La storia deformativa dell'Appennino meridionale ricostruita in un affioramento metrico nei dintorni di San Fele (Potenza)

L’evoluzione di un ciclo orogenico generalmente comprende due stadi distensivi separati da uno stadio compressivo. Questa succes- sione di eventi ha caratterizzato la storia deformativa di molte catene collisionali, ed il record di ogni singolo stadio può essere rico- nosciuto tramite l’analisi di strutture a scala diversa, da microsco- pica a regionale. Tuttavia, mentre la storia completa di una catena viene generalmente dedotta dall’integrazione di osservazioni con- dotte su numerose esposizioni e sezioni naturali, sono invece estre- mamente rare le situazioni nelle quali è possibile ricostruire l’intera sequenza deformativa attraverso lo studio un singolo affioramento. Il doppio cambiamento di regime tettonico (inversione tettonica) che ha caratterizzato l’evoluzione dell’Appennino meridionale, da di- stensione pre-orogenica a compressione sin-orogenica (inversione po- sitiva) e da compressione sin-orogenica a distensione tardo/post-oro- genica (inversione negativa), ben documentato alla scala regionale nelle Unità Lagonegresi della Basilicata, è magnificamente registrato in una sezione di dimensioni metriche a Serra Manarella, nei pressi dell’abitato di San Fele (Potenza). In questa sezione affiora la parte superiore della Formazione degli Scisti Silicei del Giurassico, che passa alla sovrastante Formazione dei Galestri del Cretacico Inferio- re. Una faglia diretta sinsedimentaria che produce un rigetto di alcu- ne decine di centimetri è sigillata dagli strati sommitali degli Scisti Silicei. Questi strati sono coinvolti in una coppia anticlinale-sinclina- le, collegata allo sviluppo verso l’alto di un sovrascorrimento meso- scopico di età neogenica. La struttura composita risultante dalla sovrapposizione del sovrascorrimento alla faglia diretta sinsedimen- taria, è a sua volta troncata da una faglia diretta riferibile al Pleisto- cene Medio, che produce un rigetto verticale di alcuni metri. Lo stu- dio dei rapporti di sovrapposizione fra queste strutture rappresenta, pertanto, un’eccezionale opportunità per poter definire in maniera univoca la cronologia relativa delle principali deformazioni che han- no caratterizzato l’evoluzione tettonica delle Unità Lagonegresi nell’Appennino Meridionale. Il riconoscimento di situazioni analoghe in altri settori della catena sud-appenninica, od in altre catene oro- geniche, può essere difficile, in quanto richiede una rara combina- zione di esposizione perpendicolare, o almeno ad alto angolo rispet- to alle strutture analizzate, ed una marcata coassialità strutturale; tuttavia esso potrebbe porre importanti vincoli all’interpretazione della cronologia relativa fra le strutture macroscopiche affioranti in una determinata area, fornendo in tal modo un’utile strumento per la realizzazione di sezioni di importanza regionale.The repeated opening and closure of oceans along the same trends has long supported the concept, known as the Wilson cycle, of periodical recurrence in orogenic events, and a certain repetitivity in mountain building processes is recognised in the geological record worldwide. Orogenic cycles may be viewed as comprising two extensional stages that are separated by a stage of contraction. This sequence has characterised the evolution of most mountain belts resulting from continental collision, and the structural signature of individual stages is recognised on a wide variety of scales, i.e. from microscopic to regional. The history of complete orogenic cycles, including pre-and post-orogenic extension, is generally inferred from numerous observations carried out at different localities scattered over wide areas, whereas the entire sequence of structural stages is very rarely preserved within individual exposures. The paucity of clear overprinting relationships among contractional and extensional structures may, in fact, prevent from a correct reconstruction of the local deformation history, especially at sites where the stratigraphic controls of syn-tectonic deposits have been lost due to erosion. This has often led to contrasting interpretations on the relative timing of development of given, map-scale structures in orogenic belts. A clear determination of the sequence of deformations inferred from outcrop-scale structures within exposures of limited size, instead, may help enhance our understanding of the time-space relationships of their kinematically related, larger host structures. The double switch in tectonic regime that led to the development of the Southern Apennines of Italy, from pre-orogenic drifting through syn-orogenic thrusting to post-orogenic extension, is widely recognised within macroscopic structures in the vicinities of San Fele, in Lucania. The outcropping stratigraphic section consists of Triassic-Cretaceous pelagic sediments, namely the Upper Triassic Calcari con Selce Formation, the Jurassic Scisti Silicei Formation and the Cretaceous Galestri Formation, that were deposited within the Mesozoic Lagonegro Basin. The Scisti Silicei Fm. is characterised by remarkable thickness and facies variations, that reflect the activity of syn-sedimentary normal faults. These extensional structures are often obscured by later deformations of Tertiary and Quaternary age. However, compilation of a detailed 1:5.000 scale field mapping made it possible to recognise remarkable thickness variations within the Scisti Silicei Fm. across a major WSW-ENE trending syn-sedimentary normal fault at Mt. Fieno, that was active during the Jurassic. This fault was slightly reactivated during the subsequent tectonic history because it offsets an anticline developed during the Apenninic contractional events. The Mesozoic sediments are affected by both contractional and extensional structures. The main contractional structure consists of a NW-SE trending, NE-verging thrust-related anticline. This fold can be traced for over 4 Km, lies in the hanging-wall of a gently WSWdipping thrust surface, and represents the uppermost feature of a complex hinterland-dipping duplex structure. The distribution of bedding data from the anticline limb and crest regions indicate that the fold plunges 15° towards N326°. The growth of the thrust-related anticline was accompanied by development of minor, metre-scale parasitic folds, that are recognised at many sites, and that are particularly abundant in the SW-and NE-dipping limbs. These minor folds mean plunge 20° towards N319°, i.e. broadly parallel to the larger host structure. The general parallelism of both macroscopic and mesoscopic folds suggests that contractional structures developed in response to a SW-NE direction of maximum shortening. Based on the available stratigraphic information, thrust-related folding in the San Fele area has been referred to the Neogene (Late Miocene-Pliocene). The thrust-related anticline appears truncated and offset by an array of NW-SE trending normal faults, that extend for over 5 Km with opposite dip, defining a fault-bounded graben. Mechanical striations and slickensides on the normal fault surfaces indicate a mean WSW-ENE direction of extension, i.e. at a low angle to the general SW-NE direction of orogenic contraction inferred from the orientation of macroscopic and mesoscopic folds. The available stratigraphic information makes it possible to refer these structures to Middle Pleistocene time, and to ascribe them to post-orogenic extension. An analogue deformation history is also magnificently preserved in a superb metre-scale outcrop at Serra Manarella, ca. 1 Km from San Fele. A Late Jurassic, syn-sedimentary normal fault is sealed by strata affected by a Neogene thrust-related fold. This composite structure, in turn, is truncated by a Middle Pleistocene normal fault. Observation of these relationships represents a unique opportunity to unequivocally establish the relative chronology of deformations in the Lucanian Province of the Southern Apennines. Moreover, the systematic recognition of analogue evidence at different localities is difficult, because it requires a rare combination of a suitably oriented outcrop and a very clear structural chronology. However, if this information is achieved, it may be used to test and improve regional cross-section restoration, thus ultimately providing a critical tool to elucidate the tectonic history of orogenic belts