High-resolution mapping of North America suggests numerous low-velocity zones above and below the mantle transition zone

We investigate seismic discontinuities across the middle of Earth's mantle beneath a large seismic array that spans the North American continent. We provide robust constraints on the depth distribution, sharpness, and spatial variation of seismic discontinuities by processing high-resolution Ps-converted seismic waves (∼0.5 Hz) through a novel denoising filter called CRISP-RF (Clean Receiver function Imaging with Sparse Radon Filters). In the upper mantle, above the mantle transition zone (MTZ), we observe a sharp velocity decrease at depths that vary from ∼290 km to ∼390 km. In the lower mantle, below the MTZ, we observe another sharp velocity decrease at depths that vary from ∼800 km to ∼1400 km. The lower-mantle discontinuities cluster at a depth of ∼940 km, while deeper converters (> 1100 km) are less likely. We evaluate our results against two leading hypotheses: basalt enrichment due to incomplete mixing and dehydration melting in which MTZ water is transported into the upper or the lower mantle, but rarely simultaneously. We conclude that our results are more consistent with a scenario where the complex interplay between composition, volatile content, and subduction history, produces disconnected regions of velocity-inversion interfaces when past convective motions bring hydrated MTZ rock into the hydrophobic upper and lower mantles.