Viscosity estimates for the crust and upper mantle from patterns of lacustrine shoreline deformation in the Eastern Great Basin

Abstract

The deformed shorelines of Lake Bonneville constitute a classic source of information on lithospheric elastic thickness and upper mantle viscosity. We describe and apply a new model to a recently augmented data set. New data better constrain both the complex spatio‐temporal pattern of the lake load and the crustal deformation response to that load. The history of lake level fluctuations has been significantly refined and somewhat modified. This is due to both more radiocarbon dates from within the Bonneville basin and to an improved calibration of the radiocarbon timescale itself. The data which constrain the crustal deformation pattern consist of ages and shoreline elevations from several hundred points which sample three major levels of Lake Bonneville and corresponding elevations from the high stands of three smaller lakes situated to the west of Lake Bonneville. The data from the smaller lakes help elucidate the pattern of deflection which occurred beyond the edge of the big lake. The geometry of the Earth model incorporates an arbitrary number of layers overlying a half‐space, and the rheology of each level can accommodate an arbitrary number of Maxwell viscoelastic elements in parallel. The inverse modeling comprises three complementary approaches: for the simplest configurations, we performed a direct search of the parameter space and delineated the irregular boundary of the subspace of acceptable models. For more complex configurations, we constrained the elastic parameters to their seismically determined values and then solved for viscosity versus depth profiles by either expressing the log(viscosity) versus log(depth) profile as a series of specially constructed orthogonal polynomials, or by allowing each of 8–10 layers (plus the half‐space) to have an independently determined viscosity. We found that the data do not strongly support (nor can they conclusively exclude) a more complex rheology than simple Maxwell viscoelasticity. The orthogonal polynomial solution exhibits an essentially monotonic decrease in viscosity with depth. The most rapid change occurs at shallow depths, decreasing from 1023 Pa s at 3 km to 1020 Pa s at 30 km. The decrease is much more gradual below, with only another factor of 5 decrease between 30 and 300 km depth. The unconstrained solution exhibits a rapid decrease in viscosity with depth from 2×1024 Pa s in the top 10 km to 4×1017 Pa s at a depth of 40 km. A nearly isoviscous asthenospheric region extends from 40 to 150 km and is underlain by a mantle lithospheric region with increased viscosity (2×1020 Pa s) extending from 150 to 300 km depth and by a uniform viscosity (1019 Pa s) half‐space below.