I conoidi nell'evoluzione delle conche intermontane umbre

The alluvial fans in the evolution of the umbrian intramontane basins

Abstract

An extensional tectonic phase from the Late Pliocene to present with a maximum value in Lower-Middle Pleistocene strongly controlled the evolution of the central Apennines area and the Umbria region as well. Together with this extensional condition, isostatic uplift occurred in Lower Pliocene. As a consequence, two large and continuous basins crossed the area from NW to SE. Along the eastern margin of the region, parallel to the first ones, other smaller basins are present. The depressions are recognizable as intramontane basins crossed by superficial drainage networks belonging to River Tevere catchment. Alluvial and colluvial deposits, eroded from neighbouring slopes, fill the basins. Alluvial fans are one of the most common deposition landforms in this environment. In this paper, we started from a mapping inventory of alluvial fans in Umbria region. Identification and mapping of fans were carried out in the field, by using topographic maps and interpreting two sets of black and white aerial photographs. The boundaries of the fans were also crosschecked using a digital derivate hillshade from the 90 m DEM. The fan boundaries were drawn interactively on the shaded relief map based on the spatial distributions of slope, local relief and curvature. The fans are divided in inactive delta and fan delta and in active alluvial fans. Alluvial fans in tectonically active areas are related to the ratio of sedimentation to uplift rate. Geometric and sedimentary characteristics of the fans allow to understand the fault activity distribution in the region. We supposed a more recent evolution of the fans going from west to east, according to the extensional stress field migration.