Determining subsurface temperature & lithospheric structure from joint geophysical-petrological inversion: A case study from Ireland

Abstract

High quality maps of the geothermal gradient and temperature are essential when assessing geothermal potential for a region. However, determining geothermal potential is a challenge as direct measurements of in situ temperature are sparse and individual geophysical methods are sensitive to a range of parameters, not solely temperature. Here, we develop a novel approach to determine the geothermal gradient using a new joint geophysical-petrological inversion where seismic velocities and density in the mantle are related to temperature and bulk composition within a thermodynamic framework. Large datasets of phase velocities of seismic surface-waves are now incorporated into the inversion, and provide essential constraints on the lithospheric thickness and temperature, which shape the crustal geotherms to a significant extent. We also include all available measurements of the surface heat flow, radiogenic heat production (RHP) and thermal conductivity within the crust, to further constrain the temperature and geothermal gradient, in particular in the top few kilometres of the crust. We use Ireland as a case study and show how our new methodology can reproduce the results of previous work but also improve on them, thanks to the complementary sensitivities of the full range of data. Lithospheric and crustal thicknesses have primary controls on the temperature gradient, with areas of thinner lithosphere showing higher geothermal gradients. In some locations, anomalously warm geotherms result from high RHP within crustal granitic rocks, mudstone and shales. RHP is above continental averages across all Ireland, likely due to a crust with mostly felsic lithology. The new methodology provides a robust workflow for determining the geothermal potential in areas with limited direct temperature measurements, facilitating knowledge creation for the transition to sustainable energy sources and energy self-sufficiency.