Shear zones can contain various structures. The structures discussed here are veins which are found on blocks of karst limestone in the Jebel Shams mountains, Oman (Fig. 1).

Figure 1. Karst topography in limestone - white calcite veins can be seen in the blocks in the foreground and are prominent top left of the view.

The veins in Fig. 2 are arranged in a zone. Some have a characteristic sigmoidal shape (Fig. 2) showing that they rotate from they moment they form (simple shear mechanism) - this geometry can be used to determine the sense of shear - in this case it is sinistral (to the left).

There are some younger structures which cross-cut the veins - these have a relatively small displacement as indicated by the blue arrows on Fig. 2. Interestingly, the movement at this later time is right lateral or dextral.

Figure 2. MAP VIEW: Calcite veins in limestone. some have characteristic sigmoidal shape indicating a sinistral shear direction (red arrows). Later dextral faults are shown alongside the blue arrows.

Another example from the same locality is given in Fig. 3. The sigmoidal shape is not as well developed as in the previous example - neverthess a sinistral sense of shear can still be established.

Figure 1. Calcite filled veins in Limestone are good indicators for direction of shear — Figure 3. MAP VIEW: Another example, although the shapes are not sigmoidal like some of those in Fig. 2.

The most pervasive structures in this outcrop are branch-like markings on the surface of the limestone beds (Fig. 4) - possible plumose markings. Shells are also locally present (Fig. 5).