The interaction between telescopici alluvial fans and normal faulting in the Terni Basin (central Italy)

Alluvial fans in tectonically active areas are related to the ratio of sedimentation to uplift rate. This paper examines the telescopic alluvial fans evolution in an extensional basin (Terni basin, Umbria) of the central Apennines along the contact zone between the calcareous basement and the fluvial-lacustrine adjacent area. An extensional tectonic phase controlled all the central Apennines area from the Late Pliocene to present with a maximum value in Lower–Middle Pleistocene. Together with this extensional condition isostatic uplift occurred in Lower Pliocene. The Terni basin is a pre-Apennine intramontane basin of the Umbro-Sabino area. It is a semi-graben, with a rectangular shape, 15 km long (E–W direction) and 8 km wide (N–S direction). A normal fault (Martana fault) bounds the northern part of the basin along the southern margin of Martani chain. A large apron outcrops along the contact between the limestone ridge and the Terni basin over fluvial–lacustrine (Plio-Pleistocene) and alluvium deposits. Along the joint area two additional faults parallel to the Martani Mountains fault are recognisable by geomorphologic evidences, with ESE–WNW trend. Faulting led to the uplift, erosion and re-sedimentation of the sedimentary infill in the formerly active basin. In this paper, we started from a mapping inventory of eleven alluvial fans. The identification and mapping of fans was 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 25 m DEM. The fan boundaries were drawn interactively on the shaded relief map based on the spatial distributions of slope, local relief and curvature. Then an analysis of the morphogenetic processes influencing the alluvial fans evolution was used to define a correspondence between fans and catchments. The primary results of the research show differences in morphometric and morphologic characteristics of the fans. We suggest that the climatic conditions are not relevant due to the substantial uniformity in the entire basin. In this area the fans evolve by normal faulting activity. The interaction between rivers and faults leads the base level downward. Thus the attempt to balance the differences along longitudinal profile rivers brings to diversification in fans evolution. As a consequence of the presence of the two faults a major westward opening is generated with fanhead trenching and telescopic fans particularly for the central ones.