Exploring the origin of shallow anisotropic mantle fabric beneath New England

The tectonic units in northeastern U.S. record a multi-stage accretion during the Paleozoic Appalachian Orogeny. The footprint of amalgamated terranes in the lithospheric mantle may manifest as abrupt velocity discontinuities detectable by receiver function (RF) techniques, with the evidence of anisotropy signifying past deformation. The characterization of layering and their relationship to the surface tectonic features therefore holds implications for evolution of lithospheric mantle through continental collisions. An intriguing observation that emerged from a recent survey of upper mantle discontinuities by Li et al. (2021) is the presence of shallow (60 km) velocity boundaries beneath the New England region. The boundaries are localized despite being in the depths of favorable sensitivity by P-to-S RF analysis. Additionally, these boundaries are spatially correlated with the extent of thick (50 km) crust and a long-lived orogenic plateau, the Acadian altiplano at ca. 380-330 Ma, as indicated by geochronologic, geochemical, and petrologic data (Hillenbrand et al., 2021). Furthermore, the asthenosphere beneath New England is known to host the seismically slow Northern Appalachian Anomaly, whose correlation with shallow mantle boundaries detected by S-to-P RF analysis and local absence of shear-wave splitting have been noted. How the shallow mantle fabric might relate to these features, and their potential interplay during orogenesis remain to be explored. Preliminary characterization of directionally variant and invariant signal components in the RF wavefield shows primarily positive velocity gradients in Massachusetts and New Hampshire, indicative of downward increase in velocity. Most boundaries show evidence of anisotropy, with roughly N-S trending symmetry axes that correlate with the orientations of surficial structural lineation indicating mid-crustal flow during the orogenic collapse. However, a considerable area (42°-44° N) within the region also show roughly E-W trending axes, reflecting potentially different formation mechanisms. In this work, we will further test the axes orientations and attempt to infer timing and origin of these mantle boundaries.