Polyphase out-of-sequence thrusting and occurrence of marble detritus within the wedge-top basin deposits in the Mt. Massico (southern Apennines): insights into the late Miocene tectonic evolution of the central Mediterranean

We present a geological survey of the southwestern sector of the Mt. Massico (southern Apennines), which allowed us to reconstruct the stratigraphy and tectonic architecture of this area. The Mt. Massico is a key area to study the out-of-sequence thrusting stage that affected the southern Apennine chain since the late Miocene, which despite the wide occurrence of these structures in the whole chain, is a tectonic process up to now poorly investigated. This area deserves to be analyzed also for the occurrence of marble detritus within a wedge-top basin deposit, representing the only place in the southern Apennines, where such a kind of metamorphic rocks occurs. Historically, this area is known for the extraction of an ornamental stone, named Mondragone Marble. The marble is hosted as olistoliths and clasts within the deposits of the Caiazzo Fm. unconformably covering a Jurassic–middle Miocene succession, mostly made of shallow water-to-deep-basin carbonates. The wedge-top basin deposit mainly consists of a chaotic assemblage of conglomerates, olistostromes, and olistoliths of Meso-Cenozoic limestones, deep-basin rocks, and marbles, embedded in a quartzose matrix. The structural setting results from a polyphase deformation related to the superposition of two thrust systems. The first thrust system includes a duplex verging to east, with the Cenozoic carbonates thrusted onto the Caiazzo Fm. and the roof thrust cut by several breaching thrusts. The late thrust system encompasses ramp-dominated faults verging to the north. Folds, minor thrusts, and S–C structures are associated with the major compressional structures. To date the Caiazzo Fm. and put a temporal constraint to the thrust fault activity, we performed a nannoplankton content analysis that furnished, for the base of this deposit, an age not-older than the upper Tortonian. These out-of-sequence thrusts hence have acted since the late Miocene. Similar structures are widespread in the southern Apennines and are interpreted as the surficial expression of envelopment thrusts formed at deeper structural levels with respect to the thrust front. We envisage that the marble and plutonic supply was provided by the Mesomediterranean continental crust in the late Tortonian when this microcontinent was passing close to the Apennine Platform domain during its orogenic migration toward SE.