Hydrothermal silicification and active extensional faulting: Insights from the Kornos-Aghios Ioannis fault, Lemnos Island, Greece

Abstract Fault-related diagenesis critically influences the petrophysical and mechanical properties of fault zones, affecting fluid circulation and seismicity in the upper crust. Questions remain about the composition of the mineralizing fluids and the role of fault architecture in controlling the distribution of hydrothermal silicification. This study addressed these questions by establishing the source of hydrothermal fluids and documenting the spatial variability of synkinematic hydrothermal products along the Kornos-Aghios Ioannis fault (Lemnos, Greece). The Kornos-Aghios Ioannis fault is an ~10-km-long extensional fault system that deforms, at shallow depth (<1 km), Eocene–Miocene sandstone turbidites and volcanic rocks. Our multidisciplinary approach, combining structural analysis, petrography, mineralogy, geochemistry, and fluid inclusion microthermometry, revealed that silicification occurred in an epithermal environment assisted by the circulation of acid-sulfate hydrothermal fluids (temperature ~160–170 °C). Mineralizing fluids were predominantly meteoric in origin, with a possible residual magmatic component. The intensity of silicification decreases with distance from the principal slip surface, forming up to 250-m-thick rock volumes that, locally, outsize the width of the fault damage zone. Along the fault strike, only an ~4-km-long fault segment exhibits silica cementation. This along-strike variability is best explained by the long-term evolution of the fault system. Kilometer-scale soft- and hard-linkage patterns combined with second-order geometric complexities (e.g., breached relay ramps and tips) localized fluid flow and promoted extensive alteration. Our findings have implications for the understanding of the permeability structure of silica-cemented fault zones, with relevance for the assessment of seismic hazards and exploration of natural resources in siliciclastic rocks.