The multi-Ma history of the Fucino lacustrine basin (Central Apennines, Italy) inferred from the cores of the deep Avezzano borehole: implications for post-orogenic extension and active tectonics

A 271 m-deep borehole was drilled within the town of Avezzano, near the NW margin of the Fucino basin, in the core of the central Apennines of Italy. The basin is a post-orogenic extensional structure controlled by two, nearly orthogonal normal fault systems: one along the northern side of the basin, SSE-dipping, and one along the eastern side of the basin, SW-dipping. The SW-dipping normal fault system sourced the devastating 1915 M7.0 Fucino earthquake. This investigation, conducted as part of a seismic microzonation study, provides critical insights into the temporal evolution of post-orogenic extension and active tectonics in the central Apennines.

In W-E direction, seismic reflection data reveal a semi-graben geometry characterized by a wedge-shaped sedimentary infill thickening eastward toward the main SW-dipping normal fault. The borehole penetrated the western, thinner sector of this sedimentary wedge, reaching the pre-graben bedrock at approximately 270 m depth. The succession comprises continental deposits extending to at least 255 m depth, underlain by marine siliciclastic bedrock. From the surface to ~140 m, continuous and well-preserved lacustrine deposits were recovered. Between 140 and 186 m, core recovery was discontinuous, revealing low-energy lacustrine sediments interbedded with coarser layers. Below 186 m, predominantly coarse-grained deposits caused difficult core recovery. The bedrock, encountered at 269.5-271 m, consists of overconsolidated clays with thin sandy interbeds displaying ~45° dipping bedding planes characteristic of flysch-like rocks.

Comprehensive lithostratigraphic, tephrochronologic, paleomagnetic, and palynological analyses were conducted. Nine tephra layers were identified within the uppermost 40 m, three of which were numerically dated. Paleomagnetic and palynologic sampling was performed at high resolution (2-10 cm intervals), complemented by magnetic susceptibility measurements at 2 cm resolution.

The integrated multiproxy dataset yields a preliminary age model indicating that the lacustrine basin is at least 4.6 Ma old, providing unprecedented temporal constraints on the onset of post-orogenic extension in the central Apennines. This age significantly predates previous estimates. The chronostratigraphic model enables reconstruction of the sedimentary infill history and constrains variations in the activity of the Fucino fault systems through time, providing data for evaluating the temporal evolution and long-term behavior of post-orogenic normal faulting over a multi-Ma timeframe. Micropaleontological analyses of the bedrock core helps in bracketing the age of the transition from compressional tectonics to post-orogenic extension. The results have implications for assessing long-term activity, slip rate and slip rate variability for the active fault system, for seismic hazard applications, while offering insights into the broader late- and post-orogenic evolution of the Apennines. These data support the ICDP Fucino paleolake project proposal MEME (Giaccio and the MEME Team, EGU 2026 Session ITS5.1/CL0.6).