Understanding the crustal structures of southern Tuscany: The contribution of the CROP18 Project

The CROP 18 Project (1995-2002) has dealt with the relationships between crustal structures and geothermal resources in southern Tuscany. The acquisition of a 120 km-long deep seismic reflection survey is also included in this Project. Here we discuss the compressional and extensional regimes, both proposed as explanation for the Neogene-Quaternary structures of southern Tuscany. We also discuss the close relationship of seismic reflectivity, CROP 18-lines orientation and Pliocene-Present tectonic structures. The main results are: (a) the extensional framework, which has been active since the Early-Middle Miocene, explains the structural features and tectonic evolution of southern Tuscany better than compression; (b) the upper crust of southern Tuscany is characterised by widespread boudinage; (c) the mid-lower crust is largely affected by magmatic intrusions; (d) NE-dipping extensional shear zones are imaged in the upper and lower crust; these are connected through a mid-crustal shear zone, located at the brittle/ductile transition. These shear zones could act as preferential pathways for upward migration of metamorphic and magmatic fluids and mantle elements, now characterising the geothermal fluids of the Larderello area.