Alpine evolution of the western Mediterranean: new insights from the Curinga-Girifalco fault zone (Serre Massif, Calabria).

The Calabria-Peloritani terrane represents a fragment of the Alpine chain drifted in the present position during the opening of Western Mediterranean (or Ligurian-Provencal and Thyrrenian basins) basins since the Oligocene. The kinematics of the Alpine deformation in Calabria has been previously defined by some authors (Faure, 1980). However, the kinematic data have never been restored in the original position and orientation, before the opening of the Western Mediterranean. In this work we present new structural data on a major tectonic contact of Alpine age. The main objective is to place in the correct orientation the Calabria-Peloritani segment of the Alpine chain at the Oligocene. The Calabria-Peloritani terrane mainly consists of Hercynian basement rocks, partly affected by an Alpine metamorphic overprint, that overthrust Mesozoic ophiolites in Northern Calabria. In Central Calabria (Serre Massif) the Hercynian basement is sliced into three nappes emplaced during the Alpine orogeny. The upper nappe is formed by a nearly continuous section of the Hercynian crust, consisting of medium- to high-grade metamorphic rocks in the lower portion. The intermediate nappe mainly consists of orthogneisses with some paragneisses, micaschists and metabasites, whereas the lower nappe is mainly composed of phyllites. The contacts between the Alpine nappes are outlined by well-developed mylonitic and cataclastic rocks. In the Serre Massif, one of these major Alpine tectonic contacts, the Curinga-Girifalco Line, is particularly well exposed. This shear zone overprints mainly metapelitic rocks of the lower part of the crustal section in the hanging wall, and granitoid orthogneisses of the intermediate nappe in the footwall. Mineral assemblage in the mylonitic metabasites of the intermediate nappe indicates that deformation took place in the epodote-amphibolite facies. ?-type amphibole porphyroclasts typically show overgrowths of bluish tschermakitic hornblende in equilibrium with epidote+albite+chlorite+sphene+phengite. Foliation is outlined by white mica that wraps around amphibole. Granitoid orthogneisses are characterized by the presence of large K-feldspar porphyroclasts and by fragmented garnets with overgrowths enriched in grossular component (up to 45%). In these rocks white mica has a moderate to high celadonitic content. Preliminary by barometric estimates suggest that deformation occurred at pressure conditions of 0.7GPa. In the analysed area, mylonites of the Curinga-Girifalco Line are are roughly oriented WNW-ESE, with shallow dips towards SSW. Kinematic indicators in these mylonites are mostly consistent with a top-to-SE shear sense in the present geographic coordinates. Rb-Sr radiometric dating on biotite from an orthogneiss-derived mylonite (43 ± 1 Ma; Schenk, 1980) indicates that this deformation took place in the Eocene. The mylonitic belt is affected by later extensional faults outlined by S-dipping cataclasite horizons. This explains the Miocene age (18-15 Ma) of the Curinga-Girifalco Line obtained by fission-track thermochronology (Thomson, 1998). Stretching lineations and shear sense of the Curinga-Girifalco Line have been carried back in the original position and orientation before the Oligocene-Miocene, and indicate a top-to-the-S sense of shear. Considering a geodynamic model of the Western Mediterranean basins (Gueguen et al., 1998), this reconstruction is in good agreement with the regional plate tectonic regime between the Eocene and Oligocene. References Faure M., 1980. Microtectonique et charriage est-ouest des nappes alpines profondes de Sila (Calabre-Italie méridionale). Rev. Géol. Dyn. Géogr. Phys., 2, 135-146. Schenk, V., 1980. U-Pb and Rb-Sr Radiometric Dates and their Correlation with Metamorphic Events in the Granulite-Facies Basement of the Serre, Southern Calabria (Italy). Contrib. Mineral. Petrol., 73, 23-38. Thomson, S.T., 1998. Assessing the nature of tectonic contacts using fission trac...