VISCOSITY ESTIMATES FOR THE CRUST AND UPPER-MANTLE FROM PATTERNS OF LACUSTRINE SHORELINE DEFORMATION IN THE EASTERN GREAT-BASIN

The deformed shorelines of Lake Bonneville constitute a classic source of information on lithospheric elastic thickness and upper mantle vis cosity. We describe and apply a new model to a recently augmented data set. New data better constrain both the complex spatio-temporal patte rn 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 fro m within the Bonneville basin and to an improved calibration of the ra diocarbon timescale itself. The data which constrain the crustal defor mation pattern consist of ages and shoreline elevations from several h undred points which sample three major levels of Lake Bonneville and c orresponding elevations from the high stands of three smaller lakes si tuated to the west of Lake Bonneville. The data from the smaller lakes help elucidate the pattern of deflection which occurred beyond the ed ge of the big lake. The geometry of the Earth model incorporates an ar bitrary number of layers overlying a half-space, and the theology of e ach level can accommodate an arbitrary number of Maxwell viscoelastic elements in parallel. The inverse modeling comprises three complementa ry 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, w e constrained the elastic parameters to their seismically determined v alues and then solved for viscosity versus depth profiles by either ex pressing the log(viscosity) versus log(depth) profile as a series of s pecially constructed orthogonal polynomials, or by allowing each of 8- 10 layers (plus the half-space) to have an independently determined vi scosity. We found that the data do not strongly support (nor can they conclusively exclude) a more complex theology than simple Maxwell visc oelasticity. The orthogonal polynomial solution exhibits an essentiall y monotonic decrease in viscosity with depth. The most rapid change oc curs at shallow depths, decreasing from 10(23) Pa s at 3 km to 10(20) Pa s at 30 km. The decrease is much more gradual below, with only anot her factor of 5 decrease between 30 and 300 km depth. The unconstraine d solution exhibits a rapid decrease in viscosity with depth from 2x10 (24) Pa s in the top 10 km to 4x10(17) Pa s at a depth of 40 km. A nea rly isoviscous asthenospheric region extends from 40 to 150 km and is underlain by a mantle lithospheric region with increased viscosity (2x 10(20) Pa s) extending from 150 to 300 km depth and by a uniform visco sity (10(19) Pa s) half-space below