LITHOSPHERE TEMPERATURES WITHIN THE WESTERN UNITED STATES AND IMPLICATIONS FOR RHEOLOGY

The evolution, deformation, and dynamics of the U.S. lithosphere are fundamentally connected to temperature. This property ties surface processes to deep forces, affects stability of cratonic lithosphere, and modulates the forces and strengths of deforming lithosphere (among many other things). Moreover, temperature variations are the largest drivers of seismic velocity variation in the upper mantle, so characterizing other aspects of physical state such as composition requires accurate assessment of temperature.

Estimates of lithospheric temperature are challenging. A particularly useful measurement is Pn velocity mapped to temperature. This Moho temperature is often below 900C, so uncertainty in the velocity-temperature scaling is minimized compared to deeper parts of the lithospheric mantle where anelasticity is more important. These Pn-derived temperatures are coupled with newly revised estimates of heatflow and shallow subsurface temperatures to “pin” lithospheric geotherms at the near-surface and just below the Moho. Making only very limited assumptions about the rather poorly known distribution of crustal heat producing elements, tight constraints can be placed on lithospheric geotherms in much of the western U.S. Where Pn velocities and surface heatflow mismatch, information can be gleaned about transient or horizontally-advective thermal processes, or compositionally modulated Pn velocity variations.

Lower crustal temperatures in the western U.S. are high (> 850C) in the Colorado Rocky Mountains, the Rio Grande Rift, the southern margin of the Colorado Plateau, the eastern portion of the Nevada Basin and Range, the Oregon High Lava Plains, and the Yellowstone hotspot track. These locales are strongly correlated with <10Ma magmatism.

Where temperature estimates are deemed robust, elastic thicknesses are predicted using laboratory-derived flow laws. In deforming zones, elastic thickness predicted by estimated temperatures and a dry diorite/olivine rheology is too large. This suggests a weaker crustal lithology, or perhaps water, plays an important role in western U.S. lithospheric deformation.