PREEXISTING FAULTS AND FAULTS THAT SHOULD NOT EXIST: EFFECTS OF MECHANICAL ANISOTROPY ON DIFFERENT SCALES?
Another class of steeply dipping reverse faults shows no evidence of an earlier normal faulting stage. The orientation of these faults thus appears to be at odds with the Coulomb criterion for shear fracture in an isotropic medium. One solution to this dilemma is to assume that steep precursor faults must have existed despite the lack of proof. Another solution, proposed here, is to assume that the causal anisotropy has a much smaller spatial scale than the large reverse fault. The situation may be akin to what has been observed in experimental deformation of slates: A shear zone forms parallel to the cleavage planes almost regardless of the angle they make with the shortening axis. Only when cleavage planes are subparallel or subnormal to the contraction direction, the attitude of the shear zone at sample scale (cm to dm) is not controlled by the submicroscopic anisotropy provided by preferred orientation of clay minerals and quartz. A similar scaling factor would imply that a kilometer-scale fault could reflect anisotropy such as steeply dipping joints on a meter to cm scale. The problem of misorientated faults has also been discussed for low-angle normal faults and regional strike-slip faults. Steep reverse faults and these examples may represent aspects of a single phenomenon that still awaits a satisfactory explanation.