SOME TECTONIC CONSEQUENCES OF FLUID OVERPRESSURES AND SEEPAGE FORCES AS DEMONSTRATED BY SANDBOX MODELLING

Abstract

We demonstrate some of the effects of fluid overpressures and seepage forces in tectonics using the results of scaled physical models. According to von Terzaghi's principle, the total state of stress in a saturated porous medium is the sum of an effective stress which controls the deformation and an isotropic fluid pressure. In tectonics, for any variation in pore fluid pressure, it is common to assume that total stresses are constant. Such an assumption is not always warranted. In experiments where air flows through sand packs, we demonstrate that gradients in fluid overpressure cause seepage forces and that these may modify total stresses. Using these principles, we have obtained yield functions for Fontainebleau sand at very small effective stresses (>5 Pa). We have corrected our results and other previously published results for significant effects of sidewall friction. For normal stresses larger than 30 Pa, the sand has a linear Coulomb yield function. The corrected cohesion is much smaller than previously reported. In extensional tests involving vertical fluid flow, the dihedral angle between conjugate normal faults decreases as effective stresses tend to zero. For nonvertical fluid flow, seepage forces modify the principle directions of stress, producing listric faults. In a sloping sedimentary sequence, the dip of normal faults depends on the overpressure gradient. We have also obtained a reorientation of principal stresses in two-dimensional numerical models. By introducing a layer of small permeability, we have been able to induce gravitational gliding in an overpressured sloping sand pack.