Systematic Mapping of Upper Mantle Seismic Discontinuities Beneath Northeastern North America

Abrupt velocity gradients in the upper mantle, detectable by receiver functions (RF) techniques, have been known to exist down to the depths of ∼110 km beneath northeastern North America. Comparisons with the surface wave velocity models have designated some negative velocity gradients (NVGs) as the lithosphere-asthenosphere boundary (LAB), delineating a relatively thin lithosphere beneath this region. This work presents a systematic survey of upper mantle layering in seismic properties using P-S RF analysis at 62 long-running sites with dense lateral sampling. We examine both radial and transverse component RF for indicators of seismic anisotropy and adopt the notion of seismic attributes, utilized in active-source seismology, to characterize the spatial distribution of directionally variant and invariant signal components. We confirm a widespread presence of NVGs at depths 60–100 km throughout the region, consistent with previous studies using mode-converted body waves. We also find numerous converting boundaries that reflect changes in directional variation (anisotropy) of seismic velocity, indicating complexity of rock texture in the upper mantle. Some of these boundaries appear as deep as 185 km, implying that the lithosphere extends much deeper than the widespread NVGs would suggest. In this, our results agree with recent estimates of the lithospheric thickness in thermodynamically consistent models combining seismic, gravity, and heat flow constraints.