Hydrogeological coupling of epikarst dynamics and tectonic discontinuities: Impacts on water intrusion at the Holy Monastery of Kleiston, Greece

The hydrogeological regime and the persistent moisture intrusion phenomena at the Holy Monastery of Kleiston are fundamentally dictated by a complex tectonic framework within the Sub-Pelagonian Unit of Mount Parnitha, NW of Athens, Greece. This study identifies the monastery’s location as a site of intense structural deformation, where the stratigraphic sequence is governed by a series of successive tectonic nappes. The structural architecture is defined by two primary thrust faults: an upper thrust (No 1) that positions carbonate rocks over volcanosedimentary formations, and a lower, sub-parallel thrust (No 2) that repositions the volcanosedimentary sequence atop an underlying lower carbonate series. Central to the water intrusion mechanism is the identification of two low-angle fault surfaces, designated as No 3 and No 4, which act as primary hydraulic discontinuities. Crucially, both of these tectonic surfaces are situated within the mass of the lower carbonate rocks and are oriented sub-parallel to the lower thrust, effectively mirroring its geometry. These intra-lithic discontinuities serve as the primary structural controls for groundwater movement, with the upper surface (No 3) coinciding with the main foundation level and the roof of the Catholicon, while the lower surface (No 4) dictates the base of the local epikarst hydrogeological system.

The interaction between this tectonic fabric and the carbonate lithology has facilitated the development of an extensive epikarst zone, exceeding 10 meters in thickness, characterized by high secondary porosity. This zone is defined by two principal discontinuity systems -striking NNW-SSE and NE-SW- alongside secondary fractures that have undergone significant karstification. The mechanical widening of these joints is further enhanced by the deep penetration of root systems, which extend up to seven meters into the rock mass, creating vertical conduits. Hydrogeologically, the epikarst functions as a perched aquifer, recharged through a combination of direct autogenic precipitation and lateral allogenic contribution from upstream debris and weathered volcanosedimentary mantles.

The manifestation of water and humidity within the monastery's functional spaces is the direct result of epikarst spring fronts emerging at the intersections of these specific tectonic surfaces with the building infrastructure. The upper fault surface (No 3) directs groundwater discharge into the Catholicon and the adjacent storage caverns, a process exacerbated by the thin carbonate cover which offers minimal lag time between precipitation events and intrusion. Simultaneously, the lower fault surface (No 4) facilitates discharge into minor caves and areas beneath the monastery’s retaining walls and communal spaces. This structural control explains the persistence of moisture even during arid periods, as the complex network of tectonic voids and karstified joints within the epikarst serves as a shallow reservoir. Consequently, the study concludes that the water intrusion at the Holy Monastery of Kleiston is a structurally driven phenomenon, where sub-parallel tectonic discontinuities within the lower carbonates serve as the primary conduits for the localized hydrogeodynamic discharge of the epikarst aquifer.