Geology Reference
In-Depth Information
Figure 1.9.
Sheet fractures exposed in coastal cliffs, Pearson Islands, Investigator Group, eastern Great
Australian Bight.
complex and apparently random or nonsystematic, though others form repeated patterns and are
said to be systematic. The origin of many joints is still unclear. Explanations based on desiccation,
fluid pressure, Earth's tides, and stress have been suggested.
In granite as in other rocks, many joints are nonsystematic, but one system and one set of frac-
tures are widely developed (Kendall and Briggs, 1933; Davis, 1984). An orthogonal or, frequently,
a rhomboidal system, consisting of three sets intersecting approximately at right angles to one
another is characteristic of granite but also of many other massive rocks (
Fig. 1.7a).
The set consists
of arcuate partings essentially, and most commonly, disposed parallel to the land surface and
is known as sheet fracture (Fig. 1.9). Both sets or types are widely developed and they commonly
co-exist. In granitic terrains they are of prime importance because of their direct impacts on landform
development. In addition, their origins carry implications for the genesis of various granite forms,
major and minor. For this reason these major fracture systems are discussed separately.
1.6
ORTHOGONAL FRACTURE SYSTEMS
Orthogonal systems in granite occur at various scales from the regional to the site (Figs 1.7 and
1.8b).
Cloos (1923, 1931) related orthogonal systems to flow banding in batholiths (Figs 1.10a and
b and Balk, 1937) has suggested that some steeply dipping fractures may be radiated or fan joints.
Orthogonal or rhomboidal patterns of fractures related to shear stress are called conjugate joints,
or shears if they are demonstrably faults. The fractures develop at an angle to the plane of greatest
principal stress (s1 -
Fig. 1.11),
with the axis of s1 splitting the acute angle between the conjugate
fractures. Cloos (1931) showed experimentally that orthogonal or rhomboidal systems of fractures
develop as a result of compression, the conjugate shears forming two sets each disposed at roughly
45° to the direction of stress, but in reality the angular relationship between the two shear planes
varies according to the force applied and the nature of the country rock. Any tension fractures
caused by brittle failure develop at right angles to s3, the plane of least main stress. In many areas,
orthogonal systems are geometrically related to regional structures, such as lineaments and known
major faults, and are probably due to crustal stress. Thus, the regional orthogonal sets of Eyre
Peninsula (South Australia) and the adjacent Gawler Ranges (Hills, 1956; Twidale and Campbell,
1990) are disposed at 45-60° to ancient lineaments, such as the Hinge Zone or Torrens Lineament
