Structure of Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 13

Figure 2.9 Classification of faults with regard to the direction of relative displacement (Wittke 1990)

Faults may be large, pervasive features of several meters in thickness or may be of
relatively limited local extent of millimeters in thickness. Displacements up to several
kilometers may arise.
The formation of faults is frequently associated with high stresses of tectonic origin (Section
9.3). As a consequence, the intact rock may be decomposed into small pieces. Thick fillings
of pulverized rock material such as mylonites are therefore often to be found within a fault.

2.5 Folds

Folding of rock mass is a consequence of regional deformation due to tectonic forces.
A systematic overview of folds, including a genetic interpretation of their origin is given
by Ashgirei (1963).
As an example, in Fig. 2.10 a regularly folded rock mass with two fold axes is shown
(Wittke 1990). A fold with downward diverging limbs is known as an “anticline” and a
fold whose limbs diverge upward is denoted as “syncline” (Murawski 1983).

The shear and tensile stresses initiated in the rock mass during folding, in most cases lead to
fractures, i.e. to the forming of joints. In the example represented in Fig. 2.10, these are de-
noted with respect to the position of the fold axes as diagonal, transversal and longitudinal
joints and are oriented perpendicular to the folded rock layers. Thus, homogeneous areas
with closely spaced parallel joints appear. The joints are frequently interrupted by discon-
tinuities that are parallel to the bedding or the schistosity (Fig. 2.10). However, in highly
ductile rocks such as rock salt, folding widely takes place without fracturing and jointing.