Cenozoic Evolution of the Thermal Regime of the Continental Mantle in Western USA: A Comparative Study Constrained by Petrologic Data

Broad regional trends in variations of Cenozoic magma composition with time indicate a progressive uplift of the depth of magma generation during intracontinental tectono-magmatic activity. These evolutionary trends are interpreted in terms of temporal changes in the lithosphere thickness over the past 40-60 Ma. A nonsteady-state thermal conductivity boundary problem for lithosphere thinning due to melting of its lowermost part is used to quantify the dynamics of the mantle thermal regime required to produce the observed trends in magma composition. An advantage of the approach is a complete independence of the results on the values of crustal heat production. The calculated mantle geotherms for hydrous peridotite melting for the time span from the beginning of Cenozoic magmatism until present are in a good agreement with data on the age of metamorphic complexes, apatite fission track data, orthopyroxene-garnet thermobarometry, available xenolith P-T data, data on the depth to Curie isotherm, and temperature constraints based on interpretations of regional seismic tomography models.

The results clearly indicate a significantly different evolution of the thermal regime of the mantle beneath the Kenya Rift, Massif Central, Basin and Range Province, on one hand, and the Colorado Plateau, the Central Asia, and the Hoggar plateau, on the other hand. The former are expected to have the present-day upper lithospheric temperatures ca. 300-400 oC higher than the latter. The results support the conclusions of a recent analog modeling results [Jurine et al., 2005] that asthenosphere-lithosphere interaction can have different modes depending on the lithosphere composition and plume temperature.