LOADING AND FLEXURAL STRENGTH OF THE LITHOSPHERE: A NEW MAP OF EFFECTIVE ELASTIC THICKNESS FOR THE WESTERN UNITED STATES

Effective elastic thickness T_e (or equivalently, flexural rigidity D) is an integral measure of strength of the lithosphere that depends on temperature, composition, and the history of mass loading. These same factors (strength, temperature, composition and mass flux) also strongly influence tectonic evolution of the lithosphere. Estimates of T_e are commonly used to model surface processes such as basin formation and erosional uplift, and they can also inform our understanding of variations in mantle buoyancy, mantle rheological strength, and convection. The Earthscope Transportable Array seismic network is providing an unprecedented areal coverage and spatial density of measurements of subsurface structure at lithospheric-scale, which enables better constraint of mass fields and improved estimates of flexural strength. Mapping lateral variations in lithospheric strength and mass density can lead to a better understanding of how deformation is distributed at the surface.

We examine the flexural strength (effective elastic thickness) of the western United States using estimates of crustal thickness, crustal seismic velocity ratio V_P/V_S, gravity, and heat flow data from the Earthscope transportable array. We invert for the moho density contrast, the density variation associated with changes in crustal-averaged V_P/V_S (which is primarily sensitive to rock composition), and the coefficient of thermal expansion in order to estimate mass variations within the lithosphere. We then use these mass fields to model gravity and topography and to estimate effective elastic thickness from the relationship between predicted and observed gravity and topography across the western half of the United States.