ANALISI MORFOTETTONICA ED EVOLUZIONE QUATERNARIA DELLA VAL D’AGRI, APPENNINO MERIDIONALE

Abstract

The Agri River extends for about 100 km from the axial zone of the Southern Apennines to the Ionian coast of the Basilicata region,
Southern Italy, flowing with a mean E-W direction and crossing both the internal and external geological domains of the NE-verging
fold-and-thrust Apennines belt. The Agri Valley develops along one of the most tectonically active regions of southern Italy, which has
been struck by recurrent and large seismic events in historical times, such as the 1857 Basilicata earthquakes. Based on the great sensitivity of the alluvial rivers to the variations in topographical gradient, a morphotectonic analysis has been carried out on the Quaternary alluvial, erosional and marine terraces occurring along and at the mouth of the Agri River. The preliminary results allow us to divide the Agri Valley into three reaches (sectors A, B and C), bounded by two major tectonic structures, the Armento Thrust and the Scorciabuoi Fault and characterized by different deformation and morphogenic processes. The orders of terraced surfaces have been labelled with roman numbers, starting from the youngest and lowest terrace (I) to the older ones. Except terrace I that has been observed throughout most of the valley, all other terraces are followed by a letter, indicating the sector of the Agri Valley where they occur (a, b or c) if of fluvial origin, or by the letter "m" when they represent marine terraces. The Quaternary morphological and sedimentary evolution of high Agri Valley (sector A) has been controlled mainly by the persistence, until Present time, of an important threshold occurring at the nucleus of the anticline associated to the E-verging Armento Thrust. During the Middle-Late Pleistocene, the beginning of the entrenchment of the threshold caused the progressive outpouring of the ancient Pertusillo lake, previously generated by the growing anticline, with the formation of the wide terrace (III-a) representing the top of the lacustrine deposits. On the other hand, the alluvial terraces I and II-a are possibly related to as many, probably fault-related, uplifting phases which occurred in Late Pleistocene-Holocene time. The middle Agri Valley (sector B), crossing the Middle Pliocene-Middle Pleistocene S. Arcangelo Basin, is eastward bounded by the NWSE-trending strike-slip Scorciabuoi Fault. Four orders of asymmetrical Middle Pleistocene-Holocene alluvial terraces, showing a clear antiform warping, have been recognized in this sector. The deformed terraces, together with the shifting of the entrenching channel toward the opposite northern bank of the river, suggest that the uplift and the warping processes are likely to be associated to a blind thrust or, alternatively, to the tectonic activity of the Scorciabuoi Fault. The occurrence of a seismogenic fault in the central part of the Agri Valley would account for the strong damages (I=X MCS) undergone by the villages of Aliano and Alianello during the 1857 Basilicata earthquake (M=6.5). Finally, the low Agri Valley (sector C) is characterized by the occurrence of both alluvial and marine terraces showing a good morphological correlation. The morphotectonic analysis of the marine terraced sequence, considered as the result of the interaction between tectonic uplift and Middle-Late Quaternary eustatic changes, pointed out a non-uniform tectonic uplifting rate of the coastal area, the magnitude of which clearly decreases toward the front of the Apennines orogenic belt. Moreover, the vertical distribution of the palaeo-shorelines of the marine terraces allowed us to estimate uplift rates ranging from about 0.2 mm/a to more than 2.5 mm/a, which are more typical of fault activity rather than regional uplift.