Geology Reference
In-Depth Information
in compression. Similar conclusions were reached with respect to a granite exposed in a quarry at
Quenast in Belgium. In some areas, e.g. New England, there is evidence of what Dale (1923) called
double-sheet structure, that is, two sets of sheet joints, the strikes of which intersect at oblique
angles. This, surely, is explicable only in terms of shortening of the massif.
It has been shown by
in situ
stress measurements that the Australian continent is in a state of
substantial horizontal compression, and many other examples of pronounced compressive stress
in the horizontal plane, stress far greater than suggested by theoretical considerations, have been
recorded. The excessive stress may be attributed to relic compression derived from past orogenies,
though the role of continuing or modern earth movement of compressive type should not be over-
looked; there is, indeed, much evidence of contemporary compression in the crust (Müller, 1964;
Denham, Alexander and Worotnicki, 1979). For example, Isaacson (1957) reported that at a depth
of 1056 m in one of the shafts of the Kolar Goldfield, southern India, the theoretical stresses ought
to be 313.538 kg/cm
2
vertically and 134.98 kg/cm
2
horizontally, whereas, in reality, the measured
stresses were 409.15 kg/cm
2
and 471.01 kg/cm
2
respectively. Expansion consequent on the release of
inherent stress caused a shaft 3.81 m diameter at a depth of some 3048 m to decrease by 0.5334 cm
in a north-south direction and by 1.16 cm east-west. Similar results have been obtained in the Snowy
Mountains of New South Wales (Moyé, 1964).
Coates (1964) reported deformation in a tunnel some 90 m below the surface in southern Ontario.
There was rapid expansion of the walls (contraction of the tunnel) in the first forty days after exca-
vation followed by a period of several months of slower but similar changes so that, 240 days after
exposure, there had been up to 4.6 cm of lateral expansion. Vertical movements were noted, but
they were consistently small.
Many of the data and arguments outlined above as inconsistent with the offloading hypothesis
can be used in support of the suggestion that sheet fractures and structures are associated with
shortening. In addition, experimental work by Holzhausen (1989) has shown that lateral compression
of partly confined blocks produces arcuate upwards strain trajectories (Fig. 2.14a). This finding
Strain lines/fractures
(a)
regolith
(b)
(c)
Figure 2.14.
Result of compression of partly confined block: (a) laboratory situation (After Holzhausen,
1989), (b) result of compression of partly confined block in terms of the possible field reality,
(c) suggested alternative result, possibly resulting from rapid or massive application of stress.
