The two principal suggested modes of facilitating overthrust faulting are (1) lubrication at the sole by evaporite beds or micaceous shales and (2) flotation due to anomalously high (> hydrostatic) pore-water pressures. Past rapid sedimentation and tectonic compression have been suggested as important causes of anomalously high water pressure (Hubbert and Rubey, 1959). We suggest osmosis as another important possibility.
Anomalous water pressures of at least 400 bars above hydrostatic have been measured in oil wells; many of these wells penetrate evaporites and/or shales which separate formation waters of differing salt concentrations. These pressures are explicable by assuming osmotic equilibrium across a membrane which separates saturated halite solution from solutions up to 10 weight per cent NaCl. Thus, osmotic equilibrium may be an important mechanism for floating thrust sheets. Lubrication of thrust sheets by shales or evaporites and flotation by anomalously high pressures may be simply different manifestations of the same geologic milieu.