Evolution of karstic, intermontane plateaux and basins mimicking fault activity: the Rocca di Mezzo case study in the central Apennines, Italy

This study investigates the Rocca di Mezzo area in the central Apennines (Italy) to evaluate the presence of a hypothesized active and capable normal fault underneath a school edifice. A multimethod approach was applied, including geologic field data, borehole data analysis, paleoseismological trenching, stratigraphic analysis. Field observations indicate the absence of structural features affecting the Meso‑Cenozoic bedrock associated to extensional faulting in the sector where the supposed fault should be located. Only structural evidence of reverse faulting related to an inactive compressive tectonic phase was found. Stratigraphic data also indicate that the Meso‑Cenozoic bedrock dips gradually westward, buried by Quaternary sediments in the area where the fault was hypothesized, so that the lateral contact between the bedrock and the Quaternary sediments is just related to the stratigraphic setting. Trench excavations across the southern sector of the suspected fault trace revealed lacustrine sequences, with interbedded paleosols, undisturbed by any fault planes. Radiocarbon and archaeological dating confirm the Holocene age of these lake deposits, consistent with other nearby lacustrine sequences, indicating that multiple sectors of the plateau hosted small lakes during the Holocene whose oscillations have been controlled by karstic‑related processes. This oscillation has determined phases of deposition and erosion that have strongly conditioned the evolution of this part of the plateau. The present‑day geomorphic characteristics of the area, including ponds and numerous dolines, suggest that karstic drainage and local erosion controlled the apparent deepening of the bedrock. The deepening is not therefore caused by the activity of the presumed fault, but it is just caused by local erosion led by karstic processes. These results demonstrate that indirect data or few hints of supposed fault activity can lead to misinterpretation of the presence of active faults. Misinterpretation can be solely ruled out by integrating multiple lines of geological evidence framed in a comprehensive neotectonic analysis, which allows resembling the whole geological evolution of an area over long time spans.