ORIGIN OF THE CALCARE CAVERNOSO AND THE ABSENCE OF TECTONIC FABRIC IN A SHEAR ZONE; ALPI APUANE METAMORPHIC CORE COMPLEX, ITALY

The Alpi Apuane is widely recognized as a metamorphic core complex; however the shear zone exhuming the metamorphic core is absent. Instead of a tectonized carapace typical of a cordilleran type metamorphic core complex, the boundary between the metamorphic footwall and sedimentary hanging-wall is occupied by the Calcare Cavernoso, a fine grained calcareous rock with local conglomerate lenses of unknown age and origin, with only locally preserved tectonic fabric. Similar rock types are observed elsewhere throughout the Alps and Apennines (e.g. Rauhwacken, Carniole, Cornieules, Cargneules), and several authors have suggested that these similar rock types are the result of tectonic reworking and dissolution of Triassic evaporites. The structural position of the Calcare Cavernoso is in agreement with the notion of a tectonic origin, and existing models for the exhumation of the Apuane metamorphic core place extensional structures along this horizon (e.g. Carmigniani and Kligfield 1990; Fellin et al., 2007). We present field and laboratory evidence (⁸⁷Sr/⁸⁶Sr, microscopy, cathodoluminescence) that indicates that the Calcare Cavernoso is a fault rock formed in the Apuane shear zone, and that the present texture and composition was formed in the presence of abundant fluids most likely not derived from sea-water. Our results imply that fluids may play a significant role in low angle normal faulting, and that fluid rock interaction may erase, or inhibit the development of tectonic fabrics.