Evaluation of the erosion rate in the Southern Apennines (Italy) based on geological-geomorphological data / Valutazione del tasso di erosione in Appennino Meridionale da dati geologico-geomorfologici

The object of this study is the estimation of the long-term erosion rate in the southern portion of the Apennines chain (southern Italy). This was done with the aim of validating the hypothesis, proposed by MAZZOLI et al.. (2008), that thin-skinned extensional tectonics played a major role in the exhumation of formerly deeply buried tectonic units. More in detail, the study aimed at providing data on the erosional component of the rock exhumation which, based on Apatite Fission Tracks data reported in MAZZOLI et al. (2008), has affec- ted in the last 10 Ma the deepest units in the southern Apennines fold–and-thrust belt. The erosion rate was estimated based on the evaluation of the volume of deposits delivered by a large hydrographic basin to a conti- nental environment sedimentary basin. The accumulation basin is the Sant’Arcangelo basin (hereinafter BSA), which developed as a marine environment wedge-top basin (originally connected to the Apennines foreland basin) during the Middle Pliocene and evolved as an alluvial-lacustrine basin in the late part of the Early Pleistocene in response to the thrusting related uplift of a ridge located to the E of it (VEZZANI, 1966, 1967; CARBONE, 1991; PIERI, 1994; ZAVALA, 2000; PATACCA & SCANDONE, 2001; GIANNANDREA & LOIACONO, 2003; BENVENUTI et al., 2006). The drainage basin correlated to the BSA covers a large portion of the eastern slope of the Southern Apennines, and includes elevations formed of formerly deep-seated structural units (namely, Lagonegro units and Apulian platform unit) exhumed during Pliocene-Quaternary times. Literature data and field surveys allowed the identification of the sedimentary unit (Castronuovo-San Lorenzo unit, CSL, composed of lacustrine deposits passing laterally and upwards into alluvial plain sediments) which was deposited within the BSA when the basin was isolated from the neighboring foreland basin. The CSL ranges in age from around 1 to 0,7 Ma (PATACCA & SCANDONE, 2001; MAT- TEI et al., 2004; SABATO et al., 2005). G.I.S. elaborations allowed the reconstruction of the top and bottom surfaces and volume of the CSL. The calculated volume value was corrected accounting for the erosion which affected the CSL top surface, for the original width of the deposition area, for the suspended load loss, and for the sediment porosity. The paleo-hydrographic basin coeval to the CSL was reconstructed based on geomorphological/geological evidences, which include the compositional analysis of pebbles of different units of the BSA succession. The obtained 0,25 ± 0,07 mm/a erosion rate value is comparable to the values, averaged over the last 0,7 Ma and spanning from 0,2 to 0.3 mm/a, estimated for the eastern chain margin-foreland basin area in both the southern and northern Apennines (AMATO et al., 2003; CYR & GRANGER, 2008). These values, which can be considered as representative of erosion which has affected the Apennines over the Quaternary, are much lower than the Pliocene-Quaternary rock exhumation of formerly buried units estimated by AFT data, and this points to the important contribution of the extensional tectonics in the exhumation processes in the Southern Apennines.