Upper mantle viscosity derived from the difference in rebound of the Provo and Bonneville Shorelines: Lake Bonneville Basin, Utah

Abstract

Twenty‐four new field measurements of elevation of Provo level and Bonneville level shoreline terraces provide data for reanalysis of isostatic rebound in the Lake Bonneville basin. Analysis of the differential rebound between the Provo shoreline (maximum rebound of 43 m) and the Bonneville shoreline (maximum rebound of 69 m) requires that the latter be an equilibrium shoreline. Within the possible measurement errors, the Provo shoreline formed after the elastic lithosphere had attained at least 91% equilibration to the removal of the water load between the Bonneville and Provo shorelines; within the errors the Provo may also represent an equilibrium shoreline. From the new data presented in this paper, the best estimate of the upper limit of effective viscosity of the uppermost mantle, assuming a half‐space model and a 2000‐year time interval between the Bonneville and Provo shorelines, is 2 × 1019 N s m−2 (2 × 1020 P). In addition, comparison of shoreline rebound profiles, for both shorelines, with theoretical plate flexure models indicates that the mean flexural rigidity of the Basin and Range lithosphere in this region is 1 × 1023 N m, or slightly less.