Pliocene-Quaternary fault kinematics in the Larderello geothermal area (Italy): Insights for the interpretation of the present stress field

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In geothermal areas the evaluation of the active stress field from local earthquakes is generally contradictory, mainly suggesting normal to strike-slip tectonic regime. This feature has been investigated in the Larderello geothermal area comparing information from field data and focal mechanisms from the area of the Pliocene-Pleistocene Lago Basin, where the most effective exploitation occurs today. The methodology follows the classical approach of structural geology, inversion of kinematic data from slip-fault surfaces and comparison with focal mechanisms of local earthquakes. The results indicate strong similarities in the orientation of the main stress axes as obtained from paleo-stress analysis and double couple focal mechanisms solution, suggesting the same stress field since Pliocene, at least. The analysed structures are explained as part of the deformation developed within a NE-oriented transfer zone, and the Lago Basin is explained as a pull-apart basin. The transfer zone is characterised by the coeval activity of NE-striking left-lateral oblique- to strike-slip faults and by NW-striking normal faults, thus indicating a key for the understanding of the different focal mechanisms. Moreover, field data analysis indicates that both the NE- and NW-striking faults can be reactivated as normal faults dominantly, as also suggested by the local focal mechanisms. It is therefore concluded that the vertical movement is active when surface uplift, reasonably induced by heat flow, is more effective than crustal extension. Finally, the switch of the local intermediate stress axis from vertical to horizontal is considered a key-factor to channel deep geothermal fluids toward lateral structural traps.