Cross-strike structures in fold-and-thrust belts: tectonic evolution and circulation of fluids

Fold-and-thrust belts (FTBs) represent the typical structural expression of crustal shortening in the frontal domains of orogenic wedges, where deformation is mainly accommodated by thrusts and folds striking perpendicular to the orientation of regional compression. The linearity of these structures is often interrupted by high angle cross-strike structures (CSS) roughly parallel to the shortening direction. These features frequently inherit pre-orogenic elements and can exert a first-order control on shortening patterns and fluid migration within orogenic wedges, segmenting the belts into discrete structural domains. They can represent potential sites for geothermal exploitation or strategic mineral accumulation, act as structural or stratigraphic hydrocarbon traps or as zones of fluid leakage. CSS include belt-scale transverse zones (TZs), transfer faults confined vertically and horizontally by thrust surfaces, and early- to late-orogenic strike-slip faults. These types of structures strongly differ in terms of dimensions, geometry and maturity. Although their influence in the structural evolution of FTBs and in controlling fluid circulation within them is well established, temporal constraints on CSS tectonic activity, the nature of circulating fluids within them, and the architecture of the associated flow systems remain poorly documented. This PhD thesis explores the tectonic evolution and fluid-flow systems of CSS through the study of two study areas characterized by end-member CSS in terms of longevity, dimension, and tectonic role: the Central Southern Alps (CSA, Lombardy, Italy) and the Monte Conero Anticline (MCA, Northern Apennines, Italy). The CSA represent the retrobelt of the Alps, where large-scale, basement-rooted TZs originated during Mesozoic rifting stages and were subsequently reactivated during Alpine shortening, partitioning the belt into discrete structural blocks characterized by different shortening modes. In contrast, the MCA is the outermost exhumed anticline of the Northern Apennines, crosscut by limited-displacement strike-slip faults which do not influence the overall deformation style of the orogen at the belt scale. A multidisciplinary approach was adopted to constrain the tectonic evolution and the hydrological architecture of the study areas, by combining geological mapping, balanced cross-section construction, meso- and microstructural analyses, carbonate U–Pb geochronology, basin analysis, stable (δ¹³C–δ¹⁸O) and clumped isotope (Δ₄₇) analyses. Results from the CSA show that major TZs—such as the Faggio and Lecco Lines—record a polyphase evolution. U–Pb geochronology and tectono-stratigraphic observations reveals that these structures formed during Mesozoic rifting stages (Ladinian, Late Norian, Early Jurassic) and remained tectonically active throughout the entire Alpine orogeny, with U-Pb ages spanning over a period longer than 100 Ma. This temporal persistence underscores the fundamental role of TZs in accommodating deformation through multiple orogenic stages. Isotopic data from syn-tectonic carbonates reveal that fluids circulating along TZs derived from both deep and meteoric sources, defining open and vertically connected circulation systems. Inverted normal faults perpendicular to the shortening direction localized the ascent of warm fluids, likely guided by deep-seated fault zones inherited from the pre-orogenic rifting. Conversely, thrusts acted as closed systems dominated by formation waters buffered by host rocks, with local rising of deep, warm fluids. In the MCA, the integration of structural and geochemical data reveals a multistage deformation history where the limited-displacement strike-slip faults cutting the anticline do not sufficiently offset clay-rich, impermeable stratigraphic layers separating distinct reservoir to enable leakage or communication between them. In fact, syn-tectonic calcites mainly precipitated from locally buffered fluids under near thermochemical equilibrium conditions, confirming compartmentalized and poorly connected closed fluid systems. By comparing these two contrasting tectonic settings, this study demonstrates that the dimensions, geometries, inheritances, and tectonic roles of CSS fundamentally control both the shortening modes and the distribution of deformation and fluids within orogenic wedges. Large and long-lived TZs behave as efficient, permeable corridors connecting crustal levels through multiple tectonic stages, whereas young, poorly developed transverse strike-slip faults lack the continuity and displacement necessary to connect different fluid reservoirs. Overall, the findings of this thesis highlight the exceptional longevity of TZs, their inheritance and their fundamental control on deformation modes and fluid circulation throughout fold-and-thrust belts. Understanding these processes provides crucial insights into the role of CSS in governing deformation patterns and the hydraulic architecture of orogens, with broader implications for predicting fluid pathways during deformation, assessing ore deposit localization and seismicity, and evaluating carbonate reservoirs for resources exploitation and CO₂ storage.