Cooperative Institute for Research in Environmental Sciences

Buoyancy sources in the Western U.S.

Will Levandowski (1), Craig Jones (1), Weisen Shen (2), Mike Ritzwoller (2)

(1) CIRES, (2) CIEI, Dept. of Physics, CU

The heterogeneous geologic history of the western U.S. invites a broad spectrum of possible ways of supporting modern, widely varied topography ranging from orogen-scale thermal support to highly variable combinations of crustal buoyancy, mantle temperature, lithospheric composition, and convective effects. We explore the need for diverse means of support in two areas: the Sierra Nevada region with P-wave tomography and the Utah-Colorado-Kansas region spanning several provinces imaged with ambient noise tomography. Expected topography is initially calculated from crustal densities derived from seismic wavespeeds combined with mantle densities inferred from wavespeeds assuming a chemically homogeneous but thermally varying mantle. This expected topography has internal uncertainties of a few hundred meters and compares well with observed topography in broad terms, but substantial deviations reflect the presence of melt and compositional variations. Predicted topography is too high in the Cascade backarc, the eastern Basin and Range, and in much of the Southern Rocky Mountains. This difference we attribute to the presence of melt of less than 0.5% of total volume, which lowers wavespeed with little effect on density. Reducing the density anomaly to account for melt can reconcile this discrepancy. Conversely, parts of the Wyoming craton and Great Plains have predicted topography lower than observed, a discrepancy most plausibly related to iron depletion of the Archean and early Proterozoic mantle lithosphere. No results to date require sublithospheric loads and probably preclude variations in topography from such loads exceeding ~0.5 - 1 km in each region.