Structural consequences of seepage forces in overpressured thin-skinned thrust belts
Résumé
In thin-skinned thrust belts, fluid overpressure can facilitate detachment. If the overpressure has a gradient, it causes fluid flow, according to Darcy's Law. Where fluid migrates upward, through a sealing layer of small permeability, the pressure profile has a ramp. Where fluid generates within a source layer, the profile has a parabolic peak. The migrating fluid imparts seepage forces to all elements of the solid framework and these forces have structural consequences. We discuss experiments on thrusting in multilayered granular models, where permeability is a function of grain size. At the base of a sealing layer, if the pressure reaches lithostatic values, it facilitates detachment. Fore-thrusts and back-thrusts form in alternation and the resulting prism acquires a small apical angle. The faults curve up, from the detachment, through the sealing layer, becoming straighter in the overburden. Similar structural styles are common in overpressured deltas and accretionary prisms. For example the outer thrust belt of the Niger delta has a very small apical angle. It detaches on sharp surfaces within Eocene shale. Fore-thrusts and back-thrusts alternate. They curve up, from the detachment, through the shale, becoming straighter in the overburden. According to seismic velocities and pressure data from wells, the fluid pressure reaches high values in the shale, but not in the overburden. Probably the overpressure is due to hydrocarbon generation in the shale, which is a recognized source rock. Currently, hydrocarbons reach the sea bottom through hanging-wall vents above thrust faults.