Structural control on karst water circulation and speleogenesis in a lithological contact zone: The Bossea cave system (Western Alps, Italy)

Geological surveys, structural mapping, and thin section analysis helped to unravel the speleogenesis and water circulation in the Bossea cave (Northern Italy), an unconventional karst system at the contact between carbonates and underlying insoluble rocks. The surveys within the cave and in the surrounding surface outcrops revealed a complex structural setting caused by the coupling of lithological-mechanical heterogeneities with two major regional left-lateral ESE-WNW strike-slip faults. Transpression associated with the faults led to a disharmonic deformation, accommodated by a detachment surface, between the buckled basement meta-volcanics and the disharmonically-folded-marble sequence laying on top. Low-angle shear zones, recumbent or drag folds, and small stacks of duplexes are localized next to this detachment. The preferential pathways for water circulation inside the vadose zone are the folded bedding interfaces in the meta-carbonate sequence. Fracture clusters at slipping bed terminations, on the other hand, provide through-sequence connectivity. Once underground stream water flow reached the cataclastic and highly deformed meta-volcanics of the detachment core and damage zones, erosion started, removing large volumes of rocks. The downward erosion of the Permian basement rocks caused gravitational instability and subsequent roof collapses, aided by structural control in the damage zone above the detachment surface (slip surfaces in between duplexes or drag folds). This gravitational instability ultimately led to the formation of the giant halls that are characteristic of the downstream part of the Bossea cave.