Quaternary evolution of the "Castelluccio di Norcia" basin (Umbro-Marchean Apennines, central Italy)

Abstract

Castelluccio di Norcia is a closed depression, 12 km in length and 8 km in width, located in the southern part of the Umbro-Marchean Apennines, westward from the highest reliefs of the chain. The Umbro-Marchean sequence outcropping in the area was folded and shifted eastward, along major overthrusts, during Late Miocene-Early Pliocene times. During the Early Pliocene, a very flat "erosion surface" was modelled on the folded and thrusted bedrock. In Upper Pliocene times, major uplifting movements began in the chain, the highest values (> 2000 m) being reached in the highest present-day summits of the southern part of the chain and to the east of the study area. A "young landscape", with many preserved smoothed parts, developed in the Apennines up to the late Lower Pleistocene wide paleovalleys, remnants of which are today watersheds, formed during this period. Successively, in the most uplifted parts of the chain a "mature landscape" started to develop. Extensional tectonic movements, which in part originated the Castelluccio-di-Norcia Basin, affected the western part of the chain. A series of NW-SE to N-S trending faults with >1000 m vertical displacement became active. Elongated valleys developed in the western sector of the basin while only minor erosional features are present on the Costa-del-Vettore Fault scarp, probably due to the continuous activity of this major fault system. The oldest sediments outcropping in the basin are coalescent alluvial fan deposits, debris fan and stratified slope-waste deposits, sometimes interfingered with one another and containing thin beds of pyroclastic deposits, attributed to late Middle Pleistocene times. The emplacement of these old deposits is linked to a widespread glacial and periglacial morphogenesis, the former being represented on the northernmost exposures where large nivation and glacial cirques can be seen. At Piano Perduto, (1340 m a.s.l.), there are cemented remains of a frontal glacier moraine developing from the Canatra valley. Late Middle Pleistocene deposits are deeply dissected and terraced, especially along the basin eastern escarpment. During Upper Pleistocene times, no glacial sediments reached the plain, whereas large alluvial fans were being deposited at the foot of slopes. These fan deposits would subsequently be moderately dissected in the Holocene. A depositional phase followed, consequent to deforestation and intense soil erosion in historical times. The dense network of faults delimiting the plain became active before the late Middle Pleistocene, but evidence of small displacements are present along the eastern main fault scarp. Along this fault huge landslides - e.g. the Colli Alti e Bassi landslide - occurred. A very flat debris slump at the base of the main scarp suggests its close relationship to earthquakes.