Structure of South Peruvian Upper Crust Revealed by Magnetotellurics and Shallow Seismicity: Implications for Geothermal Fluid Circulation and Crustal Temperatures

Active orogenic zones such as the Andes are rarely considered as potential geothermal resources, despite their high topographic gradient responsible for driving meteoric fluids and the presence of numerous hydrothermal springs. This study explores the different contexts of the emergence of hydrothermal springs in the Andean forearc in Southern Peru (Tacna region). New magnetotelluric data and the location of the 2022-2024 seismicity reveal the compartments of the upper crust where hydrothermal fluids circulate into two different contexts: (i) the volcanic system: beneath the volcanic arc, a seismic cluster (13-20 km deep) separates two conductive bodies, one spherical (5 km deep) and the second elongated (0.5-2 km deep), revealing a magma chamber and hydrothermal fluids over-pressurized and trapped under a clay cap. (ii) the orogenic system: beneath the forearc range (5 km high), an elongated conductive body (0-18 km deep) marks the location of the Calientes thrust, while the alignment of a triangular-shaped resistive body (1-18 km deep) and of a seismic cluster (down to 20 km deep) reveal the cold basement uplifted by the active faults of Incapuquio. The forearc basin and the underlying basement are revealed by a conductive and resistive body, respectively. Based on these results, we propose a conceptual model in which fluids infiltrating the forearc range use a network of crustal faults to be transferred at depth, dramatically disrupting the geothermal gradient laterally over distances of kilometers (18-150°C.km-1). They are then transferred to the forearc basin, where the main population centers are located.