Thermal structure of the North American uppermost mantle inferred from seismic tomography

[1] We map the thermal state of the North American mantle between depths of 50 and 250 km by inverting P and S velocities of three recent seismic tomographic models. In the well-resolved regions, temperatures derived from P velocities agree with those derived from S velocities within the estimated uncertainties, and generally, the seismic temperatures are in agreement with those inferred from surface heat flow. Adiabatic mantle temperatures are found as shallow as 50 km under most of the Basin and Range. Warm, subsolidus mantle and known crustal structure can account for the high average elevation and large-scale variations in topography of western North America. In the cratonic mantle beneath the stable eastern part of North America, temperatures at 50-100 km are on average 500 degreesC cooler than under the tectonic western part of the continent and adiabatic mantle temperatures are not reached until 200-250 km depth. To balance the effect on topography of the thermally implied density increase for the North American craton, we infer a compositionally induced density decrease equivalent to a 1% depletion in iron over a depth interval of 50-250 km. In regions where T-P differs significantly from T-S we drop our assumption that variations in seismic velocity are only due to thermal structure. A discrepancy between T-P and T-S between 50 and 150 km depth under the Cascades and the Gulf of California can be accounted for by the presence of 1 to 2 vol % of fluids and/or melt. Another such discrepancy beneath Wyoming remains enigmatic.