PREEXISTING FAULTS AND FAULTS THAT SHOULD NOT EXIST: EFFECTS OF MECHANICAL ANISOTROPY ON DIFFERENT SCALES?

Deformation of the lithosphere, especially in the brittle realm, is often demonstrably influenced by older anisotropy. Some of the best documented examples are of inversion tectonics, where regions that have undergone extension – mostly sedimentary basins - become shortened after the regional stress field has changed. The mechanical weakness that induces inversion has been attributed to (1) thermal weakening, in particular of the mantle, (2) replacement of basement by weak sedimentary rocks in grabens, or (3) weakness of the normal faults. I present an example from Central Europe where widespread, low-magnitude normal faulting was followed by equally widespread shortening, often involving fault reactivation with reverse kinematics. The low magnitude of extension suggests that in this case the faults that were mechanically weak and moved as reverse faults in spite of unfavorably steep dips. The existing anisotropy was therefore on the same spatial scale as the newly formed structures.

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.