Teleseismic receiver function analysis of data from six dense arrays in the western U.S. is used to investigate mantle transition zone (MTZ) discontinuities and the prevalence of a low‐velocity layer atop the 410 km discontinuity (410‐LVL). Negative polarity Ps arrivals indicative of a low‐velocity layer with a top 25–60 km above the 410 are identified in 8–11 out of 18 stacks of receiver functions from highly sampled back azimuth corridors. The 410‐LVL is interpreted as partial melt resulting from upwelling of hydrated mantle across a water solubility contrast at the 410. The 669 km mean depth of the 660 km discontinuity (660) and the magnitude of 660 topography suggest variable hydration, locally approaching saturation, in addition to >150 K lateral temperature variations beneath five arrays. Mean amplitudes of P410s and P660s increase monotonically with period from 2 to 10 s; however, greater variations are observed in the frequency dependence of P410s compared to P660s implying 410 thickness is more heterogeneous. Variable 410 thickness is attributed to changes in hydration modulating the width of the olivine‐to-wadsleyite transition interval. Frequency dependence of P660s amplitudes suggests a broad velocity gradient consistent with multivariate phase changes in the olivine and garnet systems. Sporadic detection of the 410‐LVL, the magnitude and length scales of MTZ discontinuity topography, and inferred variations in hydration support the occurrence of vigorous small‐scale convection in the western U.S. mantle. Comparison of receiver functions with body wave tomography suggests small‐scale convection driven by sinking slab segments and lithospheric instabilities contributes to the intermittent nature of the 410‐LVL.



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