Viscosity Structure of the Crust and Upper Mantle in Western Nevada from Isostatic Rebound Patterns of Lake Lahontan Shorelines

Large lakes can both produce and record significant crustal deformation. We present an analysis of the isostatic rebound pattern recorded in the shorelines of paleo-lake Lahontan, in western Nevada, using a layered Maxwell viscoelastic model. The inferred viscosity structure depends on loading history. We use three variants of a well documented lake surface elevation model as input and recover corresponding estimates of viscosity and density structure. A simple two layer model, with an elastic plate over an inviscid half-space, fits the observed elevation pattern quite well, with a residual variance of 30% of the data variance. Using muli-layered, finite viscosity models, the residual variance is reduced to 20% of the data variance, very nearly to the noise level. In the higher resolution models, the viscosity is below 10 Pa s over the depth range from 80 to 160 km. The minimum viscosity is very similar to the value that has been seen in the eastern Great Basin, from similar analyses of Lake Bonneville shorelines but the low viscosity zone is a thinner beneath Bonneville. Making small adjustments to a seismically derived density structure allows an improved fit to the shoreline observations. Additionally, we find that small variations in proposed loading models can result in presumably spurious density inversions, and suggest that this modeling approach provides a test for loading histories.