Effective elastic thickness Te depends primarily on temperature, composition, and state of stress of the lithosphere. In this paper, we examine high-resolution spectral estimates of Te and their relationships to regional heat flow, age of the lithosphere, seismic properties, stress orientations, and earthquake focal depths of the western U.S. Cordillera. The relationship of Te to heat flow indicates that ductile flow accommodates long-term (~106 to 108 years) isostatic response at different levels of the crust and upper mantle, depending principally on age (and, by implication, bulk composition) of the lithosphere. Isostatic response is primarily controlled by the upper mantle in Archean lithosphere of the middle Rocky Mountains, whereas Te depends on lower crustal flow in Early Proterozoic lithosphere of the Colorado Plateau. The Yellowstone-Snake River Plain volcanic field and significantly extended regions in the Basin-Range and northern Rocky Mountains are associated with latest Proterozoic aged lithosphere and indicate middle to upper crustal control of long-term Te. We also show that azimuthal variations of Te reflect deviatoric stress in the lithosphere. Te is found empirically to approximate the 95th percentile focal depth of background seismicity. The latter relationship is inconsistent with brittle-ductile control of focal depth, indicating that another rheological transition (e.g., from stick-slip to stable sliding frictional behavior) is responsible. Tectonic and structural relationships expand upon the hypothesis that the geographic distribution of tectonic features depends fundamentally on spatial variations in strength of the lithosphere. Moreover, we find a spatial correlation of the Intermountain Seismic Belt to a marked transition in Te, implying that forces responsible for this active seismic zone are derived from local buoyancy anomalies rather than from current-day plate boundary interactions. ¿ American Geophysical Union 1995