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Fault inversion may lead to significant obliteration of earlier tectonic structures, thus preventing the straightforward interpretation of the complete kinematics and deformation history of faults. We adopt a multidisciplinary approach to: (1) reconstruct the tectonic evolution through space and in time of the extensionally-inverted Mt. Tancia Thrust (Central Apennines, Italy); (2) understand the deformation mechanisms and the transition and reactivation of compressional structures during negative tectonic inversion; and (3) constrain the origin of fluids involved during tectonic processes. To this end, we combined: (1) detailed geological mapping and multiscale structural analysis; (2) illite-smectite paleothermal indicators; (3) C, O, and clumped isotopes on calcite mineralizations and (4) K–Ar dating of authigenic and/or syn-kinematic illite from the Mt. Tancia Thrust fault rocks. We show that shortening occurred between ~9 and ~7 Ma, during possibly multiple events of fluid overpressure and shear rupturing involving fluids entrapped over long term within the host rocks. Post-compressive tectonic inversion occurred at ~3 Ma under fluid pressure fluctuations during shear events with an input of meteoric-derived fluids. Tectonic inversion is spatially confined within the first few metres below the thrust surface in a volume dominated by the partial overprinting, folding, transposition, and re-utilization of the earlier compressional fabric.

The Mt. Gorzano Fault (MGF) is a major seismically active extensional fault of the central Apennines, responsible for the destructive Mw 6.0 Amatrice earthquake in 2016. The MGF developed during post-orogenic extensional tectonics, generating a continental basin in the hanging wall. The age of the onset of the MGF and the relationship among faulting, fluid circulation, and the seismic cycles are unknown. We investigate these issues by studying the footwall damage zone of the MGF (exhumed from ~2-3 km depth), where extensional structures and related mineralizations cut and partially overprint the pre-existing compressional orogenic fabric. Structural and geochemical analyses (REE, O, C and clumped isotopes) combined with U-Pb dating of calcite mineralizations show that extensional deformation along the MGF began at least ~2.5 Myr ago. Its activity has continued to the present-day with a mean slip rate of ~0.9 mm/yr, consistent with other seismically active extensional faults of the central Apennines. During Apennine contractional tectonics, orogenic structures and tectonic overburden hindered the ascent of deep fluids, which, therefore, may have been progressively overpressured. We postulate that such overpressure occasionally or cyclically triggered impulsive deformation (earthquakes). During the subsequent extensional phase (younger ~2.5 Ma), pre- or co-seismic crustal dilation opened pathways for the ascent of deep mineralizing fluids, which impregnated the fault damage zone and may have triggered earthquakes. The crustal reservoir of carbonate-rich fluids nowadays is the source of water for the Acquasanta thermal springs close to the Mt. Gorzano seismic area.