Crustal Geothermal Properties and Evidence of Laramide Thermal Perturbation of the Western United States

Abstract:

The relative importance of water, lithology, temperature and grain size as controls on lithospheric deformation is not well-understood. Regions that have experienced repeated deformation events often have relatively abundant quartz in the crust (Lowry and Pérez-Gussinyé, 2011), suggesting lithology plays a role, but there are two possible pathways for that to play out. One is a direct pathway: Quartz has the lowest viscosity of any of the common crustal minerals for given temperature and water fugacity, so will deform more readily. The second is an indirect effect on temperature: In crystalline basement, quartz affiliates with granites, which tend to have higher radiogenic isotope concentrations than more mafic rocks. Heat generated by these isotopes thermally insulates the deeper lithosphere, resulting higher temperatures and decreased viscosity of the deep crust and uppermost mantle, and hence more deformation. Here we examine the effect of radiogenic heating from decaying isotopes on crustal deformation. Using data from USArray including crustal thickness estimates from Lowry and Pérez-Gussinyé (2011), we compare Moho temperature estimates derived from Pn-phase velocities by Schutt et al. (in prep) with geotherms modeled using surface heat flow measurements and various assumptions about the distribution of radiogenic heating in the crust. RMS misfits of predicted to measured Moho temperatures are minimized for models that assume low surface radiogenic heat production, small variation with depth, and temperature-dependent thermal conductivity parameters consistent with laboratory measurements of crustal lithologies. Misfit between the model and measurements is dominated by relatively low measured Moho temperature in regions deformed by thick-skin tectonism during the Laramide orogeny. The surface heat-flow model assumes steady-state geothermal conditions, and thermal perturbations introduced more recently than the time required for a conductive thermal pulse to traverse the lithosphere (~100 Myr) would not be observed in surface heat-flow. Hence the temperature discrepancy may reflect thermal perturbation by proposed Laramide flat-slab subduction. Shallow subducted slab would serve both to chill the base of the lithosphere and insulate it from deeper convective heating.