Multiple transition zone seismic discontinuities and low velocity layers below western United States

With P-to-S converted waves recorded at seismic stations of the US Transportable Array we image the fine structure of upper mantle and transition zone (TZ) beneath the western US. We map the topographies of seismic discontinuities by stacking data by common conversion points along profiles. Systematic depth and amplitude measurements are performed not only for the well-known ‘410’ and ‘660’ interfaces, but also for minor seismic discontinuities identified around 350, 590, and 630 km depths. The amplitude of conversion suggests shear wave velocity (Vs) increase by 4% at the ‘410’ and the ‘660’. The observed ‘660’ velocity contrast is smaller than expected from the 6% in IASP91 but consistent with a pyrolitic model of mantle composition. The Gorda plate, subducted under Northern California, is tracked to the TZ where it seems to flatten and induce uplift of the ‘410’ under Northern Nevada. Maps of ‘410’/‘660’ amplitude/topography reveal that the TZ is anomalous beneath the geographical borders of Washington, Oregon, and Idaho, with: (i) a thickened TZ, (ii) a sharp change in depth of the ‘660’, (iii) a reduced ‘410’ conversion amplitude in the North, and (iv) a positive ‘630’ discontinuity. Such anomalous structure might be inherited from the past history of plate subduction/accretion. A thinned TZ under the Yellowstone suggests higher-than-average temperatures, perhaps due to a deep thermal plume. Both the ‘350’ and the ‘590’ negative discontinuities extend over very large areas. They might be related either to an increased water content in the TZ, a significant amount of oceanic material accumulated through the past 100 My, or both.