Dynamic elevation of the Cordillera, western United States

We introduce a methodology that synthesizes topography, gravity, crustal-sc ale seismic refraction velocity, and surface heat flow data sets to estimat e dynamic elevation, i.e., the topography deriving horn buoyancy variations beneath the lithosphere. The geophysical data independently constrain the topographic effects of surface processes, crustal buoyancy, and thermal bou ndary layer thickness. Each of these are subtracted from raw elevation of t he western U.S. Cordillera to reveal dynamic elevation that can exceed 2 km and is significant at > 95% confidence. The largest (similar to 1000 km di ameter) of the dynamic elevation anomalies resembles a numerical model of a hypothetical Yellowstone hotspot swell, but the swell model does not accou nt for all of the significant features seen in the dynamic elevation map. O ther dynamic elevation anomalies are spatially correlative with Quaternary volcanism, but partial melt call contribute no more than a few hundred mete rs of elevation. Hence much of the dynamic elevation likely derives from ot her thermodynamic anomalies. Possible alternative mechanisms include both s uperadiabatic upwelling and adiabatic phase boundary deflections maintained by latent heat effects. Comparison of seismicity and volcanism to effectiv e viscosity gradients, estimated from lithospheric flexural rigidity to fac ilitate the numerical swell model, suggests that tectonism focuses where li thosphere with negligible mantle viscosity abuts lithosphere with significa nt uppermost mantle viscosity.