Chapter 12 Generalized Upper Mantle Thermal Structure of the Western United States and Its Relationship to Seismic Attenuation, Heat Flow, Partial Melt, and Magma Ascent and Emplacement

Abstract

The crust and the upper mantle of the western United States are characterized by low seismic velocities and high seismic attenuation values, as well as high heat flow and high electrical conductivity values. Generalized temperature distributions inferred for the region provide partial constraints for understanding these anomalous and apparently fundamental features. Laboratory seismic measurements of rocks from the upper mantle at high pressures and high temperatures are compared with seismic observations to estimate the overall thermal structure of this portion of the upper mantle. This chapter presents these estimated temperature distributions beneath the western United States and estimates the laterally averaged degree of melting as a function of depth. Beneath the eastern Rockies, the resulting upper mantle temperature is below the dry solidus, and no melting is generally expected. The chapter discusses the available heat-flow data of the western United States and calculates conductive temperatures within the lithosphere. The temperatures inferred from surface heat-flow values are consistent with those from laboratory seismic data for a dry peridotite. Peaks in partial melt content occur at ≈165 km (≈3%) and at ≈145 km (≈10%) beneath the Intermountain (IM) and Western Margin (WM) regions, respectively. Elastic dislocation treatments of veins and deep dikes yield magma-filled fracture heights that range from 500 m to 11 km, thereby suggesting that the magma ascent pathway is highly disconnected.