Erosional landsurfaces of the Campano–Lucano Apennines (S. Italy): genesis, evolution, and tectonic implications

Abstract

Within the Campano–Lucano Apennines small but widespread relics of gentle erosional landsurfaces (GELs) occur that are elevated several hundred metres above the present valley floors. On the basis of our observations, the old idea that all these relict landforms can be traced back to a single phase of quasi-planation that affected the entire belt after thrust stacking and before a phase of final uplift must be rejected. New field evidence and chronological constraints support the conclusion that the GEL relics belong to various, uncompleted cycles of erosion that occurred just during the construction of the chain (i.e. Late Miocene to Early Pleistocene). As many recent geological studies have pointed out, the Southern Apennines Chain is an arc-shaped accretionary wedge that migrated as a thrusting system towards its retreating foreland by means of addition of thrust units to the front and loss of rifted portions at the rear (Tyrrhenian back-arc basin). Also, the spatial distribution of the various generations of GELs, their ages, and their relations to the main structures and syn-orogenic formations of the belt seem to agree with this geodynamic scenario. Here we propose a ‘first approximation’ conceptual model that considers each generation of Campano–Lucano Apennines GELs as largely formed when the thrust units on which they rest were located at low elevation near the coastline of the foredeep basin, i.e. shortly after the entrance of those thrust units into the tectonic wedge. Then each GEL (as a matter of fact the compartment on which it rested) was taken towards more and more internal parts of the wedge as frontal accretion continued. Owing to this relative migration, the fate of each GEL-carrying compartment was to reach the part of the belt that was affected by extensional block faulting, i.e. the Tyrrhenian side of the chain. Each GEL eventually reaching this domain was tectonically fragmented, deeply cut by waves of fluvial dissection proceeding from the steep tectonic coast toward the NE and finally also rifted under the Tyrrhenian Sea. Other causes of GELs fragmentation and dissection were probably the breaching of the chain's outer flank by out-of-sequence thrust and the up-arching of the roof thrust system caused by the formation of deep anticlinal stacks. On the contrary, when forward propagation of thrusts continued for long periods, the GELs being carried toward the inner zone of the wedge could not have experienced deep dissection because the increase of their elevation a.s.l. was compensated by their increasing distance from the foredeep coastline (maintenance of low gradient along the river valleys thanks to a very low critical taper).