An alternative method of azimuthal data analysis to improve the study of relationships between tectonics and drainage networks: examples from southern Italy

A detailed morphotectonic study of two intermontane basins located in the axial zone of the southern Apennines (Auletta and Valico di Prestieri basins) has been carried out to assess the relationships between Quaternary tectonics and fluvial network geometry. The comparison of morphological and structural features has been aided by an alternative method of morphotectonic data analysis based on the treatment of azimuthal populations by cumulative diagrams. The Auletta basin coincides with the present-day lower valley of the Tanagro River and is filled by upper Pliocene to lower-middle Pleistocene marine to continental sediments. Such deposits are tilted towards SW and dislocated by the 20 km-long NW–SE-trending fault bordering the huge carbonate morphostructure of the Alburni Mts. The complex evolution of the basin during the Quaternary has controlled the geometric arrangement of the main rivers of the area. The two major streams (i. e. Tanagro and Bianco rivers) appear to be structurally controlled by N120–140°-striking faults, as suggested by morphometric and geometrical (i.e. right-angled confluences and elbows, trellis-type fluvial pattern) characters of the net. In the southern flank of the valley, the preferred orientation of the I and II order fluvial channels (orthogonal to the Alburni Mts) indicates that the high relief and steep slopes of the Alburni positive morphostructure have been able to reduce the influence of the bedrock fracture pattern on the arrangement of minor streams. The Valico di Prestieri basin is a narrow trough located to the south of the Lauria Mts carbonate ridge in the western sector of the Calabria-Lucania border. This basin is filled by continental middle-upper Pleistocene alluvial deposits organized in several sedimentary bodies. Such depression was generated along a N120°-striking brittle shear zone and modified by subsequent extensional tectonics. Structural controls on the evolution of the fluvial net appear more marked here than in the Auletta basin. As a matter of fact, the minor channels of the Valico di Prestieri basin show a good geometric correspondence with the outcrop-scale fracture pattern of the Mesozoic carbonate which form the Lauria Mts. The major streams of this area follow the main N120–130°-trending tectonic lineaments. Relationships between Quaternary tectonics and fluvial network geometry are traditionally investigated by means of both classical morphostructural analysis – largely based on statistical treatments of azimuthal data – and morphometric calculation. In this study, orientation data have been represented through the azimuthal cumulative curve, a simple complementary method that facilitates the graphical plot and the morphotectonic interpretation of complex azimuthal datasets. Apart from resolving the well-known problems stressed by the use of rose diagrams, the method here adopted allows to display several statistical populations in the same graph, thus favouring the visual comparison between structural and geomorphological features and better emphasizing vertical-axis block rotations.