Plio-Quaternary tectonics and possible implications for geothermal fluids in the Marche Region (Italy)

Structural features such as faults and fractures play an important role in fluid circulation within the crust, and influence geothermal exchange potential. Based on this consideration, we examined the subsurface structural setting of the Marche Region (Central Italy) in terms of detailed data we obtained on the groundwater in wells; detailed analyses of its physical properties, particularly electrical conductivity, revealed some anomalies in several localities which appear to be related to the local tectonic structures. In addition, we interpreted data from seismic reflection profiles provided by ENI S.p.A, and kindly shared with us, for the SW-NE area crossing the Apennine chain and extending to the Adriatic Sea. Our interpretation indicates the presence of important sub-surface and deep Plio-Quaternary structures linked to outcropping ones with possible hydrogeological implications. Our interpretation of these seismic profiles enabled us to identify some high angle structures affecting the whole sedimentary sequence and routed at depth (>10 km), thus allowing us to gain an understanding of the recent structural evolution of the Apennine Marche sector. We interpreted these mainly NW-SE trending structures to be transpressive structures, related to lower depth SW and NE-dipping high-angle reverse faults (positive flower structures), probably involving the upper crust basement. We identified them along three main parallel alignments (transects) from SW to the coastline, crossing the Mio-Pliocene Apennine range and its external Plio-Quaternary sector. Our analyses of the groundwater in wells throughout the study area indicated possible relationship with deep geological structures. It seems that the high degree of fracturing that accompanies these complex and recent fault systems could facilitate the exchange between superficial and deeper fluids. This is supported by the observation that there is a direct relationship between the electrical conductivity of the water in wells located along the calcareous-marly Apennine Marche ridge, and the amount of rainfall.