Joints at high angles to normal fault strike: an explanation using 3-D numerical models of fault-perturbed stress fields

Structural methods based on homogeneous stress states predict that joints g rowing in an extending crust form with strike orientations identical to nor mal faults. However, we document a field example where the strikes of genet ically related normal faults and joints are almost mutually perpendicular. Field relationships allowed us to constrain the fracture sequence and tecto nic environment for fault and joint growth. We hypothesize that fault slip can perturb the surrounding stress field in a manner that controls the orie ntations of induced secondary structures. Numerical models were used to exa mine the stress field around normal faults, taking into consideration the e ffects of 3-D fault shape, geometrical arrangement of overlapping faults, a nd a range of stress states. The calculated perturbed stress fields around model normal faults indicate that it is possible for joints to form at high angles to fault strike. Such joint growth may occur at the lateral tips of an isolated fault, but is most likely in a relay zone between overlapping faults. However, the angle between joints and faults is also influenced by the remote stress state, and is particularly sensitive to the ratio of faul t-parallel to fault-perpendicular stress. As this ratio increases, joints c an propagate away from faults at increasingly higher angles to fault strike . We conclude that the combined remote stress state and perturbed local str ess field associated with overlapping fault geometries resulted in joint gr owth at high angles to normal fault strike at a field location in Arches Na tional Park, Utah. (C) 1999 Elsevier Science Ltd. All rights reserved.