Survival and Demise of Thick Continental Lithosphere under Highly Extended Crust

Survival and Demise of Thick Continental Lithosphere under Highly Extended Crust Mantle contribution to surface elevation (color), overlaid by topography (shading) for orientation. Pink triangles (seismic stations) are location where Moho depth was measured, white stars (refraction profile centers) mark crustal velocity and density values from refraction experiments used to convert receiver function Moho times to depth and to calculate isostatic contribution of the crust. Mantle buoyancy is well constrained in areas where both seismic stations and refraction data exist in the vicinity. Circles mark locations of basalts with measured Neodymium isotope ratios and are plotted so that younger samples overlay older ones.
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We find an unusually thick lithosphere under a highly extended portion of the Basin and Range. We determined Moho depths using P-to-S receiver functions from the EarthScope Transportable Array and the Southern Great Basin Digital Seismic Network. As one would expect from isostasy, the crust is thicker under the high-elevation Northern Basin and Range and thinner in the low-lying Southern Basin and Range. However, we find an unexpected crustal keel under the highly extended Death Valley extensional area in between, underlying low surface elevations and putting the crust well outside of isostatic equilibrium. Dense crust is not found in this area in refraction results, but a lithospheric mantle keel fits the seismic observations as well as the observation of anomalous isotope ratios in basalts from this area that require an old, enriched lithospheric source [Farmer et al., 1989]. Unusually low εNd values indicating an old lithospheric mantle source were replaced by asthenospheric basalts in the Lake Mead extensional area, but persist during the entire history of basaltic magmatism (12-0 Ma) in the Death Valley extensional area, matching the geographical extent of dense mantle. If the anomaly is isostatic and a lithosphere-asthenosphere density contrast of 50 kg/m3 (0.05 g/cm3) is assumed, the lithosphere-asthenosphere boundary lies at a mean depth of 120 km within the anomaly, compared to a mean of 40 km outside the anomaly. We conclude that deformation was highly incoherent between the surface and the lower crust and mantle lithosphere, and that an island of Precambrian lithosphere has survived beneath the Death Valley extensional area and may currently be sinking.
</p><p>References
</p><p>Farmer, G.L., F.V. Perry, S. Semken, B. Crowe, D. Curtis, D.J. DePaolo, Isotopic evidence on the structure and origin of subcontinental lithospheric mantle in southern Nevada, J. Geophys. Res. 94, B6, 7885-7898, 1989
</p><p>Acknowledgements: This research is funded by NSF grant EAR-0838509. Seismic data with the exception of those from the Southern Great Basin Digital Seismic Network were obtained from the IRIS DMC.</p>

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