An example of river pattern evolution produced during the lateral growth of a central polygenic volcano: the case of the Alcantara river system, Mt Etna (Italy)

Analysis of numerous geognostic soundings and geophysical investigation dat a, indicates the presence of a wide depression, identifiable as the ancient valley of the Alcantara River, below the northern flank of Mt Etna. Surfac e and subsurface data were used to reconstruct the complex stratigraphic su ccession filling the paleo-Alcantara valley, consisting of superimposed lav a flows, interlayered with thick lentiform elastic bodies. The evolution of the Alcantara river system was defined with reference to the different pha ses of growth of Mt Etna. The first correlation between volcanic activity a nd the drainage system refers to the initial phases of alkaline volcanism ( 170-100 ka). During this period, the southern watershed of the paleo-Alcant ara valley acted as a morphological barrier to the products of the various volcanoes which gradually grew in the sector now occupied by the Valle del Bove. The paleo-Alcantara riverbed was sporadically invaded by lava flows g enerated by peripheral activity of the central edifice. A radical modificat ion of the hydrographic pattern of the region began about 45-40 ka as a con sequence of the growth of the Ellittico volcano, the main eruptive centre o f the Etnean edifice. The gradual expansion of this volcano led, over a per iod between about 30 and 25 ka, to the northward deviation of the paleo-Alc antara riverbed. The consequent erosion of a section of the watershed with the adjacent hydrographic basin of the paleo-S. Paolo river resulted in the capture of the latter. From then on, the hydrographic pattern of the area gradually took on the characteristics it has today. A water circulation sys tem developed within the thick volcanic pile filling the paleo-Alcantara va lley, with the formation of an aquifer, which is today one of the most impo rtant hydro-structures of the whole volcanic edifice and is, in part, explo ited in the eastern side of Sicily. (C) 2001 Elsevier Science B.V. All righ ts reserved.