Australia: The Land Where Time Began
Tectonic landforms are associated with active folding and faulting, recent development of features such as arches, troughs, domes and basins, as well as fractures.
A silcreted land surface of southwestern Queensland, and parts of New South Wales and South Australia that are adjacent, dating from the Miocene, has been folded and subsequently dissected, though much of the original land surface remains intact. The folding, dating from the Miocene, at least 5 million years old, has resulted in an area of gentle dips. A series of broad simple anticlines, synclines, domes and basins were formed when the silcreted surface was squeezed. The tilted or dipping flanks remain, though the stretched crests of the anticlines have been breached. Northeast of Perth repeat levelling demonstrates gentle folding or warping.
Fractures are of 2 kinds, differential motion occurs along faults, but no movement occurs along joints, though may joints are actually faults in which there is only a small amount of displacement. Faults can range from a few centimetres to hundreds of kilometres long, joints are not often found that are more than a few kilometres long. Both faults and joints allow the access of water that can lead to erosion.
Features generated by faults
Faults are fractures in the rock that involves displacement of the block on one side relative to the other, which cause earthquakes and earth tremors. Scarps, that are produced by faulting during earthquakes, are the most common landform that results directly from faulting. An example of linear dirt scarps forming was seen in 1968 when an earthquake struck near the town of Meckering in the southwest of Western Australia. Following the earthquake the dirt scarps could be seen in the landscape of the area. At the time of their formation these scarps were from 1.5 to 3.0 m high, and crevasses were also present. Being in soil instead of rock, the scarps and crevasses were gradually subdued and eventually completely removed by erosion.
In 1990 an earthquake struck Tennant Creek in the Northern Territory. This quake resulted in a arcuate scarp, a fault scarp, about 10km long. As such features are soon removed by weathering and erosion, some regard these formations as rare, though others believe all scarps, having been produced by dislocation, should be referred to as fault scarps, even when they are heavily dissected and eroded down. Subsequent movement of the earth in earthquakes tends to be centred on fault lines as they are lines of structural weakness once they have formed, the existing fractures being used to accommodate the motion. The result is that faulting 'tends to be recurrent or repeated'. Triangular facets are formed when prominent scarps have formed by uplift along faults, the scarp then being dissected by streams. Another result of recurrent movement is the rejuvenation of streams and the formation of wineglass or bottle-shaped valleys. Scarps often persist as tectonic landscape features, though they are dissected by the streams.
Fault scarps often form in groups, some of the groups being so complex that it has not been possible to discern a simple pattern, some forming en echelon patterns, fault scarps that are offset but parallel, though in some areas they can extend over great distances running parallel to each other.
Long, narrow uplands that have been upthrust along faults are called 'horsts', such as the Mt Lofty Ranges near Adelaide that is a complex example involving several fault zones. The fractures mostly run north-south, but swing to the southwest, forming an en echelon pattern to the south of Adelaide. On this horst, the summit, a lateritised planate surface, has been preserved, and the margins have been downfaulted, which allowed entry of the sea into the basins that resulted. As these are of Eocene or younger age, it indicates that some of the faulting must have occurred in the latest Cretaceous or earliest Tertiary, though it is evident dislocation has taken place more recently. Half-grabens or fault-angle valleys are present on the backslopes of blocks such as the Para, Eden and Clarendon blocks, near Adelaide and to the south, as a result of the fault blocks dipping to the east and to the south.
When a long narrow block subsides between faults that are parallel, it is called a graben or rift valley. Spencer Gulf and Gulf of St Vincent, in the Gulfs region of South Australia, are examples of a complex downfaulted feature that has been invaded by arms of the sea. A compressional setting is indicated by the presence of some reverse faults. Adjacent structures partially override blocks occupied by Spencer Gulf and Gulf of St Vincent. A half-graben is occupied by Lake Torrens and the authors suggest a similar structure may be occupied by Lake Eyre, as a fault scarp that ruptured the gypsum crust (gypcrete) forms the western side of the lake. 80 km to the west, faults bordering Mt Margaret displaced the gypcrete.
The authors suggest the compressional reverse faulting seen at the margins of the faulted blocks in the Gulfs region may be unusual. Most grabens result from crustal tension, as occurs at mid-oceanic ridges. The largest known from the continents is Great Rift Valley of East Africa, the Red Sea and the lands of the Mediterranean. Smaller examples occur in the American west, Iceland, Lake Baikal, Lake George in New York State, as well as the valleys of the Rhine and the Rhone. Normal, or gravity, faults border these features, which indicates extension. Many terminate in bifurcations.
Some grabens are very deep, such as some from northern Arabia that have at least 10 km of alluvial fill. In the southwest of Western Australia, the Darling Fault is the wall of a half-graben that runs parallel to the coast that is 12 km deep. There are not many examples of classical rift valleys in Australia, a situation that the authors suggest is probably a result of the continent being in compression. An example is present in Tasmania, where the lower Tamar River flows in this type of structure.
Faults & changed drainage patterns
In the northeastern part of the Eyre Peninsula, South Australia, the way drainage systems can be changed by faulting is illustrated by the Ash Reef fault block that was uplifted across the pre-existing drainage system, with the result that the water backed up to form a lake or swamp. Wrench faults sometimes cause rivers flowing across them to form an offset, of a dog-leg pattern, continuing in their pre-existing bed when they pass the blockage, the result being that the formerly straight-flowing river then has 2 straight sections with an offset where the river flows along the fault zone section, that can be up to 90o, between them. If the fault continues to be active, the offset section of the river can increase in length. Outside Australia good examples of these features can be been in Sumatra, Indonesia, and the San Andreas Fault Zone in California.
The Eucla district in Victoria has a spectacular example of fault-related drainage disturbance to the south of Deniliquin, where the Murray River swings sharply to the south. The deflection of the original course of the river was caused by the uplift of the Cadell Fault or Tilt Block that was uplifted across the river course between 20,000-30,000 years ago. The river continued to flow by flowing around the block, mainly around the southern end of the block.
In New South Wales the Wakool River has a similar origin. There is a depression, Green Gully, on the gentle western slope of the fault block that has been described by W.J. Harris as one of several 'Well-marked tortuous depressions with a general east-west direction ... it is a wide shallow valley with relatively steep sides, and with every appearance of being a disused, partly infilled river course'. It is believed to be the channel, or possibly one of the channels, of the ancestral Murray River. A distinct ridge, with an escarpment facing east-west, results from the Cadell Fault continuing southward, that has blocked the local drainage causing the formation of Lake Cooper. In the Mulculca area of the Barrier Ranges, western New South Wales, small streams have been blocked by faulting in more recent times.
Neotectonic forms are the fractured and folded rocks at the surface of the Earth that have formed less than 5,000 years ago, postdating the Miocene. It was neotectonic features that were associated with the Meckering earthquake. Contemporary warping and folding has been demonstrated by repeat levelling in Australia, as well as in areas thought to be stable such as France and European Russia, and in the areas known for frequent, severe earthquakes, such as the European Alps, the Himalayas, and in particular along the circum-Pacific rim, Japan, western US, among others.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|