Inferred rheology and upper mantle conditions of western Nevada and southern California-northwest Mexico

Haylee L Dickinson, Purdue University

Abstract

Understanding the viscous strength (rheology) of the mantle is essential for understanding the dynamics and evolution of the Earth. Rheology affects many geologic processes such as mantle convection, the earthquake cycle, and plate tectonics. This study uses tectonic (postseismic) and non-tectonic (lake unloading) events that have induced differential stress changes within the crust and mantle, which in turn, create surface deformation. The viscoelastic relaxation is constrained using geodetic methods, such as GPS, InSAR, or measurements of shoreline rebound. We can use these observed surface displacements to constrain numerical models of the relaxation processes that can be used to infer a viscosity structure. These studies allow us to infer the mechanical nature of the lithosphere and asthenosphere using 3D finite element models. When we combine our inferred viscosity structure with calculations of conductive geothermal gradients and models of mantle melting, we can infer environmental conditions of the upper mantle like water content, mineralogy, and degree of melt. In our first study, we seek to reduce non-uniqueness issues that plague in situ rheology studies by simultaneously modeling the response of the crust and mantle for a single region of western Nevada to multiple processes constrained by multiple observational data sets. Western Nevada has experienced a series of Mw >6.5 earthquakes over the last ~150 years, from the 1872 Owen’s Valley earthquake to the 1954 Dixie Valley event, as well as the loading/unloading of Pleistocene-aged Lake Lahontan. Our goal was to answer whether a single Newtonian viscosity structure can explain all of the geodetic constraints. We found a strong lower crust underlain by a relatively weak upper mantle can explain all observational constraints. We also infer the decreases in viscosity we observed are due to hydration possibly from the subduction of the Farallon slab and melt content. In the next study, we investigate the 2010 El Mayor-Cucapah earthquake. This provides a fairly unique region because the postseismic deformation extends across multiple distinct geologic provinces, giving us the opportunity to study lateral heterogeneity using five years of cumulative GPS-measured postseismic deformation. The surface deformation is best explained by a laterally heterogeneous and depth dependent viscosity structure with the Salton Trough having a weaker viscosity than the surrounding region, consistent with the inferred thermal structure of the region and the seismologically observed LAB. We infer a region of hydration with possible melt for the Peninsular Ranges and suggest the Salton Trough has dehydration within the upper mantle, creating the lateral heterogeneity.

Degree

Ph.D.

Advisors

Freed, Purdue University.

Subject Area

Geophysics

COinS