EXPLORING THE ORIGINS OF SHALLOW MANTLE BOUNDARIES 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 and spatially coincide with anomalous geological features recognized in the region, such as the extent of long-lived Acadian altiplano at ca. 380-330 Ma (Hillenbrand et al., 2021), as well as the seismically slow Northern Appalachian Anomaly in the asthenosphere beneath. How these shallow mantle boundaries might relate to these geological 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 axes that correlate with the orientations of surficial structural lineation indicating mid-crustal flow during the 330-310 Ma orogenic collapse. However, a considerable area (42°-44° N) within the region also show roughly E-W trending axes, reflecting potentially different origins. Additionally, while some observed signal attributes remain consistent at higher frequencies, others show differential arrivals of signal components that suggest presence of small-scale boundaries that comprise the observed signal at lower frequencies. Here we will further quantify key characteristics of observed signals in order to better assess the relationship between the observed boundaries in the context of regional tectonic history.