Recent numerical modeling of normal faults has demonstrated a number of important consequences for the effect of the 3D fault geometry on fault evolution and mechanical interactions. However, existing 3D models only address faults that are either frictionless or contain a cohesive end-zone along the fault tipline. We include frictional behavior on the fault related to the effect of an increasing lithostatic load with depth. This addition of frictional behavior is important because the manner in which a normal fault slips affects both its subsequent evolution in terms of propagation tendency, and deformation in the region around the slipping fault. A field example where joints are related to normal faulting in Arches National Park, Utah, demonstrates the range in possible orientations of joints that form in the perturbed stress field of a normal fault.
We model normal faults using a 3D linear elastic, displacement discontinuity, boundary element method program. The faults have a circular tipline geometry, and a dip of 50 degrees. An increasing lithostatic load with depth resolves normal tractions on the fault plane that increase in magnitude in the down-dip direction along the fault. A Coulomb frictional slip criterion governs the slip, which is driven by a remote tectonic load. Incorporating this frictional behavior into the fault model results in asymmetric slip distributions on the fault that are skewed towards the upper fault tip. Slip gradients are greatest towards the upper extent of the fault tipline, and differ from predicted slip distributions on frictionless normal faults. Our predicted slip distributions indicate that fault propagation tendency is enhanced in the vicinity of the upper tip. The high slip gradient provides an increased likelihood for joint growth in the tipline vicinity at the top of the fault, assisted by the lower confining stress in this region. The principal stress trajectories, which were examined using a 3D graphical visualization program, vary along the fault tipline and indicate a range of possible joint orientations with respect to the fault.
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