Mono- and poly-orogenic merged chain sectors framed within the Mediterranean Alpine Mountain Belt: the example of the Calabride Composite Terrane

The present-day framework of the Calabrian orogenic segment is mostly the result of Palaeozoic orogenic processes, renewed by the Alpine-Apennine large-scale nappe and strike-slip tectonics, locally generating exclusive Alpine metamorphic complexes as well as weakly to pervasively overprinted basement blocks. This evolution led to the formation of a composite terrane (i.e. Calabride Composite Terrane - CCT) constituted by basement rocks presently merged in several Hercynian (Atzori et al., 1984) or possibly older (Micheletti et al., 2007) sub-terranes. These rocks were locally overprinted during the different stages of the Alpine metamorphic cycle, which also affected part of the Mesozoic oceanic-derived units and sedimentary sequences (Liberi et al., 2006; Cirrincione et al., 2008). These metamorphic terrains were definitively stacked during the Alpine-Apennine thin skinned thrusting event (Ortolano et al., 2005; Pezzino et al., 2008). In this scenario, several questions are still debated about the geodynamic history of this southern Alpine sector chain such as, for example, the unclear correlation between Northern and Southern Calabride Composite Terranes. The former is characterised by Europe- and Africa-verging tectonic transport and by the presence of Alpine ophiolitic units, while the latter by an exclusive Africa verging tectonic transport and by the absence of ophiolitic units. Furthermore, not reliable correlating data are already available among the different tectonic units which compose the Southern Calabride Composite Terrane (SCCT), as revealed by the PT trajectories, constrained by means of pseudosection tool and microstructurally derived PT estimates (Cirrincione et al., 2008, 2009; Angì et al., 2009), here integrated in order to envisage a more reliable tectonic framework for the different sectors of SCCT. In particular the northern part of the SCCT (i.e. the Serre Massif) represents one of the few places in the world where it is possible to observe a nearly complete preserved Hercynian crustal section, formed as a result of an initial prograde Hercynian orogenic cycle followed by a retrograde decompressional one evolving toward an extensional deep seated shearing evolution, which favoured the intrusion of huge masses of granitoid bodies, giving rise to a gradually distributed thermal metamorphic overprint. By contrast the adjoining Aspromonte Massif is characterised by an Alpine derived nappe-pile structure composed by the superposition of three tectonic units with different tectono-metamorphic history. The deepest one is the result of a single Alpine metamorphic cycle, characterised by an early HP/LT stage evolving to a late mylonitic shearing event. The overlying metamorphic rocks can be described as the result of a relic metamorphic cycle, in which HT/LP mineralogical assemblages (i.e. Hercynian-type evolution) were overgrown by the same late Alpine mylonitic shearing parageneses identified in the deepest unit. Finally, the uppermost unit of the tectonic pile, constituted by low- to high-grade metamorphic rocks, are characterised by an exclusively Hercynian-type evolution not overprinted by the successive Alpine metamorphic cycle. The integrated application of inferred petrological and microstructural PT estimates here presented, proved to be a particularly powerful approach for investigating the tectono-metamorphic evolution of basement rocks and obtaining reliable geodynamic constraints. The results of this integrated study suggest that the Southern Calabride Composite Terrane is characterised by the presence of both mono- and poly-orogenic chain sectors presently framed within the same belt probably as the result of the juxtaposition of both internal and external Alpine basement units along deep seated transcurrent faults, which likely driven since Oligocene the present-day framework of this Alpine sector chain.