Alpine structural and metamorphic signature of the Sila Piccola Massif nappe stack (Calabria, Italy): Insights for the tectonic evolution of the Calabrian Arc

Combined structural and petrographical investigations, coupled with Ar-40/A r-39 geochronology, were carried out in the Sila Piccola Massif of the Cala brian Arc in order to define the structural geometry and map out the major structural and metamorphic breaks within the exposed nappe sequence. On the basis of the contrasting Alpine pressure-temperature (P-T) and structural signatures the nappe stack can be divided in two major tectonic complexes, bounded by a flat-lying ductile to brittle extension shear zone. The upper complex consists of a nappe-like structure, where a major top to the east c ompressional shear is recorded. The lower tectonic complex consists of an o phiolite-bearing sequence showing typical high-P/low-T parageneses (Mg-carp holite and Na-amphibole). The Ar-40/Ar-39 geochronology on phengites in equ ilibrium with blueschist minerals provided a minimum age estimate for the b lueschist event in the lower complex rocks at the Oligocene-Eocene boundary (around 35 Ma). Ductile to brittle top to the west extensional shear accom panied the nearly isothermal retrogression and exhumation of the lower comp lex rocks, reworking the previous nappe contacts with shear localization al ong the upper/lower tectonic complex discontinuity. The Ar-40/Ar-39 dating indicates that this postnappe stacking tectonic evolution took place from 3 0 Ma onward. It is proposed that exhumation of the deep-seated rocks occurr ed below a top to the west extensional detachment active during convergence and orogenic complex formation (synorogenic extension). The age of this de tachment is bracketed between 30 Ma and the post-orogenic Neogene basin sed imentation (middle-upper Miocene). The revised structural and metamorphic s cenario is here integrated into a new tectonic evolutionary reconstruction, which involves an early high-P/low-T top to the east crustal thickening ep isode during the construction of the Apennine orogenic wedge (Eocene-Oligoc ene), followed and overprinted by a top to the west extensional shear, prob ably active from the late Oligocene.