Inferring Mantle Reology in Western Nevada Using Postseismic Relaxation and Lake Lahontan Rebound

Abstract:

Western Nevada has experienced two episodes of loading: tectonic loading due to a series of M_w≥6.5 earthquakes between 1872 and 1954 in the Central Nevada Seismic Belt and non-tectonic loading/unloading due to the formation and evaporation of Pleistocene-aged Lake Lahontan. Prior studies have used observed surface deformation (InSAR, GPS, or lake rebound measurements) to constrain numerical models of lower crustal and mantle relaxation to infer depth-dependent viscosity structures. However, four separate studies have inferred four different viscosity structures, highlighting non-uniqueness and other issues that plague in situ rheology studies. Here we combine the postseismic and lake unloading processes into a single self-consistent study to reduce non-uniqueness issues and determine whether a single Newtonian viscosity structure can explain all of the observations. We use finite element models to show that all of the observational constraints can be explained by a Newtonian rheology beneath Western Nevada marked by a strong lower crust at a depth of 20-30 km with a viscosity of at least 5x10²⁰ Pa s and a strong mantle lid at a depth of 30-40 km with a viscosity of at least 5x10¹⁹ Pa s overlying a relatively weak upper mantle of order 10¹⁸ Pa s. Our ability to fit both postseismic and lake unloading by a single Newtonian viscosity structure may be a result of the limited resolution of the data sets, none of which record expected transient behavior early in the unloading processes.