Abstract - Although the 3-D geometry of tectonic, geological faults is well constrained from both seismic reflection surveys and outcrop studies relatively little is known about their growth. In particular, stress and strain distributions within layered sequences and their control on the localisation and nature of deformation have not been satisfactorily modelled. We perform 2-D numerical experiments that focus on the growth of tectonic normal faults in multilayered sequences using the Particle Flow Code (Itasca Consulting Group). A linear contact model with frictional sliding is used. Competent rocks (e.g. limestone, sandstone) are modelled as coarse, bonded particles and incompetent rocks (e.g. shale) as fine, non-bonded particles. The rheologies of the two model materials are calibrated by means of biaxial compression tests with varying confining pressure. The incompetent material is near perfectly plastic, i.e. the material flows at constant stress after yielding. At intermediate confining pressures the competent material deforms by localised shearing, i.e. faulting, following failure at peak strength. At high confining pressure deformation is less localised. The deformation displayed by PFC model materials is consistent with those derived from experimental data.
Localisation of a single through-going fault is achieved by introducing a pre-cut 'fault' at the base of a multilayer sequence; this boundary condition means that a single fault, rather than several faults, are localised within the model. The down thrown side of the pre-cut fault moves with constant velocity; a constant overburden pressure is applied. The modelling replicates many of the features seen in natural faults. Initially a monocline develops above the ‘basement’ fault in the competent units. Folding is accommodated by flow within the incompetent layers and by slip along the layer interfaces. Faults in the multilayer sequence generally grow from bottom to top in a staircase geometry caused by refraction across mechanical interfaces. The model faults reproduce the bifurcation of fault surfaces and subsequent evolution to a throughgoing fault observed in nature.
Abstract of talk given to:
1st International PFC Symposium, Gelsenkirchen, Germany, November 2002