Structural control on drainage network of the southwestern side of the Agri River upper valley (Southern Apennines, Italy)

Abstract

Relationships between low order streams and fracture pattern of the upper part of the Agri Valley (Southern Italy) have been analysed by comparison of azimuthal diagrams to infer the existence of structural controls on recent evolution of drainage network. A computer-aided counting procedure based on DEM and GIS methods, greatly improved the quantitative analysis, allowing rapid treatment of a large number of orientation data. The upper valley of the Agri River is an intermontane Quaternary basin located in the axial zone of the southern Apennines. Tectonics has strongly controlled shape, morphology, and sedimentary evolution of the basin up to the present. Pleistocene extensional tectonics is commonly envisaged as broadly responsible for the basin evolution, but many data suggest that the Agri valley is a more complex structure than a simple extensional graben or than a pull-apart basin. The study area coincide with the south-western flank of the valley, which is prevalently made of Meso-Cenozoic shallow-water carbonates thrust on coeval pelagic successions and broadly affected by Plio-Quaternary high-angle faults. The basin floor is filled by middle Pleistocene faulted alluvial deposits. Evidences of deformation in younger sediments as well (i.e. upper Pleistocene to Holocene) have been recently documented by radiocarbon dating of faulted palaeosols. The outcrop-scale fracture pattern of the south-western side of the Agri River upper valley is characterized by the presence of several sets of sub-vertical joints. Two of them are well-defined sub-orthogonal sets and correspond to N150°±10° and N60°±10° trends. Other sets show with N-S and N120°±10° trends, which represent the orientations of regional high-angle faults. In many areas of southern Apennines the N150°-striking minor faults and joints and associated orthogonal fractures represent the youngest structures of Quaternary brittle deformation, produced by NE-SW extension. The basic elaboration of the hydrographic net map has been made on 1:25000 scale IGMI topographic maps, on which all streams have been recognized and digitized with Arcview GIS software. In this way, the fluvial net is geo-referenced and the related dataset is enriched with additional information. As a matter of fact, every single stream is linked to a table with sub-basin pertinence, hierarchic order and length. Further, for the I to III order streams also the orientation in azimuthal notation is reported. In this study only the I and II orders have been considered because of their better susceptivity with regard to tectonic influence. The minor fluvial network of the south-western flank of the valley is arranged according to the same trends of the fracture systems. In particular, the rose diagram related to the first order streams shows a clear maximum corresponding to the N160°-170° orientation class, with a minor sub-orthogonal trend. The second order streams are arranged according to N160°- 170°, N50°-60° and N80°-90° trends. Both diagrams do not show significant statistical dispersion or background noise. The fracture pattern of the upper Agri Valley strongly controlled genesis and arrangement of minor streams, which traced the youngest sets of joints and minor faults (NW-SE and NE-SW trends). E-W-trending structures also exerted a structural control, but only on the II order streams: this fact may depend on the age of these features, which seem to pre-date the other trends.