The structural evolution of the Radicondoli-Volterra Basin (southern Tuscany, Italy): Relationships with magmatism and geothermal implications

The Western Tuscany, in the inner sector of the Northern Apennines, is characterized by a thinned con-tinental crust (~20-25 km), high heat flow (>100 mW m-2), and the presence of magmatism. The latterproduces the current geothermal activity, which is mostly localized at the large Larderello-Travale fieldsettled above the granites with the same name. These features and the several Late Miocene sedimen-tary basins that developed in the area are normally related to an extensional tectonics that affected theback-arc area since the Early-Middle Miocene (~16 Ma), although, some geophysical studies image thecontinental crust deformed by W-dipping thrust faults. Here we present a new field survey conductedin the Radicondoli-Volterra Basin, which bounds the Larderello-Travale pluton system to the northeast.The results support former conclusions that the basin infill has been intensely shortened. Integratedanalysis of other surrounding hinterland basins suggests that a phase of regional shortening affected theback-arc area approximately between 7.5 and 3.5 Ma. This period partly overlaps with the emplacementof the Larderello-Travale granites that started no later than 3.8 Ma, thus raising the possibility that com-pressive structures may have - at least in part - controlled magma emplacement, and/or also that theearly emplacement of the Larderello-Travale granites occurred when the compressive stresses startedto decay. In some cases, isotherms localize and elongate in the direction of subsurface anticlines in thebasement, as occurs in the Travale area. Comparison with the results of analogue models indicates thatthe core of active and inactive fold anticlines in the basement may focus the upraising of magmatic fluids,with obvious implications for the geothermal exploration of the area.