Large-scale, massive burial dolomitization affected Upper Carboniferous carbonates of the southwestern Variscan Cantabrian Zone (Spain). Replacive and void-filling dolomites formed and were post-dated by calcite cementation. The process of dolomitization was due to the circulation of hypersaline and hydrothermal marine-derived brines, controlled by rock anisotropies. The dolomites probably formed in Early Permian time during post-thrusting orocline formation in an extensional setting. Lithospheric delamination induced increased heat flow and allowed thermal convection of the fluids. The dolomites from the Cantabrian Zone are excellently exposed and represent an interesting outcrop analogue for hydrocarbon reservoirs elsewhere. In comparable geotectonic settings of the European Variscan Orogen large masses of similar dolomites could have formed if Mg-rich brines were available.