Control of fluid pressures on the formation of listric normal faults

•We develop Limit Analysis to investigate the formation of listric normal faults.•Listric faults can occur without viscous/ductile behaviours, nor flexural stresses.•Disequilibrium-compaction fluid pressures control listric fault formation.•We apply the method to the offshore Niger Delta and the Gulf of Mexico.•The observed listric faults are due to shallow fluid-retention depths. Listric normal faults are widespread in the extension of the upper crust. Despite major advances in understanding the formation of listric faults through various experiments, the mechanical conditions that allow their formation are highly debated. In particular, Anderson's faulting theory predicts that newly formed normal faults are planar and are dipping at least at 45∘, and in practice, at 60–65∘ for most rock types. Here, we develop Limit Analysis to investigate the formation of a listric fault at the onset of slip linking a deep detachment to the topographic surface. We find that listric normal faults can occur in the brittle upper crust without appealing to viscous or ductile behaviours, nor to flexural stresses. The disequilibrium-compaction fluid pressures, typically observed in many sedimentary basins, are essential for the formation of listric faults. The fluid pressure is hydrostatic down to the fluid-retention depth ZFRD and sustains a higher gradient below this depth. Parametric studies show that the surface slope is also essential for the formation of listric faults even with a gently dipping surface slope (≤4∘), whereas a flat topographic surface leads to a simple Andersonian geometry at the onset of slip. The method is applied to two field examples in order to determine fluid overpressures that best match the fault shapes interpreted from seismic data. For the offshore Niger Delta, the simulated normal faults match the observed listric faults with a very shallow ZFRD = 0.5–0.75 km, and below ZFRD the fluid pressure has a lithostatic gradient, consistent with the observed fluid-pressure profiles. To reproduce a series of listric faults joining on one common low-angle detachment in the NW Gulf of Mexico, we demonstrate that a shallow ZFRD = 0.7–1.1 km is required, below which the fluid pressure increases to the lithostatic pressure on the detachment, in agreement with the fluid-pressure observations.