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Detection of a Lithospheric Drip beneath the Great Basin
John D. West (Arizona State University), Matthew J. Fouch (Arizona State University), Jeffrey B. Roth (Arizona State
University/ExxonMobile), Linda T. Elkins-Tanton (Massachusetts Institute of Technology)
Using a combination of shear-wave splitting and seismic
P-wave delay time tomography, we investigated a region of
greatly diminished shear-wave splitting times, collocated with a
sub-vertical cylinder of increased seismic velocity in the upper
mantle beneath the Great Basin in the western United States.
The localized reduction of splitting times is consistent with a
rotation in flow direction from predominantly horizontal to
sub-vertical, and the high velocity cylinder is characteristic of
cooler lithospheric mantle. We suggest that the reduced splitting
times and higher than average seismic velocities are the result of
a cold mantle downwelling (a lithospheric drip). The cylinder of
higher seismic velocities is approximately 100 km in diameter,
extends near-vertically from ~75 km depth to at least 500 km,
and plunges to the northeast. Near 500 km depth, the cylinder
merges with a separate zone of high-velocity material, making
resolution of a distinct cylinder difficult below this depth.
We generated geodynamic numerical models of Rayleigh-Taylor
instabilities originating in the mantle lithosphere, using structural
constraints appropriate to conditions in the central Great Basin.
These models predict downwelling lithospheric mantle in the form
of a strong, focused lithospheric drip developing over time periods
of 100mWm; yellow and red). d, Seismic tomography horizontal slice at
200 km depth. e, Shear-wave splitting times surface showing the strong drop
in the central Great Basin. f, Isosurface at +0:95% velocity perturbation for
NWUS08-P2 showing the morphology of the drip, which merges with a larger
structure at ~500 km depth. The black arrows denote the inferred mantle flow
direction; the white arrow denotes the flow direction of the Great Basin drip.
West, J.D., M.J. Fouch, J.B. Roth, and L.T. Elkins-Tanton, (2009), Vertical mantle flow associated with a lithospheric drip beneath the Great
Basin, Nature Geosci., 2, 439-444
Holt, W., M.J. Fouch, E. Klein, and J.D. West, (2010), GPS measured contraction in Nevada above the Great Basin mantle drip, in preparation.
Acknowledgements: Thanks to the USArray Transportable Array team for the instrumentation which made this study possible, and to the IRIS
Data Management Center for providing access to the data. Partial support for this project came from US National Science Foundation
grants EAR-0548288 (MJF EarthScope CAREER grant) and EAR-0507248 (MJF Continental Dynamics High Lava Plains grant).
II-206 | 2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics