INSIGHTS FROM LAYERED ANISOTROPY BENEATH SOUTHERN NEW ENGLAND: FROM ANCIENT TECTONISM TO PRESENT-DAY MANTLE FLOW

Seismic waves with different propagation and oscillation directions can exhibit different velocities when going through a medium with some directional properties; this phenomenon is called seismic anisotropy. Seismic anisotropy observed beneath eastern North America is often attributed to present-day flow in the upper mantle. The mantle flow causes shear waves oscillating in the direction of flow (e.g., in the direction of North America plate motion) to travel faster than those that travel in other directions. However, this pattern does not hold true for some regions along the Appalachian orogen, suggesting that past tectonic events can result in long-lived, ‘frozen-in’ anisotropy in the lithosphere, which modifies the predicted anisotropic behavior beneath these regions. In this study, we investigate sources of seismic anisotropy beneath southern New England using a method based on directionally dependent variations of P-wave to S-wave conversions at interfaces with contrasts in anisotropy. This method can separate signals caused by different anisotropic features and constrain the depth distribution of anisotropy. Within the crust there are multiple features that may be correlated with stratification in the Hartford Basin, faults in the Taconic thrust belt, shear zones formed during Salinic/Acadian terrane accretion events, and orogen-parallel crustal flow in the Acadian orogenic plateau. We apply a Bayesian inversion method to obtain quantitative constraints on the direction and strength of intra-crustal anisotropy beneath the Hartford Basin. In the upper mantle, we identify a fossil shear zone possibly formed during oblique subduction of Rheic Ocean lithosphere. We also find evidence for a plate motion-parallel flow zone in the asthenosphere that is likely disturbed by mantle upwelling near the southern margin of the Northern Appalachian Anomaly in the eastern part of the study area.