Integrating geophysical, geochemical, petrological and geological data for the thermal and rheological characterization of unconventional geothermal fields: the case study of Long Valley Caldera

We propose a novel multidisciplinary approach to image the thermo-rheological stratification beneath active volcanic areas, such as Long Valley Caldera (LVC), which hosts a magmatic-hydrothermal system. Geothermal facilities near the Casa Diablo locality supply 40 MWe from three binary power plants, exploiting about 850 kg s⁻¹ of 160–180 °C water that circulates within the volcanic sediments 200 to 350 meters deep. We performed a thermal fluid dynamic model via optimization procedure of the thermal conditions of the crust. We characterize the topology of the hot magmatic bodies and the hot fluid circulation (the permeable fault-zones), using both a novel imaging of the a and b parameters of the Gutenberg-Richter law and an innovative procedure analysis of P-wave tomographic models. The optimization procedure provides the permeability of a reservoir (5.0 × 10⁻¹⁴ m²) and of the fault-zone (5.0 · 10−14 – 1.0 × 10⁻¹³ m²), as well as the temperature of the magma body (750–800°C). The imaging of the rheological properties of the crust indicates that the brittle/ductile transition occurs about 5 km b.s.l. depth, beneath the resurgent dome. There are again deeper brittle conditions about 15 km b.s.l., agreeing with the previous observations. The comparison between the conductive and the conductive-convective heat transfer models highlights that the deeper fluid circulation efficiently cools the volumes above the magmatic body, transferring the heat to the shallow geothermal system. This process has a significant impact on the rheological properties of the upper crust as the migration of the B/D transition. Our findings show an active magmatic system (6–10 km deep) and confirm that LVC is a long-life silicic caldera system. Furthermore, the occurrence of deep-seated, super-hot geothermal resources 4.5 – 5.0 km deep, possibly in supercritical conditions, cannot be ruled out.