse component receiver functions at broadband seismic stations in eastern North America to constrain sharp con trasts in seismic anisotropy with depth. We examined data from six permanent seismic stations, including three that are just to the east of the Grenville Front and three that are within the Appalachian Mountains. A harmonic stacking modeling method was used to constrain the presence of anisotropic interfaces within the crust and mantle lithosphere. A comparison among stations located to the east and to the west of the Grenville Front reveals evidence for different lithospheric anisotropy across the front, which in turn argues for significant lithospheric deformation associated with the Grenville orogeny. Stations located in the Appalachians exhibit a striking signature of strong and multi layered anisotropy in the lower crust, consistent with observations in modern orogens, as well as the lithospheric mantle. Our observations constrain the existence and approximate depths of contrasts in anisotropy within the lithosphere and may be used for future testing of spe cific hypotheses regarding lithospheric deformation associated with orogenesis.