D. L. Schutt

Crust and upper mantle velocity structure of the Yellowstone hot spot and surroundings

[1] The Yellowstone hot spot has recently been shown to be a plume that extends into the transition zone. At roughly 60–120 km depth, the plume material rising beneath Yellowstone Park is sheared SW by North America Plate motion, producing a profound low velocity layer emplaced beneath the thin lithosphere. To constrain the absolute seismic velocity of the plate-sheared plume layer, fundamental mode Rayleigh wave observations have been inverted for phase velocity using the two plane wave technique. The resulting phase velocity models are inverted with Moho-converted P to S arrival times to better constrain crustal thickness and absolute S wave velocity structure to � 120 km depth. A regionalized S wave velocity model has an extremely low velocity minimum of 3.8 ± 0.1 km/s at 80 km depth beneath the hot spot track. Nonregionalized 3-D velocity models find a velocity minimum of 3.9 km/s beneath the hot spot track. Below 120 km depth, our resolution diminishes such that the lateral spreading of the plume track is not resolved. The volume of the low velocity plume layer is small and the estimated buoyancy flux for the Yellowstone plume is <0.1 Mg/s which contrasts with the � 9 Mg/s value for Hawaii. In addition, a notable region of thick crust and high lower crustal velocities is found around Billings, Montana, consistent with previous refraction and receiver function studies that interpret this as evidence for a massive Precambrian underplating event. Citation: Schutt, D. L., K. Dueker, and H. Yuan (2008), Crust and upper mantle velocity structure of the Yellowstone hot spot and surroundings, J. Geophys. Res., 113, B03310, doi:10.1029/2007JB005109.

Anisotropy of the Yellowstone Hot Spot Wake, Eastern Snake River Plain, Idaho

Over the last 10 million years, the Yellowstone hot spot has passed beneath the eastern Snake River Plain, both magmatically modifying the Snake River Plain crust and creating a wider, wake-like “tectonic parabola” of seismicity and uplift. Analysis of SKS arrivals to a line array of 55 mostly broadband stations, distribution across the tectonic parabola, reveals a nearly uniform orientation of anisotropy, with an average fast axis orientation of N64E. The back azimuth of null splitting events is parallel to the measured fast axis, suggesting that anisotropic material consists of a single layer. Splitting parameters are independent of backazimuth, suggesting that anisotropy is constant beneath each station. Thus station-averaged split parameters are representative of the anisotropy beneath the station. Station-averaged split times range from 0.6–1.5 s, and define a pronounced depression in split times centered about 80 km southeast of the axis of the Snake River Plain.

Evidence for a deep asthenosphere beneath North America from western United States SKS splits

The uniform southwesterly trend of fast-traveling split SKS waves that traverse the upper mantle beneath the Yellowstone swell provides good evidence for both southwesterly motion of North America over a relatively stable deep Earth interior, and young strain accommodation within the Yellowstone swell mantle via dislocation creep of olivine. These results contrast with the many SKS splits recorded across the western United States, the splitting behavior of which often is very complex at individual sites, and the orientations of which, as a set, cannot be attributed simply to plate motion or to small-scale convection beneath North America lithosphere. These data suggest that, away from Yellowstone, (1) mantle fabric is complex, and (2) North America motion is accommodated by strain at depths greater than those typically associated with asthenosphere.

Temperature of the plume layer beneath the Yellowstone hotspot

Recent studies show that the Yellowstone hotspot is associated with a plume-like lowvelocity pipe that ascends from the transition zone to the lithosphere-asthenosphere boundary, where the plume is sheared to the southwest by North American plate motion. Rayleigh wave tomography shows this plate-sheared plume layer has an extremely low S wave velocity of 3.8 ± 0.1 km/s at 80 km depth, ~0.15–0.3 km/s lower than the velocity observed beneath normal mid-ocean ridges. To constrain the temperature of the plume layer, a grid search with respect to grain size and temperature is performed to fi observed Rayleigh wave phase velocities. This search fi nds that the excess temperature of the plume layer is >55–80 °C at 95% confi dence for two different temperature-velocity and two different melt-velocity models , confi rming that a thermal mantle plume exists.

3-D multiobservable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle: III. Thermochemical tomography in the Western-Central U.S.

We apply a novel 3-D multiobservable probabilistic tomography method that we have recently developed and benchmarked, to directly image the thermochemical structure of the Colorado Plateau and surrounding areas by jointly inverting P wave and S wave teleseismic arrival times, Rayleigh wave dispersion data, Bouguer anomalies, satellite-derived gravity gradients, geoid height, absolute (local and dynamic) elevation, and surface heat flow data. The temperature and compositional structures recovered by our inversion reveal a high level of correlation between recent basaltic magmatism and zones of high temperature and low Mg# (i.e., refertilized mantle) in the lithosphere, consistent with independent geochemical data. However, the lithospheric mantle is overall characterized by a highly heterogeneous thermochemical structure, with only some features correlating well with either Proterozoic and/or Cenozoic crustal structures. This suggests that most of the present-day deep lithospheric architecture reflects the superposition of numerous geodynamic events of different scale and nature to those that created major crustal structures. This is consistent with the complex lithosphere-asthenosphere system that we image, which exhibits a variety of multiscale feedback mechanisms (e.g., small-scale convection, magmatic intrusion, delamination, etc.) driving surface processes. Our results also suggest that most of the present-day elevation in the Colorado Plateau and surrounding regions is the result of thermochemical buoyancy sources within the lithosphere, with dynamic effects (from sublithospheric mantle flow) contributing only locally up to ∼15–35%. ©2016. American Geophysical Union. All Rights Reserved.

Moho temperature and mobility of lower crust in the western United States

We use measurements of mantle P-wave velocity from the Moho refracted phase, Pn, to estimate temperature within the uppermost few km of the western U.S. mantle. Relative to other approaches to modeling the deep geotherm, using Pn velocities requires few assumptions and provides a less uncertain temperature at a tightly constrained depth. Assuming a homogeneous mantle composition, Moho temperatures are lowest in an arc that extends from the High Lava Plains through western Montana and the high-plains region of Wyoming and western Kansas/Nebraska. Highest temperatures are observed under recent (<10 Ma) volcanic provinces and are consistent with melting. Estimates of lower crustal viscosity suggest that the western U.S. west of the Laramide deformation front likely has regions of mobile lower crust that decouple upper crustal and upper mantle tractions.