Negative velocity gradients in the western US upper mantle from Ps and Sp receiver functions

Using three-component broad-band seismic data from EarthScope Transportable Array, PASSCAL arrays, and other regional seismic deployments in the United States west of Georgia, receiver functions are created using a multi-channel deconvolution algorithm, yielding 65,519 Ps and 34,057 Sp receiver functions. An automated depth picker was coded to pick depths and amplitudes of arrivals from a common conversion point image volume. The depth picker finds a pervasive negative velocity gradient (NVG) between the Moho and 150 km depth. The resulting NVG depth maps find depth ranges of 63 – 120 km (Ps) and 64 – 135 km (Sp). Using a surface wave shear velocity model (Shen and Ritzwoller, 2012) and an anelastic olivine model (Jackson and Faul, 2010), a temperature volume was created to assess temperatures at the NVG depths. The NVG is shown to exist across a broad temperature spectrum (Ps, 602° - 1356° C; Sp, 620° – 1398° C), with Ps modal peaks at 811° C and 1231° C, and Sp modal peaks at 732° C and 1273° C. These modal peaks correspond with two modal NVG Ps/Sp depths; 86 km (811° C) and 70 km (1231° C) for Ps, 97 km (732° C) and 82 km (1273° C) for Sp. From the NVG depths and temperatures, as well as lithospheric age, geologic history, and xenolith constraints, three species of NVG are identified. East of the Rocky Mountain Front, NVG depths are deep (82 – 135 km), with cold mantle temperatures (<950° C), and these NVG arrivals are internal to the lithosphere. This NVG species is interpreted as a metasomatic freezing front from accumulation of volatile rich low velocity phase melts (phlogopite, amphibole) over Ga time scales. West of the Precambrian rift hinge line to the Pacific, and in the Rio Grand Rift area, NVG depths are shallow (63 – 101 km) and mantle temperatures are warm (>1150° C). This NVG is at super-solidus conditions, thus melt is believed to cause this warm, shallow NVG species. Finally, in the Colorado Rocky Mountain and Colorado Plateau regions, NVG depths are intermediate (79 km – 109 km) with intermediate temperatures (950° C to 1150° C). Post-Laramide alkaline volcanism within this region (e.g., Navajo field, Great Falls Tectonic Zone, Leucite Hills, Colorado Rocky Mtns.), and local super-solidus conditions, supports the conclusion that this intermediate NVG species represents a perturbation of the metasomatic layer in the eastern areas. The multi-channel deconvolution as well as the picking algorithm, results in a laterally coherent NVG that is significantly more resolved than previous studies (Kumar et al., 2012; Levander, 2011; Lekic, 2011).