Lithosphere-asthenosphere interaction beneath the western United States from the joint inversion of body-wave traveltimes and surface-wave phase velocities
Richard M. Allen
University of California, Berkeley
US Geological Survey
National Taiwan University
Geophys. J. Int. , 185 1003-1021, doi: 10.1111/j.1365-246X.2011.04990.x, 2011.
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NW-SE cross-section across the Colorado Plateau showing the two lithospheric drips. See Fig 13.
Section through the plume pancake at 50km depth showing the low-velocity plume material spreading out beneath the lithosphere. See Fig 12.
Vertical sections orientated perpendicular to the hotspot track. Beneath Yellowstone the plume conduite is observed. Below the Snake River Plain the shallowing and broadening plume head can be seen. See Fig 12.
The relation between the complex geologic history of the western margin of the North American plate and the processes in the mantle is still not fully documented and understood. Several pre-USArray local seismic studies showed how the characteristics of key geologic features such as the Colorado Plateau and the Yellowstone Snake River Plains are linked to their deep mantle structure. Recent body-wave models based on the deployment of the high density, large aperture USArray have provided far more details on the mantle structure while surface-wave tomography (ballistic waves and noise correlations) informs us on the shallow structure. Here we combine constraints from these two datasets to image and study the link between the geology of the western US, the shallow structure of the Earth and the convective processes in mantle. Our multi- phase model provides new constraints on the extent of the Archean lithosphere imaged as a large, deeply rooted fast body that encompasses the stable Great Plains and a large portion of the Northern and Central Rocky Mountains. Widespread slow anomalies are found in the lower crust and upper mantle, suggesting that low density rocks isostatically sustain part of the high topography of the western US. The Yellowstone anomaly is imaged as a large slow body rising from the lower mantle, intruding the overlying lithosphere and controlling locally the seismicity and the topography. The large E-W extent of the USArray used in this study allows imaging the "slab graveyard", a sequence of Farallon fragments aligned with the currently subducting Juan de Fuca Slab, north of the Mendocino Triple Junction. The lithospheric root of the Colorado Plateau has apparently been weakened and partly removed through dripping. The distribution of the slower regions around the Colorado Plateau and other rigid blocks follows closely the trend of Cenozoic volcanic fields and ancient lithospheric sutures, suggesting that the later exert a control on the locus of magmato-tectonic activity today.
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