Evoluzione geomorfologica polifasica e tassi di sollevamento del bordo sud-occidentale dell'alta Val d'Agri, (Appennino meridionale)

Geomorphic and tectonic features from the western flank of the upper Agri River valley in the southern Apennines have been examined to determine regional and local uplift rates in the geodynamic framework of southern Italy. The high valley is a NW-SE trending fault-bounded intermontane basin located in the Lucanian segment of the southern Apennines, along the axial zone of the chain. The basin has an area of about 540 km2 large (a little more than 1/3 of the whole catchment basin) and formed during Quaternary times. Tectonics has strongly controlled both geomorphological and sedimentary evolution of the basin up to the present. As a matter of fact, the valley has been affected by large earthquakes such as the 1857 Basilicata earthquake and the basin floor is filled by middle-upper Pleistocene faulted alluvial deposits. On the grounds of recent geomorphological and structural studies (DI NIRO & GIANO, 1995; GIANO et alii, 1997; 2000; SCHIATTARELLA et alii, 1998; CELLO et alii, 2000) the valley appears to be a more complex structure than an extensional graben or than a simple pull-apart basin as suggested by some authors. In any case, the last generation of lineations on fault planes documents an extensional regime with a NE-SW tensile axis. Such a tectonic regime persists until the present day, as inferred by the regional seismicity and in situ stress measurements (AMATO & SELVAGGI, 1993; AMATO & MONTONE, 1997) and as proved by the occurrence of palaeosols affected by normal faulting (GIANO et alii, 2000). The pre-Quaternary bedrock is constituted of Mesozoic-Cenozoic shallow-water and slope carbonates (Monte Marzano-Monti della Maddalena Unit, after BONARDI et alii, 1988b), prevalently outcropping along the western side of the basin, thrust over coeval pelagic successions (Lagonegro units, SCANDONE, 1975; PESCATORE et alii, 1999) which crop out mainly along the eastern flank. Toward the east and south-east the bedrock is formed by Tertiary siliciclastic sediments (Albidona Fm, Gorgoglione Flysch) which occupy the southern part of the high valley (CARBONE et alii, 1991). The Quaternary sediments are entirely constituted of continental clastics, represented by lower to upper Pleistocene slope coarse grained deposits, which form coalescent fans along the flanks of the basin, and by middle Pleistocene alluvial deposits («Complesso Val d’Agri», DI NIRO et alii, 1992) in the plain. Since the plain is deeply dissected by recent fluvial erosion, it is possible to observe, at several points, more than 100 m of the alluvial sequence. The age of the Quaternary sediments has been deduced by correlating some morphostratigraphic features of the upper Agri valley with the post-Sicilian features from the nearby Sant’Arcangelo Pliocene-Pleistocene basin (DI NIRO et alii, 1992). Four orders of polygenic landsurfaces, of which relics are well preserved along the water divide and suspended on the present valley floor, are recognizable in the high valley. Two of them are erosional (S1 and S2) whereas the lower ones (S3 and S4) are both depositional and erosional. According to previous regional interpretations (BRANCACCIO et alii, 1991; SANTANGELO, 1991; AMATO & CINQUE, 1999; SCHIATTARELLA et alii, 2003a), the ages of these landsurfaces are included in a time span ranging from 1.8 to 0.125 Ma. A more ancient landsurface, probably sculptured during late Pliocene times, can be also recognized in the neighbouring areas of the Campania-Lucania Apennines. Using geomorphic features of this kind, and Pleistocene deposits involved in the genesis of erosional and depositional landsurfaces, a set of uplift rates has been calculated for the western flank of the valley. The values of the Quaternary local uplift rates may vary from a minimum of 0.3 mm/yr to a maximum of about 1 mm/yr whereas the values of the regional uplift rate are always equal or higher than 1 mm/yr in the last 1.2 Ma. All these data are in good agreement with those from other areas of southern Apennines (AMATO, 2000; SCHIATTARELLA et alii, 2003a). Due to high slip rates on fault planes (0.5 to 0.8 mm/yr in the 1.2-0.73 Ma time span) the major part of the relief energy can be ascribed to the activity of basin-border faults. Yet, the local morphostructural offsets have to be coupled with the regional uplift of the orogen to reach the total amount of Quaternary uplift. An acceleration of the local component of vertical motion starting from the early-middle Pleistocene can be also deduced. It is noteworthy that during late Pleistocene to Holocene times the same fault system was characterized by strongly reduced slip rate of up to 0.1 mm/yr.