Dynamic elevation of the Cordillera, western United States

Abstract

We introduce a methodology that synthesizes topography, gravity, crustal‐scale seismic refraction velocity, and surface heat flow data sets to estimate dynamic elevation, i.e., the topography deriving from buoyancy variations beneath the lithosphere. The geophysical data independently constrain the topographic effects of surface processes, crustal buoyancy, and thermal boundary layer thickness. Each of these are subtracted from raw elevation of the western U.S. Cordillera to reveal dynamic elevation that can exceed 2 km and is significant at >95% confidence. The largest (∼1000 km diameter) of the dynamic elevation anomalies resembles a numerical model of a hypothetical Yellowstone hotspot swell, but the swell model does not account for all of the significant features seen in the dynamic elevation map. Other dynamic elevation anomalies are spatially correlative with Quaternary volcanism, but partial melt can contribute no more than a few hundred meters of elevation. Hence much of the dynamic elevation likely derives from other thermodynamic anomalies. Possible alternative mechanisms include both superadiabatic upwelling and adiabatic phase boundary deflections maintained by latent heat effects. Comparison of seismicity and volcanism to effective viscosity gradients, estimated from lithospheric flexural rigidity to facilitate the numerical swell model, suggests that tectonism focuses where lithosphere with negligible mantle viscosity abuts lithosphere with significant uppermost mantle viscosity.