Abstract

Combined structural and petrographical investigations, coupled with <sup>40</sup> Ar/ <sup>39</sup> Ar geochronology were carried out in the Gimigliano tectonic window (Sila Piccola Massif, Calabria) in order to define the structural and metamorphic evolution of the exposed ophiolite-bearing Gimigliano-Monte Reventino unit and to provide new insights on the tectonic evolution of the whole region. Conversely to previous field studies, we found that this unit constitutes the base of the exposed nappe pile, separated by a major semi-brittle to brittle top-to-the-WNW extensional shear zone from the units above, which differ both in metamorphic grade and structural styles. The unit shows typical blueschist index minerals in metapelites (Mg-carpholite) and in metabasites (Na-amphibole). Estimated peak P-T conditions based on metapelite thermo-barometry are P = 1.0-1.2 GPa and T = 350-380 degrees C. <sup>40</sup> Ar/ <sup>39</sup> Ar geochronology on phengites in equilibrium with pelitic blueschist assemblages provided a minimum age estimate for the blueschist event in the lower complex rocks at the Oligocene-Eocene boundary (33-38 Ma). As attested by the preservation of the carpholite-bearing parageneses and structural signature, its exhumation probably occurred during active convergence and nappe stacking in a manner to preserve cool temperature conditions and to prevent thermal re-equilibration in the nappe pile during exhumation. A syn-orogenic style of extension is thus proposed for the early exhumation phases of the unit. The minimum age of 17 Ma deduced from the age spectrum indicates an early Miocene age for the final cooling and exhumation of the unit at low pressure. Subsequently during the Tyrrhenian extension (middle Miocene onward), post-orogenic extension reworked ductile extension controlling the present tectonic contacts among the exposed unit. These results suggest that tectonic thinning of the Calabrian Arc orogenic wedge may be more widespread and significant than generally recognised.

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