Detection of a Lithospheric Drip beneath the Great Basin

Detection of a Lithospheric Drip beneath the Great Basin Summary of geological and geophysical constraints for the central Great Basin. a, Shear-wave splitting with the topography background for reference. b, Post-10-Myr volcanism (black circles) shows a regional dearth of volcanic activity. c, Heat flow showing reduced values (~50mWm, blue) in the regional high (>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.
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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.
</p><p>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 <1 to ~25 Myr, triggered from local density anomalies as small as 1% and initial temperature increases of ~10%.
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Lithospheric drips are often inferred based on surface expressions of rapid uplift, subsidence, or voluminous magmatic activity, but have remained challenging to detect directly due to their relatively small size and transient nature. The Great Basin drip was detected by purely geophysical means and does not exhibit significant recognizable surface topography modifications, consistent with our numerical models.
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The interpretation of a lithospheric drip beneath the Great Basin is consistent with geophysical and geological characteristics of the region, including a distinct paucity of post-12 Ma volcanic activity and a heat flow low. Lithospheric material feeding horizontally into the drip would be expected to generate contractional forces if significant mantle/crust coupling exists, and surface contraction centered near the drip has been observed. The northeast plunge of the drip provides a unique indicator of northeast-directed regional mantle flow relative to the North American plate.
</p><p>References
</p><p>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
</p><p>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.
</p><p>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).</p>

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