Anisotropic seismic properties beneath the Northern Appalachians: a guide for past and present deformations within the upper mantle

Seismic anisotropy, the directional dependence of seismic velocity, is a proxy for deformation at the depth. Within the Earth’s upper mantle, anisotropic seismic observations likely reflect the strain-induced realignment of olivine crystals, which is the major component of upper mantle peridotite. The pervasive mineralogical fabrics form by both the present-day convection of asthenosphere, and the past deformations imprinted in the lithosphere by plate motion and orogenesis. The topography and the surface geology of the Northern Appalachians provide evidences for past tectonic events that had altered the continental margin. Thus, the observation of seismic anisotropy beneath this region should reflect contributions from both the lithosphere and the asthenosphere. We infer and characterize the seismic anisotropy beneath the Northern Appalachians using the observations of core-refracted shear phases. The multiple provenances of seismic anisotropy likely vary the anisotropic properties along the wave’s ray path. As a result, the apparent splitting parameters within each station change with respect to the wave’s propagation direction. To effectively compare the directionally varying splitting parameters across the region, we selected data based on a template list of 61 events with optimized back azimuthal coverage. In addition to single phase splitting measurements, we also obtained station-averaged splitting parameters using Splitting Intensity (SI) technique to consider both split and non-split measurements. Regionally, the trends of averaged fast axes appear coherent and align with the direction of regional absolute plate motion (average of 249°). The general disparity between the fast axes and the trend of surface tectonic features suggests dominant asthenosphere contribution for the observed seismic anisotropy. The averaged delay times, however, are laterally variable with concentrated localities of smaller delays. The visual comparisons between the datasets of neighboring stations reveal similar splitting patterns with respect to back azimuths and inclination angles, enough for them to be grouped into four regions of distinct anisotropic seismic observations. Such mode of lateral variation suggests that the layered system beneath the region’s upper mantle is not uniform but vary geographically and may correspond to localized mantle structures associated with the modification of lithosphere. The inferred domain boundaries correlate only locally with the surficial geological features, and better correlate with the variation of seismic properties in the mantle as suggested by tomography.M.S.Includes bibliographical reference