Australia: The Land Where Time Began
Passive Fractures - Access for Weathering
Fractures are formed by planar parting in rocks. They are called sets where they run parallel to each other, systems are regular patterns that are formed of multiple sets. An orthogonal system is made up of 3 sets of fractures that are at right angles to each other, that subdivide the rock mass into cubic or quadrangular blocks, the shape being determined by their relative spacing, formed by cleavage or foliation partings, such as bedding planes and sheet fractures.
Where displacement has occurred along a fracture is it called a fault. As they are due to activities of the crust, faults are of tectonic origin. As with joints or partings that are not associated with dislocation, faults play a passive role as lines of weakness that allow water to penetrate, as a result of which they tend to form zones of weathering and erosion. In this way faults are involved in landscape development on various scales. Lineaments are major landscape features, of which some related forms may be tectonic, though they are more likely to be structural. On a smaller scale, clefts and slots occur from a few centimetres to a few kilometres.
Ridges, often in box-like patterns reflecting the fracture system geometry, arise where fracture zones have been indurated by minerals that have precipitated from groundwater, or where veins or sills have been intruded by rocks of material that is more resistant than the host rock. Some occur in karst terrains and sandstone, such as in the Bungle Bungle Range, and also in the site or local scale.
Faults in passive mode
In passive mode faults are linear weakness zones where water has penetrated to initiate weathering, as well as being exploited by rivers and other agencies of erosion. The lithological contrast that results from faulting is exploited to form structural features called fault-line scarps. Offsetting, cutting out and repetition of beds result from faulting, there are direct implications for landform development where resistant formations are involved. Weak and resistant formations are brought into juxtaposition by faulting, the resistant formation being eroded more slowly, it remains long after the weak juxtaposed rocks have been weathered and eroded away. The scarp is structural, not tectonic, as it results from weathering and erosion, it is called a fault-line scarp as opposed to a fault scarp.
Where lines of weakness result form faults, streams flow along them. These streams have unusually straight patterns in plan view, making them easy to identify, as is the case with fault-line and fault scarps. Where an impervious caprock traps groundwater in an aquifer it can often find a way to the surface along lines of weakness, resulting in lines of springs along faults. In places where mound springs form by the accumulation of precipitated chemicals at points of emergence of artesian water on the beds of Lake Eyre and Lake Frome the alignment of the springs indicates their association with faults.
Linear features on many scales that involve a number of kinds of features are evident on maps. These orientations are visible in coastlines, the channels of rivers and topographic or geologic boundaries. They result from crustal structures, usually fracture zones. Lineaments are straight or gently arcuate topographic features that are believed to be expressions at the surface of deep crustal structures, such as faults or fold axes, 10-100 km in length. Similar features of more than 100 km long are megalineaments, and linears are those that are less then 10 km long.
In Australia, trends that are NW-SE and NE-SW are especially common, being seen in coastlines, the courses of rivers, as well as the outlines of major topographic features such as the Murray Basin and the Kimberley, and latitudinal and meridional trends are also prominent, though are subordinate to the first named. In the older, crystalline shield areas and in ancient orogens the trends are very well developed, also being evident in the platform areas of younger sediments, of which the Darling Lineament is an example. It is believed likely that in the Darling Lineament and the western Murray Basin, where NW-SE and NE-SW trends are prominent in the strata from the Miocene, the fracture patterns have been imposed from the older basement rocks below. It is easily understandably how the movement of basement blocks would result in similar trending fractures being transmitted to overlying sediments, that are said to be underprinted by a process of resurgent or recurrent tectonics.
In Australia the lineament trends are believed to be of at least the latest Precambrian age, though probably much older. All around the world there are similar NW-SE and NE-SW patterns, especially in the ancient shields. They have been explained as resulting from torsion (twisting) of lithosphere that was brittle, that have been suggested to be possibly related to changes in the speed of rotation of the Earth. Conjugate (genetically and geometrically related) fractures have also been explained by the same process. A global pattern or network of lineaments has been recognised. If the continental plates have moved as has been demonstrated, especially over the last 200 My, it requires a high level of coincidence for the present lineaments in all continents to conform to the global network. This implies that they and the blocks they have developed upon have maintained their original orientations, or all have rotated by 90o, 180o or 270o. According to the authors, this presents a problem. A possible implication is that the lineaments are younger than is believed, or possibly they have evolved, at least in part, during and in response to imposed stresses during the most recent movement of the plates.
As well as being affected by descending waters, weathering and erosion to produce landscape features that are linear, other forces such as magmas, fluids and gases ascending from the mantle have exploited them. In places some of the rocks that resulted have been found to include various minerals. The discovery of the Olympic Dam copper-uranium-gold-silver deposit, occurring beneath about 300 m of barren strata, was partly due it is location on a prominent linear corridor.
Various circular and concentric patterns, on a continental scale, as well as the lineaments, have been found in the Canadian arctic and adjacent parts of eastern Canada, the northeastern USA and northern Europe. Since their discovery, circular patterns have been found in east and central Africa, that are cut by the Great Rift Valley, the western US states of Wyoming, Nevada, Utah and Texas, and in Australia. These patterns have been related to mantle plumes that are analogous to plumes related to hotspot volcanicity. An explanation of some regional drainage and topographic patterns have been suggested to be the result of doming and fracturing.
Joints and their significance
Fractures affect all types of rock to varying degrees, with a wide variety of patterns and spacing of fractures. In many instances rock masses are effectively subdivided into regular patterns. The strata in most sedimentary deposits are bedded , fractures normal to the bedding being well developed in them. Orthogonal systems subdivide many rocks of metamorphic and igneous origin, and many metamorphic rocks displaying cleavage or foliation, fractures that are spaced more of less closely, that run parallel, that reflect the planar arrangement of the grains. Sheet fractures, arcuate upward sets, most of which are convex-upward, are found well developed in many rocks, especially in granite. Columnar joints are vertical systems arranged in polygonal patterns, that are typical of volcanic rocks. All fractures and joints provide avenues for weathering and erosion, regardless of their origin and pattern. The geomorphological importance of any set of joints is dependent upon their openness, spacing (density) and pattern, as well as the physical properties of the rock mass.
Granite, a crystalline rock in which the crystals are closely interlocking, and with few voids or spaces separating the crystals, any that are present are not connected to each other, is an example that is exposed in about 15 % of the continental areas. Fresh granite is said to have low porosity, the few voids present occupying a very small proportion, 0.1 %, of the rock mass, though the level of porosity is greatly increased in weathered granite. This low porosity of granite, few, unconnected voids, results in the passage of water through the rock mass being inhibited, making it impermeable. In a rock such as granite, with low porosity, the importance of fractures in such a rock type, that is typically subdivided by a series of fractures that are connected, is increased because the rock is then considered to be pervious. The same applies to crystalline limestone. Highly porous rocks such as shale has low permeability, in part because of the small size of the pore spaces, and water movement is impeded by surface tension.
Granite - composition and weathering
A number of minerals usually make up granite, quartz, feldspar, usually orthoclase and/or microcline, and a mica, often biotite. When these minerals are weathered the nature of the alteration indicating they have reacted with groundwater that is circulating. It has been found that mica shows obvious signs of alteration in less than 10 years if being exposed to water, based on observations from tropical areas such as Madagascar and southeastern Brazil, granite having been quarried and used in the facings of buildings and subsequently examined microscopically at intervals after the construction of the buildings, and the exposure of the granite to the elements. Lavas containing feldspar that had been dated physically or historically showed that the feldspar had begun to show signs of weathering after being exposed to the elements for a few centuries or millennia. Though quartz is much more resistant than either mica or feldspar it eventually weathers and is dissolved. Quartz remains in the younger, lower levels of the profile, while being absent from the upper zone that have been undergoing weathering for the longest time. Silica is present in solution in water that has drained from granite masses, having been derived from the alteration of silicate minerals, precipitating as opaline stalactites and stalagmites in many places.
granite - landform assemblages
When many fractures dissect a granite mass they provide many sites for the penetration of water, allowing the rock to decay quickly, the decayed rock being evacuated to produce a plain with low relief, as long as rivers cane incise their beds. Especially in the land on ancient shields of the continents, that are subjected to long exposure to weathering by infiltration of meteoric water, it has happened in many areas. In the Western Australian Wheat Belt, and the northern part of the Eyre Peninsula, plains of this type are typical. In time, water penetrating along fractures, and eventually into the body of the fracture-defined block, the water gaining access by the decay of mica and then feldspar, even granite with fractures that are moderately spaced, about 1-3 m apart, is thoroughly altered. In this process time is as important as the spacing of the joints.
In weathered bedrock or regolith, many granite plains are eroded, while others are above bedrock that is intrinsically fresh, some being very extensive. Platforms are others that extend from the base of inselbergs, that are of limited area. Where there are patches of regolith or isolated hills protected by a resistant caprock, below which is regolith, the base of the regolith being coincident with the level of the adjacent plain, this suggests the regolith has been eroded from most of the plain, producing an etch and 2-stage type.
The boulder is one characteristic granite form that is explained by weathering of granite that has been divided into cubic or quadrangular blocks by fractures that are moderately spaced, and it has been suggested that inselberg may also be explained by this process (Twidale & Campbell, Source 1).
Not all boulders originate by the processes mentioned above. They also form by the breakdown of sheet structures that are exposed, and many others are initiated while beneath the surface, within the regolith. When a granite mass has been subdivided into angular blocks that are of cubic or rectangular shape by fractures water that has infiltrated along the partings alters the rock it comes into contact with. Water can readily penetrate the block after they have become permeable, but this weathering is uneven, plane faces being attacked from only 1 direction, edges, that are bounded by 2 faces, are attacked from 2 directions and corners, with 3 plane surfaces, are attacked from 3 directions. The result is that some parts of the block are weathered more rapidly than others, the corners weathering faster than edges that are in turn weathered faster than plane faces. The angular blocks are eventually converted to rounded kernels, corestones, that are surrounded by a zone of altered rock. The grus ("fine gravel") is the resulting friable, weathered rock that is eroded easily. Following the removal of the regolith by erosion a cluster or field of rounded boulders remains. Such boulders are typically found in many outcrops of granite, as well as being well developed in other types of plutonic rocks such as gabbro and diorite, basalt, and to a lesser extent in sandstone and limestone. Comparing boulders that originated from cubic blocks with the slab-like penitent rocks, monkstones and tombstones, that developed on gneisses having a well developed cleavage or foliation, the fracture pattern influence can be seen. The formation of boulders by this process evolve in 2 stages, the first is subsurface weathering that is fracture-controlled, the second is differential erosion of the grus, the result being exposed and in situ.
It is believed that many landforms, minor and major, may be explained by a similar 2-stage mechanism, in other rock types as well as in granite. The Devils Marbles on the Stuart highway to the south of Tennant Creek, Northern Territory, Australia, is a group of large granite boulders, some of which form small blocky hills, are believed to have formed by this 2-stage process. The Mesoproterozoic granite mass, about 1510 Ma, that intruded into strata of Proterozoic age that here are folded to form an arch or anticline. The bevelled ridges of the Davenport Ranges are outcropping quartzite members of the sequence. It is believed this surface may be a remnant of a surface from the Early Cretaceous, about 140-100 Ma. The Devils Marbles are in a broad valley flanked by the bevelled ridges, standing at a lower level. A mesa that is capped with a ferruginous regolith, stands between the elevation of the Devils marbles and the crests of the quartzite ridges. The mesa and the bevelled ridges are remnants of a land surface that the boulders of the Devils Marbles were beneath when they were weathered to their present form by groundwater they were in contact with, as suggested by the evidence.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|