Geothermal modeling along a two-dimensional crustal profile in Southern Portugal

Abstract

Crustal temperatures along a S–N trending profile crossing the main geological units of Southern Portugal, the South Portuguese Zone and the Ossa Morena Zone, were calculated by solving numerically the heat conduction equation in a 2-D geothermal model of the crust. Use of observed surface heat flow and heat production based on the empirical relationship between radiogenic heat sources and observed seismic velocities, yields temperatures at the crust/mantle boundary, which are higher than the upper limit (about 700 °C) of crustal temperatures based on magnetotelluric surveys. The most uncertain parameter of the model is the upper crust heat production, whose low value implies a large non-crustal component of surface heat flow, which must be conducted through the whole crust and increases the vertical temperature gradient. To overcome this uncertainty in heat sources, the Moho heat flow was estimated using Vitorello and Pollack’s [J. Geophys. Res. (1980) 983] estimate of the background heat flow of 27 mW m −2 arising from below the zone of crustal enrichment (identified in our model with the upper crust) and transient thermal perturbation, represented in Variscan units by 5–10 mW m −2. Taking into account the heat production of the middle and lower crust, the mean value of the Moho heat flow should be 28–33 mW m −2. The Moho heat flow was allowed to vary (with variation restricted to 0.2–1.0 mW m −2 per 5 km) around 28 or 33 mW m −2 along the profile in order to improve the fit between the calculated and measured surface heat flows. Because this approach yields a surface heat flow lower than observed, the residual differences were minimized by increasing the heat production of the upper crust, where the production was allowed to vary along the profile (with variation restricted to 0.02–0.2 μW m −3 per 5 km). If Moho heat flow is allowed to vary by a total of about 9–25 mW m −2, the upper crustal heat production necessary for the calculated surface heat flow to match the observed one is in the range 2–3 μW/m −3, which indicates a mostly granitic upper crust. The resulting Moho temperatures are below 700 °C everywhere along the profile in all model configurations considered.