Integrated geophysical and geological surveys for 3D modeling of complex geological structures: an application to the study of active faults in the southern Apennines (MOSAICMO Project)

The Molise-Sannio region, in the axial portion of the Southern Apennines (Italy), is a fold-and-thrust belt where the Late Miocene to Early Pleistocene compressional tectonics has been overprinted by a younger extensional stress regime responsible for a significant degree of seismicity, and which is coexisting with strike-slip faulting to the north-east. Active faults in this area are known to be capable of generating M6+ earthquakes. The goal of the MOSAICMO project (Molise SAnnio integrated crustal Model) is to develop a comprehensive multiscale crustal model of the Molise-Sannio region by combining seismological, geophysical and geological data, with a specific focus on the Quaternary intramontane Bojano basin (BB). The latter is a NE-trending depression whose genesis is debated, since according to recent studies it appears to be controlled by a system of NE-dipping active fault segments present on the southern side, while other studies claim the importance of SW-dipping faults on the other side of the basin. Indeed, the subsurface geometry and deep structure of the BB are poorly constrained by available geological data, which hampers a correct recognition of the master faults and their possible seismogenic significance. Resolving this ambiguity is a priority task that can be accomplished through an integrated geological and geophysical approach. In this project framework, multi-disciplinary geophysical studies were conducted to study the BB at different scales and resolutions, by interpreting subsurface geophysical parameters (e.g. electrical resistivity, seismic velocities, etc.) in terms of lithology and mechanical properties. Electrical methods have proven to be a powerful tool in imaging complex subsurface geology. By measuring the resistance of subsurface materials to electrical current flow, these methods can differentiate between various geological structures such as faults, basin infill sediments and basement rock types, providing high spatial resolution and significant investigation depth. 3D electrical resistivity tomography has often been used in recent years to image conductive bodies covering high-resistivity structures, such as tectonic basins or hydrothermal systems in volcanic regions. Here, we present a challenging case study for 3D geoelectrical imaging: a continental tectonic basin filled with low to moderately resistive sediments emplaced on conductive clayey-arenaceous rocks. The integration of different resistivity data (ERT and ResLog) with other geophysical methods, like seismic and magnetic surveys, further refines subsurface imaging, ensuring robust and reliable geological interpretations. We present the first 3D electrical resistivity model of the BB, down to 500 m depth, complemented by several 2-D ERT profiles calibrated with shallow boreholes. Subsurface geophysical models were further constrained by a scientific drilling, 170-m-deep, that we performed also to obtain new stratigraphic and geochronological data on the basin sedimentary sequence. This represents an important contribution to the understanding of the regional seismotectonic setting and, locally, the seismogenic sources surrounding the BB.