Lake Bonneville: Constraints on lithospheric thickness and upper mantle viscosity from isostatic warping of Bonneville, Provo, and Gilbert stage shorelines

The deformed shorelines of the late Pleistocene Lake Bonneville provide an excellent opportunity to analyze the response of the Earth to surface loads of relatively short wavelengths (≤ 400 km) and short duration (≤ 30 kyr). Recently revised estimates of the lake loading history and an expanded compilation of shoreline elevations for the Bonneville (∼1550 m), Provo (∼1440 m), and Gilbert (∼1300 m) shorelines have prompted us to reexamine the deflection. The earth model we use is identical with that employed by Nakiboglu and Lambeck, and consists of an elastic plate overlying a Maxwell viscoelastic channel and a rigid base. Our calculation differs from that of Nakiboglu and Lambeck in two respects: we used a significantly more detailed load model, and we compared our results with observations from three distinct shorelines. The lake elevation history spans 32 kyr in 18 piecewise linear segments. The water load is computed at each time step from a digital terrain model (4×4 km cell size) and a specified lake elevation. We find that the use of three separate shorelines significantly reduces the acceptable region of parameter space: the effective elastic lithospheric thickness is (23 ± 2) km, the mantle viscosity is (1.2 ± 0.2) 10²⁰ Pa s, and the depth to a significant viscosity increase is no less than 300 km, though it might be much more. Examination of the residual deflections (differences between observed and computed shoreline elevations) reveals a regional tilt down to the NE of ∼6 10^-5, which we attribute to collapse of the peripheral bulge produced by the Laurentide ice sheet.