THE EFFECT OF FLUIDS ON THE GEOMETRIC AND MECHANICAL EVOLUTION OF FOLDS AT SMALL THRUST RAMPS: INITIAL RESULTS FROM 2D, COUPLED, FLUID-MECHANICAL MODELS

We use a 2-D continuum, finite-difference code to model an outcrop-scale (20 m long, 5 m thick) region of a pre-existing thrust ramp to study the first-order mechanical and geometrical effects of groundwater on the evolution of small- to meso-scale, fault-related folds. We compare both `dry' and `wet' cases, in order to illustrate: (1) The mechanical effect of fluids on deformation: effective stress-related effects, occur at the micro- and meso-scale in the porous rock and in fault-zones, in which pore-fluid pressure fluctuations affect the likelihood of failure by decreasing or increasing frictional resistance and thus yield strength, and; (2) The effects of deformation on fluid-flow: deformationally derived, seismogenic pumping either by poro-elastic loading which will pump fluid away from the area of elastic loading, or by poro-plastic dilation of saturated rock, which pumps fluid into the area of active deformation – with predicted, although transient, dilatant hardening.

The software uses an elastic–plastic constitutive relation, including a Mohr–Coulomb failure criterion and a non-associated flow rule, and coupled fluid flow, with bulk rock properties that approximate sandstone and shale, and the fluid properties of water. The influence of volumetric strains on the pore pressure is reflected in the fluid constitutive law. In turn, pore-pressure changes cause mechanical deformations to occur. The equations of motion use total stresses. However, effective stresses are used to determine whether the material has yielded The model is fully dynamic.

Results of initial models show that fluid flow occurs in 3 ways: 1) poro-elastic processes, i.e. fluid is forced out of areas of active compressional loading and into areas experiencing tension; 2) dilatant pumping into areas that are actively dilating during brittle-plastic simple shear, and; 3) where dilation may have evolved to zero, the active fluid pumping mechanism is stress-state softening, where pressure within the actively deforming shear zone is lower than the elastically loaded region outside.

The presence of fluids has distinct effects on the mechanical evolution on most models, the most obvious feature is the broad distribution of finite shear strain during wet-dilatant model runs, in contrast to focused zones of simple shear in dry runs.