Ricostruzione di sequenze morfoevolutive nei Picentini meridionali (Campania, Appennino Meridionale)

Geomorphological and neotectonic evolution of Picentini Mts. (Southern Apennines, Italy)

Abstract

The Monti Picentini massif is one of the largest and most interesting morphostructural units of the Southern Apennines. It corresponds to a roughly rectangular horst (about 37 by 25 km), that rises several hundred meters above the surrounding hilly landscape where terrigenous units of Miocene age outcrop. Since the latter are much more erodible than the Mesozoic carbonate rocks forming the massif, the height of the marginal fault scarps often appears increased by differential erosion. This phenomenon is particularly common and relevant along the eastern and northwestern sides of the massif, which also correspond to the flanks of ancient structural depressions, whose soft terrigenous infilling was eroded during the neotectonic phases of uplift of the axial zone of the chain. The tectonic events that strongly influenced the long and complicated geomorphological evolution of the Monti Picentini can be divided into two groups: those that occurred before, and those that occurred after the modeling of a relatively much more mature erosional landscape (herein referred to as Palcosurface). This surface has had a polycyclic genesis: the early phases of modeling may date back as far as the Late Miocene (i.e., they were contemporaneous with the last compressive events of the Apenninic orogenesis), while the last periods of planation were probably synchronous with the first moderate episodes of extensional tectonism (Late Pliocene and Early Pleistocene). The present paper deals with the tectonic and geomorphological events that shaped the current landscape of the Monti Picentini after the modeling of the Palcosurface; i.e., during the subsequent period of strong and rapid vertical displacement herein labeled «Neotectonics». Despite the complete lack of Pleistocene marine deposits, it was possible to identify and to order three distinct phases of neotectonic deformation. The fault scarps created by the first phase are easily distinguished from the younger ones by their greater maturity. The high relief they created in the area (a dense network of scarps, some of which are up to 500 m high) promoted the production of huge conglomeratic deposits, both within the massif (outcrops of Iumaiano, Montenero, Rotundo, La Mola, Faito) and along its southern piedmont (Conglomerati di Eboli Auct.). The facies of those conglomerates are of talus and alluvial cone in the intra-mountain depressions and of alluvial fan, braided river bed, and alluvial plain in the piedmont area. The first generation of detrital deposits is restricted to the southern portion of the Picentini massif, which is composed of highly fractured dolostones and dolomitic limestones. In the northern portion of the massif, dominated by almost pure and less fractured limestone, karstic modeling prevailed over the fault-blocky landscape created by the first tectonic phase. Once the deposition of the first generation of conglomerate had stopped or nearly so, a second period of block faulting occurred (second neotectonic phase). It was characterized by smaller throws than the first phase (generally less than 200 meters), a much lower number of faults (broad sectors of the massif moved in blocks), and a new increase in the average elevation of the massif with respect to the surroundings. The throw of said dislocation was generally in the order of one or two hundred meters. Along the southern piedmont of the Picentini Mts. the thick and extensive Eboli formation was locally broken and tilted by faults. Also within the massif, the conglomerates of the first generation sometimes appear cut by fault scarps of the second tectonic phase. Another period of tectonic quiet followed in the area. It was accompanied by erosional processes (such as slope decline, broadening of valleys cut into fractured dolostones, and karstic planation within tectono-karstic depressions) producing localized gentle landscapes (Ripiani erosionali), which are often cut also on the conglomerates of the first generation (Rotundo, Varco Sellara, Castello di Olevano). Probably in the same period, an erosional glacis developed on the Eboli formation. In a few places within the massif, the second tectonic phase was followed also by deposition (lacustrine and fluvio-lacustrine in the Tizzano basin, alluvial in the Grottelle valley). Finally, we recognized a third neotectonic phase which again moved a few lines within the Picentini massif (the tectonic lake of Acerno was formed and the Picentino River valley deepened). In the piedmont area, the erosional glacis cut on the Eboli formation was fragmented into blocks and raised up to 400 meters a.s.l. An uplift of greater magnitude affected the nearby Picentini Mts., where the remnants of the gentle erosional landscape (Ripiani erosionali), initially beveled to the above-mentioned glacis, are now to be found at 600 to 1100 m a.s.l. This increase in relief caused, among other things, the headward downcutting of the deep canyon of the Tusciano River through the Mt. Raione block, which resulted in the capture of Acerno's lacustrine basin (about 0.35 M.y.B.P.). The results of the first absolute datings (40K/40Ar on pyroclastic intercalations of the 150 m thick lacustrine sequence) allowed us to estimate that the creation of Acerno's lake (i.e., the third neotectonic phase) occurred about 0.75 M.y.B.P. Since the tectonic events that created the lacustrine depression seem to have the same age as those which triggered the fluvial dissection of the damming block, we conclude that the duration of the lake of Acerno corresponds to the time the canyon took to be eroded. The geomorphological study conducted so far indicates that the third neotectonic phase represents the last significant period of deformation in the mountainous area, while evidence of two other periods of faulting are present on their lower piedmont. Downslope of this disturbed sector, the rest of the Sele River coastal plain is covered by Upper Pleistocene to recent marine, transitional and fluvial deposits, without evidence of important faulting. However, the elevation of some dated Tyrrhenian beaches (0.13 and 0.1 M.y. old) testifies that the area has recently experienced an almost uniform uplift of 15–20 m.