The Vallo di Diano Range-Bounding Fault-System (Southern Italy): New Evidence of Recent Activity From High-Resolution Seismic Profiling

Abstract

The Vallo di Diano is the largest intermountain basin in the Southern Apennines (Italy). The basin evolution was controlled by the Quaternary activity of a range-bounding, SW-dipping normal fault system located to the east (Vallo di Diano Fault System, VDFS). Geological and oil industry data define the sin-sedimentary activity of the VDFS up to the Middle Pleistocene. However, commercial profiles do not resolve the shallower, eastern portion of the basin, due to strong lateral heterogeneities and unfavourable surface conditions. Therefore, Late Pleistocene-Holocene activity of the VDFS and its seismogenic potential are still uncertain. To better constrain the shallow structure of the basin, we performed four high-resolution seismic surveys, along its eastern side, where slope breccias and fans cover the Mesozoic carbonate bedrock and bury the VDFS. We also investigated some NW-trending flexures affecting Late Pleistocene fans, that we had previously detected and dubitatively ascribed to recent faulting. Seismic data were acquired with a dense wide-aperture geometry. Two high-resolution (HR) NE-trending profiles, about 1.5 km long, were collected using respectively 5 m and 10 m spaced receivers and sources. Two very high-resolution (VHR) NE-trending profiles, 400 and 350 m long, with densely spaced sources (4 m) and receivers (2 m) were also collected. HR profiling was aimed at imaging alluvial fan thickness and morphology of the underlying carbonate bedrock. VHR surveys targeted the flexures and their possible origin. All lines were acquired with a HR vibroseis source, except for the shortest profile, where we used a buffalo-gun, better suited for very near-surface imaging (z < 50 m depth). Seismic imaging consists of reflectivity images obtained by CDP-processing of reflection data complemented by Vp images obtained by multi-scale seismic tomography. The stack sections illuminated the basin down to 0.4-0.5 s TWT and reveal an array of high-angle, generally SW-dipping faults dissecting the bedrock and the alluvial fans. Faulting created accommodation space in the hanging-wall and displaced the different fan generations. Clear reflection truncations in the stack-sections correspond to significant Vp lateral changes in the tomographic images. VHR tomography is well defined along the shortest line down to 40 m depth, where two steps within slope breccias are visible. Moreover, two low-velocity wedges (colluvial packages) are imaged in the near surface (5-20 m depth). These data support recent faulting consistently with surface geomorphic features. We interpret these fault structures as splays of the range bounding master fault. Comparison with commercial reflection profiles nearby reveals a great improvement in seismic imaging achieved by HR surveys, which allow a detailed seismostratigraphic analysis of the basin.