Structural setting and Quaternary morphotectonic evolution of Monte Vulture Volcano, southern Apennines, Italy

The deformational history and morpho-structural evolution of Monte Vulture, a Pleistocene composite volcano located on the eastern margin of southern Apennines, have been outlined by integrating information from regional and local tectonics, geomorphology, kinematics of fault population, fracture pattern analysis, multivariate statistics of chemical data, and a revised stratigraphy of the volcanic units. Monte Vulture is a strato-volcano composed of highly undersaturated alkaline-potassic to ultrapotassic rocks belonging to the Roman Magmatic Province and ranging in age from 674+/-7 to 132+/-12 ka B.P.; it is a relatively small volcanic complex which consists of a 700 m thick lava and pyroclastic succession and is characterised by a central vent and parasitic cones, eccentric lava-plugs, and domes. The earlier volcanic morphology has been modified by summit and lateral volcano-tectonic collapses. The volcano is localised on a structural high of the pre-Pliocene and Pliocene sedimentary bedrock. Its activity developed in middle Pleistocene times, starting at about 0.73 Ma and ending at 0.13 Ma. Therefore, genesis and evolution Of Mount Vulture Volcano took place during the recent deformation of the frontal (i.e. eastern) part of the south-Apennines chain. The arrangement of Monte Vulture deposits has been widely reviewed by grouping, into basic unconformity-bounded units (Synthems), the volcano-stratigraphic units which are not separated by unconformities or long quiescence periods marked by palaeosols and erosional surfaces. The major unconformity of the volcanic complex has been observed at the contact of the upper cooling units on the basal ignimbrites, suggesting that an important tectonic deformation occurred at the passage from early to middle Pleistocene. The geological backbone of the area has been reconstructed by means of a balanced cross-section. The interplay between volcanogenic deformation and tectonic evolution of the chain during the Quaternary have been investigated by analysing fault and fracture populations occurring in both volcanic units and sediments. The fracture system consists mainly of high-angle or sub-vertical planes which, for each sampling site, are generally grouped in two main orthogonal sets. The orientation of fault and fracture sets mirrors the arrangement of the morpho-lineaments and changes with respect to the centre of the volcano, thus showing a radial-concentric pattern typically associated with volcanic doming (i.e. pulse of magmatic chamber and consequent pre-eruptive ground deformation). Evidence of tectonic control on the deformation of the volcanic complex come from the presence in the volcanic units of N120 degrees+/-10 degrees-trending strike-slip faults, which represent a regional feature of the southern Apennines. The existence of a flower structure generated by N120 degrees left-lateral shear and sutured by a palaeosol in turn overlaid by a 484+/-8 ka B.P. unit indicates that the volcano underwent a NE-SW contraction even during the middle Pleistocene. NE-SW-directed extensional faulting caused NW-SE to N-S non-Andersonian conjugate systems affecting all the volcanic products, so testifying a change of tectonic regime. The tectonic evolution of the area also influenced the chemical variation of the volcanic products. Such a control has been revealed by reorganising and statistically analysing the large available chemical database according to the new tectono-stratigraphic picture of Monte Vulture. New data have also been produced and included in the dataset to fill the lack of information relative to one of the synthems (Valle dei Grigi-Fosso del Corbo Synthem). The complete database was used as an input matrix for multivariate statistical analysis using Principal Component Analysis technique (PCA) and single variation diagrams were also produced. To avoid problems arising from comparison of chemical data obtained by techniques with different precision (sensitivity and detection limits) only major and trace elements analyses obtained by XRF (X-ray Fluorescence) were taken in consideration. Rare Earth Elements and some incompatible elements such as Ta, Hf, Th, U, Sc as well as Cs were not included in the database because data regarding these elements were not available for all the synthemic units. The variation plots give evidence for remarkable geochemical differences between the cooling units not consistent with a unique fractionation trend. The basal ignimbrites form the Foggianello Synthem - with a tephrite-phonolite and trachyte-phonolite composition - do not represent the most fractionated products of the foidite-tephrite magmas that originated the units. Also the more recent products (Monticchio Synthem) do not follow a similar trend. The final multivariate analysis output showed that the chemical dataset group into three different ensembles which reflect changes in the deformational regime acting during Pleistocene times in the Monte Vulture area. Structural analysis and geomorphic survey showed that the present-day setting of the volcano and adjacent areas derived from the interplay between volcanogenic doming and regional tectonics. Data presented in this study allow the tectonic activity, expressed by the N120 degrees-trending left-lateral strike-slip faults, to be constrained, showing that in this area the deformational regime changed From transpressional to transtensional conditions during the middle Pleistocene. it can be concluded that three main morpho-tectonic stages with different fault kinematics affected the Quaternary volcanic deposits and the adjoining sedimentary successions. An important tectonic episode, generated in a contractional regime and associated with structural transfer with local NW-SE extension, pre-dates these stages. In this scenario, a set of N40-50 degrees-trending deep faults was generated. Such a system was responsible for the magma ascent, maybe inducing the formation of several magma chambers localised at different levels, as suggested by the chemical peculiarities of the Mount Vulture products. The transfer structure - here named Vulture Line - crosses the entire chain-foreland system and may be identified in the alignment of the Sele and Ofanto rivers mouths and was active both during the upper Pliocene - lower Pleistocene contractional regime and the middle-upper Pleistocene extensional stages. The deep projection of the Vulture Line traces the discontinuity of the subducting Apulian plate which rolls back with different flexural retreats. This inherited feature controlled the emplacement of the ignimbrites (Foggianello Synthem) and was repeatedly re-activated during the eruptive, deformational and morphological history of the volcano.