Geomorphological evidence for anti-Apennine faults in the Umbro-Marchean Apennines and in the peri-Adriatic basin, Italy

Abstract

The Apennines are a relatively recent mountain chain which has been affected by uplift movements since the Upper Pliocene. In fact the remnants of an “erosional surface”, reduced close to base level, is preserved at the top of the relief. There is no general agreement on the geodynamic stress field and mechanisms which are creating the chain. However, it is largely accepted that uplift occurred together with the activation, on the western side of the chain, of extensive faults, oriented in the Apennine direction (NW-SE), which have been linked to the opening of the Tyrrhenian sea. A great debate is going on about the presence and significance of anti-Apennine faults (NE-SW) which have been observed by some authors but completely denied by others. The main evidence is represented by[ (1) block faulting of the remnants of the “erosional surface”. Along the Marchean Ridge, more elevated relief, delimiting relatively depressed areas, was created in correspondence with the Sibillini Mts. and Mt. S. Vicino. Similar evidence has been found in the Umbro-Marchean Ridge. Locally more than 1500 metres of displacement have been observed between more and less uplifted remnants. (2) Block faulting of fan deltas and related beaches, of Sicilian to Crotonian age, with more elevated sediments preserved between the Tronto and Tenna rivers and between the Musone and Esino rivers. Maximum displacement along a transect parallel to the coast is 200 metres. (3) fault-scarps affecting the Middle Pleistocene river terraces, as observed along the Esino, the Tronto, the Chienti and the Tenna river valleys. Maximum displacements are in the order of 50 metres. (4) Faulting of horizontal karst galleries and reorientation of the cave network, as in the Frasassi Gorge. Maximum displacements are about 100 metres. (5) Captures and alignments in the drainage network of the main river courses. (6) Large-scale gravitational movements, as in the Ancona landslide, and along the Chienti and Esino rivers. Their activation occurred in most cases after the Lower Pleistocene and although their displacements may be of relatively limited extent, dispite their recent activity, they played a major role in the modelling of the landscape. These faults display transtensive, extensional and trascurrent movements. Apart from the controversial geodynamic significance of these faults, from a geomorphological point of view they must be considered transverse elements of the stress field from blocks more or less uplifted along the Apennine chain. The importance and timing of activity of these faults in the Quaternary geomorphological evolution of the Umbria-Marchean Apennines is demonstrated using evidence usually underestimated by structural geologists, which can contribute to a debate based on a multidisciplinary approach.