The importance of structural complexity in the localization of geothermal systems: A case study along the Vallès-Penedès Fault in the Catalan Coastal Ranges (NE Spain)



Mitjanes, G.a,b,, Walsh, J.J.c, Roca, E.a, Alias, G.d, Queralt, P.1, Ledo, J.e, Pina-Varas, P.a
a - Departament de Dinamica de la Terra i de l’Ocèa, Universitat de Barcelona, C/ Martí i Franquès s/n, 08028 Barcelona, Spain.
b - Sustainable Minerals Institute, The University of Queensland, St Lucia, QLD 4067, Australia.
c - Fault Analysis Group and Irish Centre for Research in Applied Geosciences (iCRAG), UCD School of Earth Sciences, University College Dublin, Belfield, Dublin 4, Ireland.
d - Departament de Mineralogia, Petrologia i Geologia Aplicada, Universitat de Barcelona, C/ Martí i Franques s/n, 08028 Barcelona, Spain.
e - Departamento de Física de la Tierra y Astrofísica, Facultad de Física, Universidad Complutense de Madrid, Madrid, Spain.


Abstract - The Vallès-Penedès Fault is a Neogene normal fault marked by the presence of two established geothermal systems at La Garriga-Samalús and Caldes de Montbuí, within the Catalan Coastal Ranges (NE Spain). The analysis and collation of existing and new geological and geophysical datasets provide the basis for the development of an improved conceptual model that explains the presence and localization of hot geothermal fluid systems at relatively shallow depths (e.g., 60 oC at surface and 90 oC at 1 km). Geothermal flow is concentrated within Paleozoic granodiorites of the immediate footwall of the V-P fault, host rocks that are susceptible to faultrelated fracturing, and the generation of both extension and hybrid fracture systems in association with active fault 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 relay development and breaching characterizing host rocks by high fracture intensities and fault rock development on a fault that locally has a vertical displacement of over 1.5 km. Accentuated fracturing and deformation are consistent with strain localization predicted by existing models for the development of fault zones along normal faults. The plumbing of the geothermal systems is attributed to up-fault flow in combination with lateral flow controlled by the intersection of the V-P fault with a low-angled Paleogene thrust defining the base of the hostrock granodiorites, with the geothermal systems localized at the distal end of the thrust. Sustained geothermal flow is attributed to groundwater flow circulation associated with seismic pumping and valving of warmer and deeper fluids, and the ingress of groundwater along faults and within fractured basement rocks.

Geothermics, 116, 102855. doi: https://doi.org/10.1016/j.geothermics.2023.102855, 2023.