One-dimensional thermal models constrained by seismic velocities and surface radiogenic heat production for two main geotectonic units in southern Portugal

Abstract

New heat flow density data and radiogenic heat production values obtained in southern Portugal, along with lithologic and structural data inferred from vertical seismic velocity distributions from deep seismic refraction surveys, are used to construct one-dimensional (1D) geotherms for the crust in the two main geotectonic units of the region, i.e., the Ossa–Morena Zone (OMZ) and the South Portuguese Zone (SPZ). Surface radiogenic heat production values calculated using potassium, uranium and thorium concentrations measured in different rock types were used to constrain the heat production values. In the Ossa–Morena Zone and South Portuguese Zone the surface radiogenic heat production does not exceed 3 μW/m 3. Geotherms were constructed considering that the crust in the region has a three-layer structure, as inferred from seismic refraction surveys, but with different thicknesses in the OMZ and SPZ. For both zones two 1D models with constant and exponential heat productions were constructed. The models suggest that temperatures are higher in the SPZ than in the OMZ, the difference reaching values of about 300°C at Moho depths; however, heat production values in the former are less constrained than in the latter and so temperatures in the SPZ are probably overestimated. For each geotectonic unit, no significant difference is observed in the results when constant heat production is assumed for the upper crust or an exponential decrease is assumed for the upper crust.