Geology Reference
In-Depth Information
suggestion that A-tents are initiated in the subsurface beyond the range of either insolation or the
heat of bushfires.
If A-tents were due to offloading (see Chapter 2), their crests would be aligned parallel to the
contour of the slope on which they occur, whereas in many areas they display a preferred orienta-
tion which is geometrically related to regional tectonic trends
(
Fig. 11.11).
The suggestion that
A-tents are a manifestation of pressure release consequent on erosional offloading does not explain
why the angular A-tents are typically developed on midslope, and it is not consistent with the sur-
vival of the host masses of granite in inselbergs which have resisted weathering and erosion, because
they are massive, monolithic and in compression. Two of the three A-tents of recent origin noted
in the Wudinna district have formed without the aid of erosion, either natural or artificial, while the
third is on a surface exposed by the removal of about a metre of grus.
A-tents and associated forms are manifestly youthful, yet occur on host forms that are evidently
of some antiquity; which is inconsistent, for if due to pressure release the minor suite would surely
have formed as soon as the host forms were delineated by differential subsurface weathering.
The suggestion that tree roots have, during their growth and thickening, raised slabs of rock to
such an extent that friction has been overcome, is evidenced at many sites. Vertical displacement
of granite slabs has taken place at one site on Wudinna Hill, but no lateral displacement is
involved, the displacements are irregular, and presumably the slabs will return to, or close to, their
original positions once the tree dies and decays.
If A-tents were due to impact following slippage, there ought to be gaps whence came the slabs
upslope from the A-tents. There are no such spaces associated with A-tents (e.g.
Fig. 11.9)
.
Conversely, where slippages do occur they are not associated with A-tents. Slipped slabs and
A-tents are associated and may have a similar causation, but they are not genetically related.
Some mineral alteration causes expansion, and it could be argued that such expansion could
account for A-tents. Certainly, some of the granite in which A-tents are well-developed, for
instance at Mt Magnet, Western Australia, Kulgera, Northern Territory and Augrabies, Namibia,
is impregnated with iron, manganese and silica, but many other A-tents are developed in intrinsi-
cally fresh granite. Moreover, the solution of limestone (in which A-tents are developed - see
Chapter 12) seems unlikely to result in volume increase.
Overlapping slabs cannot be explained as due to slippage, for the lower of the two components
involved laps over the upper and not the reverse, as would be the case if downslope movement
were involved. Furthermore, there are no gaps at either the lower or upper end of the two slabs
involved. The feature could be a collapsed A-tent, but the collapse has not been caused by attrition
of the rock near the erstwhile crestal fracture, because the two opposed ends of the slab match per-
fectly, and the rock shows no signs of undue weathering. If an A-tent collapsed with the downslope
slab coming to rest on the upslope member, it must have been caused by some catastrophic event.
A similar overlapping slab occurs on the northeastern midslope of Little Wudinna Hill. It now
overlaps the adjacent slab a matter of 15 cm, and another platy fragment has slipped beneath the
raised slab. That A-tents are associated with shaking is suggested by an A-tent located high on the
eastern slope of Wudinna Hill. Here the original arrangement of slabs could not be reconstituted
even if the effects of expansion were removed, for a third slab, formerly located just upslope, has
slipped into the cavity formed by the raised slabs of the A-tent
(
Fig. 11.21)
.
On the other hand, many parts of the world are known to be in substantial compression.
Earthquakes and earth tremors are ubiquitous. Dramatic disturbances are associated with some.
Thus, large boulders, originally partly embedded in regolith, but in contact with other boulders
beneath the surface, were squeezed up
(Fig. 11.22)
and translocated laterally some 40 cm during
the Great Hanshin earthquake of January, 1995 (Ikeda, 1996). The horizontal acceleration of pres-
sure waves associated with major earthquakes is known to buckle artificial paving, and shock
waves generated by quarry blasting are known to cause A-tents and other disturbances (Twidale
and Sved, 1978). Similar contemporary arching associated with rock bursts is reported in a gran-
ite quarry in Tocumweal, New South Wales. Again, in the Mariz Quarry near Guitiriz, Galicia,
both vertical and horizontal wedges, as well as small large-radius domes, can be seen developed
in association with compressive structures (see Chapter 2 and Fig. 2.9).
