An Analysis of Thrust Faults

Program year: 1975/1976Digitized from print original stored in HDRPart I. Through a literature search on the mechanics of thrust faulting, I have found that reduction of the effective downward stress component of the overburden pressure due to abnormal fluid pressures in the subsurface is the key theory in the geologist's current explanation of the mechanics of thrust faulting. I have found five possible mechanisms for developing abnormal pore pressure in the subsurface. The five theories area (1) location of outcrop of formation up dip at a higher elevation than the surface where well was drilled, (2) rapid compaction and depositon, (3) tectonic compression of a water-saturated shale or clay, (4) Clay dehydration (Fowers, 1967), a process in which montmorillonite is changed to illite releasing chemically bound water, and (5) Aquathermal pressuring, (Barker, 1972), a process in which an isolated volume of water-filled sediments is subjected to increased temperature. The expansion of the pore fluids due to increased temperature is greater than the expansion of the minerals of the rock, and this effectively generates an excess pressure. I propose that clay dehydration and aquathermal pressuring are two steps involved in the sequence of deposition of sediments and the generation of abnormal pore pressure. After normal compaction of the shales occurs, at a depth of 6000 to 7000 feet, the montmorillonitic clays begin to dehydrate. With the effective permeability being very, very low, the release of this bound water is confined to this shale zone. After more subsidence and deposition of the basin, the temperature increases with depth. Normally the increased temperature would not generate an excess pressure, because some of the water could easily bleed off. But within this confined shale zone, the low permeability will not allow the pressure to equalize, rather it builds up due to expansion beyond that which is expected under a normal hydrostatic gradient