According to the Andersonian interpretation, joints in the vicinity of normal faults have identical strikes to the faults, perpendicular to the direction of maximum remote tensile stress. However, there are examples in nature of joints associated with normal faults that can not be explained by Andersonian theory. Some joints in the vicinity of normal faults exhibit highly angular relationships with fault strike.
Numerical models of fault growth indicate a perturbation of the surrounding stress field. Such perturbations are felt by closely-spaced faults in the form of mechanical interaction, as is evidenced by fault slip distributions that deviate from those predicted for isolated faults. The models show that stress concentrations near fault tips can prevail over the regional stresses, and would thus be a more likely driving force for the growth of near-fault joints. The perturbed stress field around faults predicts joints with orientations that deviate from those that would result from the regional stress field. We address joint development around multiple normal faults in light of such stress field perturbations, as calculated using the boundary element method based program, Poly3d. The theory of linear elastic fracture mechanics is used to numerically predict joint orientations around normal faults with various three dimensional configurations.
Model results suggest that isolated elliptical faults can produce joints along the tipline that strike at a high angle to the fault (around 55°). A joint growing in this local stress field may be expected to curve as the joint propagates further from the fault-perturbed zone into the region where the remote stress dominates. However, where more than one fault is present, mechanical interaction may result in locally homogeneous stress fields that produce nearly straight joints at high angles to the faults (> 80°), such as in the relay zone of laterally overlapping faults. The significance of this effect in nature is demonstrated using an example from Arches National Park, Utah, where interfault blocks in Jurassic Entrada sandstone contain joints striking near perpendicular to the bounding normal faults.
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