Australia: The Land Where Time Began

A biography of the Australian continent 

Sheet Fractures

Sheet structures and sheet fractures are essential features of bornhardts. Thick slabs or attenuated wedges subdivide many masses of rock by sets of arcuate fractures called sheet fractures. When they are exposed in bornhardts or domical hills, they are usually convex-upwards, and horizontal or dipping gently on the hill crests, though dipping steeply on the margins or on the hill flanks, it at this point that the partings are most noticeable. Beneath the floors of the valleys they are less visible due to the detritus cover, though are developed in such situations.

Sheet structures, the slabs, that can be up to 10 m thick, between the partings, are formed as sheet fracture subdivides the mass of the rock. It has been suggested by many that the thickness of sheet structures increases with depth, though the authors say this is not true everywhere sheet fractures occur, such fractures have been observed in quarries in the US, in Vermont, as well as on Dartmoor in the UK, to depths of 100 m, and much deeper in mine shafts. In the vicinity of major fractures, that are vertical or near vertical, they are greatly increased in steepness. They are often very well developed in granite, and granite gneiss, as well as various crystalline plutonic rocks. They have even been seen in silicic volcanic rocks, as in the Gawler Ranges, in South Australia, and in number of formations of sedimentary origin, such as limestone, conglomerate and sandstone in such places as the Navajo Sandstone of the Colorado Plateau, and at Uluru in central Australia, as arkose sandstone. . The host rock is always massive, whatever its composition and origin. Other structures and textures, such as jointing that is orthogonal or columnar, rift and grain, foliation and flow structures, are all cut across by sheet structure when they occur.

For about 150 years sheet structures have been noted by geologists, quarrymen and miners, their origin having been debated since that time. When considering Inselberg origins sheet fractures are crucial, especially bornhardts. It is also important to know if they imply compressive stress. There are 2 groups of earlier explanations for sheet fractures, one group involving external (exogenetic) factors and the other group involves internal (endogenetic) factors. All fractures, including faults, are manifestations of erosional offloading, all partings at depth being closed by the pressure of the overlying rocks, though the distortions causing the fractures are still present in the rock, the strain is expressed as fractures when erosion removes the overlying rock that had been exerting the pressure that prevented the fractures from opening. The offloading allows the strain to be expressed as fractures, it does not cause them.

Several possible explanations involving exogenetic factors or agencies have been put forward. One suggestion has been insolation, the heat of the sun being suggested, though it has been rejected, if only because sheet fractures have been found to extend far deeper than the shallow zone that is affected by diurnal, annual or longer term heating from the sun. As not all partings show signs of chemical weathering, this can be dismissed as a primary cause, and it fails to explain why regularly arranged zones within the rock are affected by weathering. The authors consider the erosional offloading hypothesis to be plausible, and it is widely accepted. According to this explanation, in rocks such as granite, e.g., erosion of several kilometres of material is implied by the exposure of the rock. Because of the large mass of rock removed, the vertical loading is reduced by a very large amount, allowing the rock to expand. Subsequent to unloading or offloading by erosion, it has been suggested leads to radial expansion, the tangential or sheeting fractures developing as a response to such expansive tendencies.

Other suggested explanations involve endogenetic factors, such as plutonic injection and metamorphic expansion, that the authors say can be rejected, based on such things as not all sheet fractures occur in plutonic or metamorphic rocks. A suggestion that sheeting fracture sets have a similar origin to synclines or anticlines in rocks of sedimentary origin, being due to compression has many advocates, and is apparently growing in acceptance. The compressional hypothesis has been supported by several lines of evidence and argument that are consistent with it, though others appear to refute offloading. Sheet fractures are younger than other fracture systems such as orthogonal fractures and columnar fractures, as sheet fractures cut across the other fracture types. The authors suggest that if a tendency to radial expansion developed, the stresses would be accommodated along pre-existing fractures.

Bornhardts that have survived as a result of their massive nature, in which there are few, tight fractures, typically have sheet fractures. The authors suggest this is not compatible with expansive tendencies, saying though the explanation sounds plausible it is flawed. An example they point to is the development of sheet fractures in such rocks as the Gawler Range Volcanics and the sandstones of Uluru, which have not been deeply buried. Several minor landforms developed on bornhardts, in particular A-tents and triangular wedges, appear to be related to compressive stress release. Basins beneath bornhardts have been reported from various places around the world, that the authors say are impossible in terms of pressure release, though in deeply eroded landscapes they are explicable.

It has been found by direct measurements that many parts of Australia, as well as on other continents, are in substantial compression. Thrust or compressional faulting has been involved in many modern earthquakes, such as the those at Meckering  in 1968 and Minnipa in 1999. Sheet fractures have been associated with compression, as indicated by much of the available evidence, though the offloading hypothesis is very much in favour, as is seen in most modern text books of geomorphology and physical geology.

Sources & Further reading

  1. Twidale, C.R. & Campbell, E.M., 2005, Australian Landforms: Understanding a Low, Flat, Arid, and Old Landscape, Rosenberg Publishing Pty Ltd
Author: M. H. Monroe
Last updated 02/04/2020



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