Is lithostatic loading important for the slip behavior and evolution of normal faults in the Earth's crust?

Normal faults growing in the Earth's crust are subject to the effects of an increasing frictional resistance to slip caused by the increasing lithosta tic load with depth. We use three-dimensional (3-D) boundary element method numerical models to evaluate these effects on planar normal faults with va riable elliptical tip line shapes in an elastic solid. As a result of incre asing friction with depth, normal fault slip maxima for a single slip event are skewed away from the fault center toward the upper fault tip. There is a correspondingly greater propagation tendency at the upper tip. However, the tall faults that would result from such a propagation tendency are gene rally not observed in nature. We show how mechanical interaction between la terally stepping fault segments significantly competes with the lithostatic loading effect in the evolution of a normal fault system, promoting latera l propagation and possibly segment linkage. Resultant composite faults are wider than they are tall, resembling both 3-D seismic data interpretations and previously documented characteristics of normal fault systems. However, this effect may be greatly complemented by the influence of a heterogeneou s stratigraphy, which can control fault nucleation depth and inhibit fault propagation across the mechanical layering. Our models demonstrate that alt hough lithostatic loading may be an important control on fault evolution in relatively homogeneous rocks, the contribution of lithologic influences an d mechanical interaction between closely spaced, laterally stepping faults may predominate in determining the slip behavior and propagation tendency o f normal faults in the Earth's crust.