Late Quaternary tectonics of the Vallo di Diano basin (southern Apennines, Italy)

Abstract

We discuss new data concerning the late Quaternary tectonics of the Vallo di Diano basin, the largest intramontane depression in the southern Apennines chain (Italy). The southern Apennines axis is currently subject to NE–SW extension. Here, active NW-trending normal faults are associated with strong earthquakes ( M > 6), with average recurrence times >1 ka. The Vallo di Diano basin is a half-graben bounded by an extensional fault system VDFS (Vallo di Diano Fault System), featuring three major NW-trending, SW-dipping, ∼15–17 km long fault segments. Holocene faulting and possible historical seismicity have been documented only for the northern fault segment. No major historical earthquakes can be associated with the other fault segments, and the recent sedimentary and structural evolution of the basin is still poorly constrained. As a consequence, its seismogenetic potential has probably been underestimated. We found evidence of late Pleistocene–Holocene faulting in the relay zone of the central and southern fault segments bordering the basin. Here, late Pleistocene volcaniclastic layers at the apex of an alluvial fan are affected by normal displacements on the order of ∼1 m. Additionally, in the same area we recognized a previously unreported, 9 km long and 0.5–1.4 km wide set of scarplets (ranging in height between ∼1 m and 2.5 m) affecting late Pleistocene–late Holocene alluvial fans. The last deformation is younger than 3 ka BP. Due to the geometric arrangement of these scarplets and the long-term displacement along the Vallo di Diano Fault System, we hypothesize that the central and southern fault segments belong to a ∼32 km long hard-linked system, capable of generating Mw > 6.5 earthquakes. The Vallo di Diano basin is thus bounded to the east for its entire length (∼45 km) by an active segmented extensional fault system. Therefore it has to be considered one of the major seismogenic sources in southern Italy.