Geological characterization of the Tertiary Piedmont Basin geothermal system: new insights from structural and stratigraphic analyses

The Tertiary Piedmont Basin (TPB) in northwest Italy is a wedge-top basin developed during Eocene—Pliocene times in the Alps-Apennines tectonic junction. It accommodates, on average, 3 km of clastic sedimentary units with significant lateral facies variations, and several basin-scale unconformities tectonically-controlled. The basin experienced deformation under markedly different tectonic regimes, developing long-lived kilometric structures that affected both the sedimentary successions, and the underlying metamorphic rocks of the Ligurian Alps. The presence of several thermal springs, relatively high surface heat-flow, and locally high geothermal gradient in the TPB, suggests a deep groundwater circulation and heating most likely in a reservoir hosted within the Alpine metamorphic rocks, i.e., the basement. The geothermal system of the basin is not fully understood, since it still lacks a comprehensive and detailed geological/geophysical model of the basin-basement present-day structure. Aiming to fulfill this gap, this study shows structural analyses performed in the TPB and its Alpine basement at different scales through field-based characterizations, Digital Outcrop Model-based fracture mapping, and seismic interpretation. The integration of these structural results coupled with the spatial distribution of the basement and overlying sedimentary cover, enables a preliminary evaluation of potential reservoir or seal units in the geothermal system. These outcomes provide an adequate conceptual model to better understand the geothermal systems of the TPB, and other systems in analogue settings, having geodynamic peculiarities like slab switches or brake-off.