Multiscale karst systems, typically below seismic resolution, are important contributors to permeability but also provide significant challenges in drilling operations and in flow simulations. A substantial portion of subsurface karsts is of hypogenic origin, that is, is associated with upward movement of chemically aggressive fluids. The Morro Vermelho Karst System and Cave developed within Proterozioc shallow water carbonates and is an inspiring analog for buried hypogenic karsts. MVKS formed in association with a regional strike-slip fault. In an initial stage, deformation in the “softer” carbonates overlying more rigid quartzites and basement was accommodated by a 100s of m wide semi-ductile shear zone. Wholesale dolomitization took place and numerous dolomite-filled veins formed. With progressing strike-slip, the fault propagated upward creating an anticline parallel to the fault zone, causing the development of m-long extensional fractures, the arrival of SiO2 rich fluids in the karts and, eventually, the formation of the cave. Vertical fracture corridors and/or faults capturing deep, generally layer parallel flow are present in different settings, from foredeeps to rifted margins and are primary candidates for widespread hypogenic karst development.@en

The Morro Vermelho Cave (MVC) (Brazil) developed within the Morro Vermelho karst system, which affected Neoproterozoic limestones (Salitre Formation). The MVC experienced little interactions with meteoric processes and is an example of a hypogenic cave formed during strike-slip deformation. The Salitre carbonates in the MVC experienced distributed deformation along an elongated domain overlying a buried strike-slip fault. Gently dipping, semiductile shear zones formed with decimeter-scale (3.9 in.) dolomitic veins. In our model, Mg-rich fluids flowing along the Salitre aquifer caused at the same time extensive dolomitization of the body of rock (100-m [328-ft] scale) experiencing distributed deformation. With progressive displacement, the deep strike-slip fault propagated upward causing the development of an anticline pop-up, steepening sedimentary layers, and steep 1-10-m-long (3.3-33.8-ft) fractures, which served as pathways for upward fluid flow. These steep extensional fractures made it possible for fluids flowing in lower, quartzitic aquifers to enter the carbonate aquifer causing silica deposition in rock cavities and in fractures and fault planes. Following the main stage of speleogenesis, silica deposition took over again depositing on the cave walls a continuous silica crust, rarely observed in other settings worldwide. The interplay between regional bedding-parallel flow and focused circulation of fluids along steep faults and dipping layers, and the associated rock-fluid interactions are not unique to the contractional settings presented but can also occur in association with similar faults in rifted continental margins.

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