The importance of structural complexity in the localization of geothermalsystems: A case study along the Vallès-Penedès Fault in the Catalan CoastalRanges (NE Spain)

Abstract

The Vallès-Penedès Fault is a Neogene normal fault marked by the presence of two established geothermalsystems at La Garriga-Samalús and Caldes de Montbuí, within the Catalan Coastal Ranges (NE Spain). Theanalysis and collation of existing and new geological and geophysical datasets provide the basis for the developmentof an improved conceptual model that explains the presence and localization of hot geothermal fluidsystems at relatively shallow depths (e.g., 60 ◦C at surface and 90 ◦C at 1 km). Geothermal flow is concentratedwithin Paleozoic granodiorites of the immediate footwall of the V-P fault, host rocks that are susceptible to faultrelatedfracturing, and the generation of both extension and hybrid fracture systems in association with activefault displacements. Flow localization is enhanced further by the presence of fault-related structural complexities,with both systems marked by 300 m wide steps in the main fault trace. These are attributed to relaydevelopment and breaching characterizing host rocks by high fracture intensities and fault rock development ona fault that locally has a vertical displacement of over 1.5 km. Accentuated fracturing and deformation areconsistent with strain localization predicted by existing models for the development of fault zones along normalfaults. The plumbing of the geothermal systems is attributed to up-fault flow in combination with lateral flowcontrolled by the intersection of the V-P fault with a low-angled Paleogene thrust defining the base of the hostrockgranodiorites, with the geothermal systems localized at the distal end of the thrust. Sustained geothermalflow is attributed to groundwater flow circulation associated with seismic pumping and valving of warmer anddeeper fluids, and the ingress of groundwater along faults and within fractured basement rocks.