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Differences in REE patterns of calcite from extensional and shear veins of the Sestola Vidiciatico Tectonic Unit in Northern Apennines suggest variations in fluid source during the seismic cycle in an ancient analogue of a shallow megathrust (Tmax 100°-150°C). In shear veins, a positive Eu anomaly suggests an exotic fluid source, likely hotter than the fault environment. Small-scale extensional veins were derived instead from a local fluid in equilibrium with the fault rocks. Mutually crosscutting relations between two extensional veins sets, parallel and perpendicular to the megathrust, suggest repeated shifting of the σ 1 and σ 3 stresses during the seismic cycle. This is consistent with: (i) a seismic phase, with brittle failure along the thrust, crystallization of shear veins from an exotic fluid, stress drop and stress rotation; (ii) a post-seismic phase, with fault-normal compaction and formation of fault-normal extensional veins fed by local fluids; (iii) a reloading phase, where shear stress and pore pressure are gradually restored and fault-parallel extensional veins form, until the thrust fails again. The combination of geochemical and structural analyses in veins from exhumed megathrust analogues represents a promising tool to better understand the interplay between stress state and fluids in modern subduction zones. Supplementary material: Cathodoluminescence microphotographs, methodological details on the microstructural analysis, microphotographs of the location of analysed spots and the geochemical data table and are available at https://doi.org/10.6084/m9.figshare.c.4842165 Thematic collection: This article is part of the Polygenetic melanges collection available at: https://www.lyellcollection.org/cc/polygenetic-melanges