Reconstructing fluid circulation and temperature evolution in fault zones in space and time

This PhD thesis focuses on reconstructing fluid circulation and temperature evolution in fault zones through space and time. The primary aim is to develop a novel approach for assessing the isotopic evolution of fluids in clay-rich faults, enabling the construction of time constrained conceptual models that capture variations in temperature and fluid composition throughout the tectonic activity of the fault systems. These models describe the evolution of fluid isotopic composition in relation to progressive fluid-rock interaction processes, considering both the timing of fault activity and the structural evolution of the rocks. The methodological framework combines field observations, microstructural analysis, X-ray diffraction, K-Ar geochronology, stable isotope (O and H) analyses, and temperature estimates. With the goal of creating models that are as broadly applicable as possible, the core of this thesis (Chapters 3, 4, and 5) is dedicated to three case studies of regional-scale fault system with different tectonic settings, fluid regimes, and kinematics, and corresponds to papers either published, submitted, or in preparation. The Lemnos Fault System (Aegean Sea, Greece) – Chapter 3 – represents an extensional environment with intense hydrothermal alteration, where faults deform sedimentary and volcanic units at shallow crustal levels. The Carboneras Fault (SE Spain) – Chapter 4 – exhibits multistage compressive and strike-slip kinematics and features clay-rich gouges recording predominantly meteoric fluids. Finally, the Mykonos detachment (Aegean Sea, Greece) – Chapter 5 – is a long-lived extensional detachment associated with the exhumation of a synkinematic pluton and hydrothermal alterations. These three main chapters are preceded by a general introduction, which provides an overview of the current state of the art, and are followed by a general discussions and conclusions. The research led to the development of PolyFrac, an Excel-based tool that quantitatively reconstructs fluid histories by integrating isotopic, mineralogical, and thermal data from polymineralic fault rocks. The main results include (i) a detailed characterisation of hydrothermal alteration and fluid-rock interaction in Lemnos, (ii) the reconstruction of isotopic fluid evolution in the Carboneras Fault, and (iii) the first absolute deformation ages and structural evolution of the brittle Mykonos detachment. By investigating diverse fault kinematics and fluid sources in long-lived regional fault systems, this research provides a robust framework for understanding fluid-rock interactions and for establishing solid basis for future studies linking fault activity to the evolution of circulating fluids.