The long-term evolution of Monte Marine and Monte Pettino seismogenic faults: tectono-stratigraphic, isotopic, and chronological constraints

The central Apennines are a Cenozoic fold-and-thrust belt that has been affected by post-orogenic extension in its axial region since the end of the early Pliocene (ca. 4 Ma). Post-orogenic extension generated several intermontane basins bounded by high-angle normal faults, striking NW-SE, subparallel to the backbone of the chain. The Monte Pettino and the Monte Marine seismogenic faults (MPF, MMF) are the boundary faults of the western portion of the late Pliocene-Quaternary L’Aquila intermontane basin. Their long-term activity is typified by exhumed fault cores that coexist with active fault strands localised at the fault hanging walls, providing evidence of a polyphase tectonic activity. The fault cores are decorated by diffuse dolomitization, which indicates structurally controlled fluid-flow and metasomatism. To constrain the long-term (space-time) evolution of the MPF-MMF faults, we integrated fieldwork, stable isotope systematics (δ¹⁸O, δ¹³C and Δ⁴⁷), carbonate thermoluminescence and U-Th dating. Our results highlight two main tectonic phases, with different structural evolution and fluid-rock interaction. The first phase corresponds to the development of a major cataclastic zone, defined by meter-thick, SW-dipping (65-70°), fault cores exposed at the piedmont of the MPF-MMF ridges. The C-O systematics of the cataclasite and of the associated calcite slickenfibers, which are in the range of the carbonate bedrock, indicate a "closed" system behaviour during fault nucleation and development. Preliminary results from Δ⁴⁷ thermometry of syn-kinematic carbonate structures indicate temperatures of 34 ± 2 °C. Thermoluminescence dating of dolomite clasts in the fault zone indicates age in the range of 3.0 – 3.4 Ma, whilst the cataclastic fault core is younger (< 800 ka). The second phase is mainly recorded in upper Pleistocene sedimentary Breccias (ca. 350 ka) which unconformably cover the bedrock and the exhumed fault cores at the SE termination of the MPF. It consists of anastomosed, high-angle WNW-ESE striking fault strands, spaced meters apart and with cm-m displacements, associated with carbonate veining and travertines. Stable isotopes measured from the fault slickenfibers, carbonate veins and travertines show negative δ¹³C and δ¹⁸O values, suggesting a depositional system dominated by meteoric fluid ("open" system) with an important contribution of organic carbon. Travertines and veins precipitated at colder temperatures (12 ± 4 °C), in the range of the average local air temperatures, thus excluding precipitation from a hydrothermal circuit. Moreover, their U-Th ages range between 182 and 331 ka, compatible with the temporal constraints from stratigraphic data. Structural and isotopic results do not support tectonic reactivation of the cataclastic core of the MPF during the middle-late Pleistocene, confirming the stratigraphic evidence. Our results provide the first absolute age constraint on the post-orogenic extensional faulting in the L’Aquila basin, demonstrating a two-stage fault activity, characterised by a change from localised (from ca. 3 to ca. 0.8 Ma) to delocalised faulting (200-300 ka to present). We infer that this change in the style of extensional faulting was consequence of the evolving rheological structure of the fault zones, primarily regulated by the feedback and interactions involving structurally-controlled fluid flow, rock metasomatism and cataclastic processes in space and time.