Multiple fault strands in carbonate rocks control groundwater circulation in tectonically active areas, Monte Marine Fault, central Apennines, Italy

Faults zones play a key role on groundwater flow, conditioning both local and regional pathways. They impact groundwater flow in different ways depending on host rock lithology, 3D architecture and internal structure. Despite fault zones in carbonates are widely studied, their impact on groundwater flow is still difficult to predict. In this study, we combined structural, hydrogeological, geochemical and isotopic investigations to experimentally validate the hydrogeological behavior of a similar to 14 km-long normal fault developed within carbonates in the Central Apennines (Italy). The fault is composed of two distinct, nearly parallel, fault strands linked in a step-over zone. The width and distribution of fault rocks along its strike strongly vary, leading to a non-homogeneous permeability distribution. Results show that the two near-parallel fault strands act as a barrier due to low permeable, 5-10 m-wide cataclastic fault core thus impeding cross flow. Instead, a significant flow-through is permitted in the step-over area, where local permeability is enhanced due to the presence of > 500 m-wide highly permeable loose fault breccia. The structural complexity controls the occurrence of the along-fault spring (few liters per second) and facilitates a significant flow-through from the carbonate aquifer towards the alluvial plain affecting the groundwater circulation and the geochemical characteristics at the local and basin scale. These findings not only provide valuable insights into the local hydrogeological dynamics, but also contribute to a broader understanding of fault zone behavior worldwide, offering critical knowledge for improving global models of groundwater flow and resource management in carbonate systems.