Tectonically driven exhumation of a young orogen: an example from southern Apennines, Italy

In many young orogens of the Mediterranean region, Quaternary tectonics and regional uplift are traditionally considered strictly correlated, but few data are actually available for precise calculations of uplift rates. Such quantitative studies are needed to define the relationships between local faulting and large-scale uplift, and to formulate correct hypotheses on the geodynamic scenario in which the regional raising occurred. In addition, data about burial depths of sediments or tectonic loadings suffered by sedimentary and low-grade metamorphic rocks are essential for comparisons with the uplift rates obtained in the same areas. Such comparisons between quite different data sources improve the comprehension and choice of the most reliable mechanism responsible for the regional uplift and exhumation. Uplift rates have been calculated for a large sector of the Lucanian Apennine (axial zone of the southern Italian Apennines), and also reviewed for the Calabrian arc (southern termination of the Italian peninsula), using both geomorphological observations (elevation values, ages and arrangement of depositional and erosional landsurfaces and other morpho-tectonic indicators) and stratigraphical and structural data (sea-level related facies, base levels, fault kinematics, and offset estimations). Such data have been compared with those deriving from clay mineralogy of Mesozoic pelagic successions (Lagonegro units), outcropping in the same sector of the chain, which gave information on tectonic loadings. The values of the Quaternary uplift rates of the southern Apennines axial zone vary from a minimum of 0.2 mm/yr near Potenza town to a maximum of about 1.2-1.3 mm/yr in Agri high valley, a severely deformed Quaternary intermontane basin still tectonically active, and in Pollino Mts, a carbonate ridge with elevation up to 2200 m a.s.l.; intermediate values (0.5 to 0.7 mm/yr) have been calculated for the other studied areas. The erosion rate from a key-area of the Lucanian Apennine, obtained from both quantitative geomorphic analysis and missing volumes calculations on a catchment basin as wide as 150 km2, has been estimated at 0.2 mm/yr for the middle Pleistocene to Present time span. Since in the upper part of that chronological interval erosion and uplift rates match well, the axial-zone landscape could have reached a steady state during the late Pleistocene. Using geomorphological features and late Pliocene to Pleistocene successions involved in the genesis of erosional and depositional landsurfaces, the same rates (? 0.6 mm/yr) have been obtained for a quite large time span (about 2 Ma) in the Melandro basin and adjacent Maddalena Mts. Therefore, during the last 2 Ma, the total uplift amount of the axial zone of the Lucanian Apennine is about 1.2-1.3 km, with local peaks of 1.5 km. On the other hand, the Mesozoic pelagic units experienced tectonic loading of 4-5 km, as estimated by means of illite crystallinity (in the range 0.6-1.1 ?°2?), percentage of illitic layers in illite/smectite mixed layers (60-90 %) and white mica polytypes (in the range of 15-35%), and confirmed by other independent data. The Quaternary uplift and the related erosion rates of the southern Apennines are unquestionably due to extensional tectonics coupled with thermal/isostatic regional raising and, in minor extent, to strike-slip faulting acting in the earlier deformational stage of the south-Apennines chain. The gap of several km deriving from the comparison between uplift rates and tectonic loading values may be explained only with different exhumation modalities starting from late Miocene times. This age can be obtained assuming a fixed rate of 0.6 mm/yr, which represents the best long-term estimate for the axial zone of the chain. Going back in time using such a conservative rate to get a denudation value of about 4-5 km, Tortonian age is reached. At that time, contractional tectonics was still active in this sector of the southern Apennines. Tectonic erosion may be taken into account for a reliable explanation of the discrepancy between the different sources of data. Such phenomena led to the exhumation of the Mesozoic core of the chain (Lagonegro units) causing low-angle extension on its Tyrrhenian side by re-activation and inversion of older thrusts, and stacking on its eastern margin of Cretaceous to Miocene pelagic and flysch units (i.e. frontal embricate fan units) by gravity mega-sliding.