STUDY OF THE GEOLOGICAL EVOLUTION AND STRUCTURAL PATTERNS AT DIFFERENT SCALES CHARACTERISING THE TRES VIRGENES REGION (MEXICO) TO EVALUATE THE GEOTHERMAL ENERGY POTENTIAL.

Faults play a fundamental role in sustaining fluid circulation across volcanic-geothermal regions, where volcanic rocks commonly exhibit low porosity and permeability. In fact, most of geothermal resources production is often connected to faults and fault zones. Thus, a precise knowledge of deformational patterns is of paramount importance to extend the assessment of the geothermal potential to areas surrounding a producing field, and to find strategies to enhance energy production. Increasing electrical energy production and using geothermal energy to desalinate seawater have recently become main concerns of the Mexican authorities with regards to the Tres Vírgenes region (Baja California, Mexico), where the Las Tres Vírgenes geothermal field represents the only producing zone across the Baja California peninsula, which nowadays struggles with energy and water supplies due to the recent growth of tourism. The herein presented PhD research places into this scenario, by means of the CeMIE Geo project P15 of SENER-CONACyT, which was appointed by the Mexican authorities to UNAM (Universidad Nacional Autónoma de México) and sponsored this Thesis. In order to evaluate the geothermal prospectivity of areas surrounding the currently producing Las Tres Vírgenes field and the role of structural patterns affecting the Tres Vírgenes region in sustaining hydrothermal fluid migration a multi-disciplinary approach has been deployed, including a wide range of methodologies and receiving contributions from several branches of the Earth Sciences. Five studies have been performed, whose results are presented in this Thesis under the form of papers published or submitted to dedicated ISI-Journals: 1) A geological study on La Reforma caldera complex, resulting in a 1:50,000 scale geological map produced with modern survey methodologies and representing an important contribution concerning the geological evolution of the Tres Vírgenes region and the Quaternary volcanic processes linked to the exploitation of geothermal resources in the Las Tres Vírgenes field. 2) The investigation of the structural patterns affecting the Tres Vírgenes region, by coupling observations at the meso- (field data) and micro-scales (image analysis of fault-related micro-fractures and veins), resulting in a structural model for the Tres Vírgenes region. This structural model highlights the strong control of regional-scale strike-slip oblique structures linked to the Gulf of California on Quaternary volcanic processes and the interplay between regional and volcanic structures in exhuming the Cretaceous basement throughout the region. 3) The assessment at the micro-scale of geometrical and chronological relationships between faulting and multiple fluid circulation events depositing mineralization along fault planes, providing further support to the proposed structural model and resulting in a deeper understanding of fault-related mechanisms across the Tres Vírgenes region. Fractal statistics applied to fault-related deformation patterns indicates and quantifies the self-similarity (fractal charcter) of regional structures at the scale of the Tres Vírgenes region, possibly extending to this sector of the Gulf of California. 4) Multi-scale petrographic and geochemical investigations, determining the composition, origin and temperature of fluids circulating and depositing calcite along regional faults. The integration of different techniques working at different scales defines a novel methodology, which was for the first time applied in a volcanic-geothermal setting in this Thesis. Fluids circulating along regional faults at different temperatures and redox conditions display a geothermal-hydrothermal component and reach maximum temperatures (100°C) in proximity of the Tres Vírgenes feeding system, acting as the heating source for the hydrothermal fluids and defining the areas where fault-sustained hydrothermal circulation is more likely to occur. 5) The quantification of physical properties of undeformed and faulted volcanic rocks through triaxial tests, X-ray computerized tomography (CT) and coupled permeability simulations, suggesting the highly unlikeliness of vertical and lateral fluid migration through the exposed undeformed Quaternary volcanic rocks in the areas surrounding the Las Tres Vírgenes field. The estimate of along- and across-fault fluid flow potential, and of changes in the interconnected pore space and fault-permeability once these volcanic rocks are involved within regional damage zones, confirms the role of regional structures in focusing hydrothermal fluid circulation across the Tres Vírgenes region. The novel and quantitative findings of the several studies constituting this Thesis bear multifaceted implications: of regional and industry-related importance concerning modelling of fluid flow through the Tres Vírgenes and other geothermal regions worldwide, and more far-reaching concerning upper-crustal deformation processes in volcanic and non-volcanic settings.