3D Model of the active extensional fault system of the high Agri River valley, Southern Apennines, Italy

The high Agri River valley (southern Apennines, Italy) hosts an active fault system, and a severe earthquake struck the area in 1857 (equivalent magnitude Me = 6.98; Imax = XI). In this study, we integrated surface geology with available subsurface data in order to propose a new 3D structural model for the high Agri River valley fault system. The model presented here is based on a series of published deep geological sections across the study area, using the software 2D Move and 3D Move (Midland Valley Exploration Ltd.). It displays the active extensional fault system dissecting the pre-existing southern Apennines fold and thrust belt. This region is characterised by the tectonic superposition of carbonate platform and pelagic basin allochthonous units onto a several km-thick carbonate platform succession representing the underthrusting foreland sequence. The detachment between the allochthon and the buried carbonate platform succession is marked by a several hundreds of metres thick mélange zone which consists mainly of intensely deformed and overpressured deepwater mudstones and siltstones. The 3D model shows that, at surface, the valley is asymmetric, with the most relevant normal faults, as well the maximum thickness of Quaternary alluvial deposits, located along its northeastern side. Locally, minor antithetic faults generate secondary grabens. All these faults occur only in the shallow brittle layer. At depth, a normal fault cuts the top of the deeper unit just below the central part of the Quaternary plain, and does not continue in the tectonic units above. This SW-dipping feature, extending for about 20 km into a WNW-ESE direction, appears to be the most probable deep structure responsible for the development of the active fault system. Therefore, it is also likely to be the source of strong earthquakes such as the 1857 event. The 3D model outlines the mechanically fundamental role played by the ductile mélange zone, which controls the modes of fault propagation from the deep brittle layer to the shallow (allochthonous) one.