A prominent joint set occurs in Arches National Park, Utah, that is bounded by, and oriented orthogonal to normal faults in map view.
Fault-orthogonal joints in the Jurassic-aged Entrada Formation are confined to blocks between sub-parallel faults that have spacings of up to a few hundred meters. The joints are confined to the vicinity of normal faulting and are not part of a regional joint set. The joints define composite zones that exhibit only mode I opening, with no subsequent reactivation by shearing. The spacing between joint zones ranges from a few meters to a few tens of meters.
Joints oriented at high angles to faults are common in natural examples, however, interpretations usually involve the inference of mode II slip along a pre-existing interface, such as a fault, joint or bedding plane, that results in mode I opening of so-called "tail cracks" or "splay cracks" in the extensional quadrants at the tips of the slipped interface. However, this model predicts a maximum angle of only 70.5° between the tail cracks and the slipped interface.
When observed in map view at the surface of the Earth, the normal faults have slip vectors into the plane of the map view. Joints oriented orthogonal to normal fault strike in map view are thus not tail cracks resulting from mode II slip, therefore, tail crack theory does not directly apply.
In order to understand the formation of joints orthogonal to normal faults, one must consider the fault evolution, fault properties, fault geometry, and slip distribution. Such effects can be modeled numerically using principles of fracture mechanics for elastic solids, and the boundary element method. Numerical modeling can be used to predict the characteristics of brittle failure about normal faults, providing an analysis that circumvents the use of Andersonian theory of normal faulting, which predicts joint development parallel to fault strike (orthogonal to the maximum remote extensional stress) as opposed to orthogonal fault-joint relationships as described from Arches National Park.
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