Lithospheric Structure beneath the Western US Using USArray Data

Lithospheric Structure beneath the Western US Using USArray Data Maps of depth to (a) Moho and (b) LAB. The thick lines illustrate the physiographic boundaries in the western U.S. The LAB map is a smoothed estimate of the shallowest event we identified as the LAB in the Ps and Sp receiver functions.
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Using a combination teleseismic data from the USArray Transportable Array, previous PASSCAL experiments, and the COARSE array in Arizona we have produced images of the lithospheric structure beneath the Western United States. We have made common conversion point (CCP) stacked Ps and Sp receiver function image volumes to determine, in more detail and higher resolution than previously obtained, the crustal thickness and the depth to the Moho and lithosphere-asthenosphere boundary (LAB) throughout the Western U.S. Individual receiver functions have been converted to depth and laterally “migrated” to their conversion point using 3D P- and S-wave tomography velocity models, with redundant signals stacked for signal enhancement. Both S and P receiver functions have imaged an unusually complex crust-mantle boundary region beneath the Colorado Plateau in comparison to most other parts of the western U.S., although we also see features that correlate with expressions of lithospheric drips in the southern Sierra Nevada and the Wallowa Mountains. The Moho shallows significantly from an average of ≥40 km to the southern Basin and Range, where the crust is ~30 km. These complications in the Moho are correlated with low upper mantle velocities observed in P and S body wave tomography and S-velocity structure determined from Rayleigh wave inversion. Throughout the model, the LAB is a negative amplitude feature that has significant topographic variation, and cannot be described as a single surface. We see a particularly strong correlation between calculated equilibration pressures of primitive basalt whole rock samples from across the western United States, extracted from the NAVDAT database (http://www.navdat.org/), and the LAB estimate from the Sp images beneath the southern Basin and Range, the Colorado Plateau, and the Sierra Nevada. The depth estimates from the geochemistry data and comparison with the PdS receiver function images for the same region allows us to interpret the lithosphere-asthenosphere boundary and its relation to the tectonic provinces in the western United States. We will present different geologic scenarios that can explain these structures.
</p><p>Acknowledgements: We would like to thank Kaijian Liu, Yongbo Zhai, Yan Xu, and Meijuan Jiang for assisting with data processing. The Sp receiver function study began as an exercise at the 2008 CIDER (Cooperative Institute for Deep Earth Research) summer school. This research was funded by Earthscope grant EAR-0844741 and 0844760. AL gratefully acknowledges a Humboldt Research Prize from the Alexander von Humboldt Foundation. The Ps study was initiated by AL while on sabbatical at the GeoForschungsZentrum Potsdam, Germany.</p>

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