Thermal gradients in the continental crust

Abstract

Summary: A set of preferred geotherms for the continental crust is calculated using assumptions of steady state conductive heat transfer. The calculations use observed heat flow as the principal constraint, temperature and pressure dependent thermal conductivity functions, and a radiogenic heat generation profile that is consistent with a generalized petrological model of the lithosphere. Properties of these geotherms include: (1) a nearly constant gradient through the upper crust because the decrease in thermal conductivity at higher temperatures for felsic rocks counteracts the decreasing heat flow caused by crustal radioactivity; (2) divergence of the geotherms amounting to temperature differences of more than 500 K between rifts and shields at Moho depths, and (3) convergence of geotherms below 250 km in the asthenosphere as convective heat transfer dominates. Emphasis is also placed on understanding the sensitivity of calculated temperatures to measureable properties. Computed geotherms are overall most sensitive to surface heat flow or reduced heat flow, and uncertainties in this quantity will ultimately limit the accuracy of temperature estimations for the deeper crust and lithosphere. Otherwise, the sensitivity to parameters such as heat production, thermal conductivity and its temperature dependence throughout the lithosphere and crustal thickness depends on depth and whether the region is characterized by high or low heat flow. Upper mantle heat production, pressure coefficients of thermal conductivity, and the position of the upper/lower crust boundary are relatively insensitive parameters for lower crustal temperature calculations. Dynamic events accompanying crust forming processes lead to significant perturbations of the steady state static temperature field described above. Pressure-temperature pairs from granulite terrains plot near high temperature limits for those steady state geotherms and imply either a transient P-T-t path or a magmatic heat input from the mantle accompanying crustal thickening.