Australia: The Land Where Time Began

A biography of the Australian continent 

Plate Tectonics - Faults

Where a narrow band is formed by several adjacent fracture surfaces of zones along which a significant shear displacement has occurred are termed Faults. Though faults are often said to signify brittle deformation there is a transition to ductile behaviour where shear zones develop instead. Shear zones display intense deformation occurring along a narrow band, a place where loss of cohesion occurs on limited surfaces that are discontinuous. Faults are often thought of as large shear fractures, though the boundary separating features that are properly regarded to be shear fractures or joints are not sharply established. Though shear fractures on a millimeter scale are called microfaults, faults may range from several decimetres to hundreds of kilometres in length, being localised features or of lithospheric scale and define plate boundaries. Varying from 10-3 m in hand specimens or outcrop scale to 105 m at regional or global scale. There are several ways of recognising faults, which indicates that shear displacement, either by scarps being present in recent faults, offsets, displacements, gaps or overlaps of rock masses with aspects on them that are identifiable as bedding, layering, etc.

Nomenclature and orientation

According to the authors1 the nomenclature of faults is often inclear, as the terms used come from sources that are widely different. An example is that many terms describing faults originate with old mining usage, including the term "fault", and the terms are often not well constrained. As with other geological surfaces, fault surfaces, that can be inclined at different angles, orientation is given by the strike and dip. The dip angle is used for the division in faults, according to which high-angle faults dip at more than 45o, and low-angle faults dip at less than 45o. Rocks are divided into 2 offset blocks, at either side of the fracture surface, by faults. In the case of an inclined fault, the block above the fault surface is the hanging wall block (HWB) and its corresponding surface is the hanging wall (HW); and the underlying block supporting the weight of the hanging wall is the footwall block (FWB), the corresponding surface being the footwall (FW). If points that were homologous prior to fracturing can be recognised at each side of the fault, reconstruction of the displacement vector or slip can be performed over the fault surface, for both magnitude and direction. Relative movements can be either parallel to the fault-dip direction, dip-slip faults, or parallel to the fault strike, strike-slip faults. Vertical displacement of blocks occurs with dip-slip faults and horizontal displacement with strike-slip faults. Movement of blocks can be oblique, oblique-slip faults, as in the case of composite faults, where the block movement along the fault surface is diagonal, which allows the recognition of a dip-slip component and a strike-slip component. The dip-slip component can be further separated into horizontal movement, the heave, and a vertical component, the throw. The block displaced relatively downwards in dip-slip faults is the down-thrown block (DTB) and the block displaced relatively upwards is the up-thrown block (UTB). Blocks involved in strike-slip faults are usually named according to their orientation, such as north block and south block, etc. Accurate deduction of movement vectors is not possible in most cases, the displacement needing to be guessed by observing the offset layers. The separation can be defined, in this case, as the distance between 2 homologous planes or features, at either side of the fault, that can be measured in some specific direction, such as the strike and dip directions of the layer.

The initial fault is of a limited extent, expanding laterally progressively, with the offset between blocks increasing over time. Where there is no appreciable displacement of blocks the fault limit, or fault termination, is the tipline. When the surface of the Earth is reached by faults, the fault trace is the intersection line between the fault plane and the topographic surface, and the yip point or tip, is the end of the fault trace. Blind faults terminate before reaching the surface of the Earth, so there is no corresponding fault trace at the surface, though they can cause deformation, such as monocline folds, at the surface to the fault bounded by termination, or tip lines, at the front and upper ends.

Fault planes come in a number of different forms, most faults appearing as surfaces that are fairly flat, though they can have inclination changes at depth. Several steps are shown by some faults - in high angle faults, stepped segments with a decreasing dip are known as flats, while in low angle faults, segments with a sudden increase in dip are referred to as ramps. At the topographic surface the flats and ramps are replaced by characteristic deformation; in the case of normal faults, e.g., bending of rocks in the section of the hangingwall situated above a ramp produces a synclinal fold, and the resulting deformation over a flat is an anticlinal fold. In faults with vertical surfaces, such as in strike-slip faults, ramps can also be present, being referred to as bends, or oriented normal (side-wall ramp) or oblique (oblique ramps) oblique to the fault strike. Listric faults, faults with a cylindrical or rounded surface, that have a steady dip decrease with depth, and end in a low-angle or horizontal detachment. According to the authors1 detachment faults can be described as low-angle faults which generally join a listric fault in the surface separating a faulted hangingwall, that has a set of imbricate listric or flat surface faults, from a footwall that is not deformed. At mechanical or lithological contacts detachments form where rocks have different mechanical properties, and the friction coefficient commonly decreases. Secondary imbricate fault sets can be either synthetic, if they have the same dip sense as the main fault, or antithetic, if they have dip direction that is opposed to that of the main fault.

Classification of faults

Several kinds of faults can be defined based on the relative displacement of blocks along any fault surface. In dip-slip faults, with relative block movements parallel to the direction of dip, are separated into normal faults and reverse or thrust faults, according to the sense of shear. According to the authors1 normal faults are generally of high-angle type, with surfaces dipping close to 60o and the hanging wall block slides down the surface of the fault as the down-throw block. It is also possible for low-angle faults to form. Faults in which the hangingwall block is forced up the surface of the fault, reverse and thrust faults, define the up-thrown block. A distinction between thrust and reverse faults has been made that is based on the surface angle, though both terms are considered by many authors to be synonymous. Relative movements along the strike of the fault surface are strike-slip faults, generally with steep surfaces that are close to 90o with the result that the terms hangingwall and footwall do not apply. Strike-slip faults have been divided into 2 kinds based on the relative shear movement; the fault is right-handed or dextral if an observer situated astride the fault surface if the block on the right approaches the observer, and left-handed or sinistral if the left block approaches the observer, and the direction the observer is facing is irrelevant. The dip and strike components derived from the relative block movements can be used to define oblique-slip faults There are 4 possible combinations, normal-sinistral, normal-dextral, reverse-sinistral and reverse-dextral. Rotational faults, , faults showing displacement gradients along the fault surface, are formed when there is rotation of a block with respect to the other block along the fault surface.

 

Sources & Further reading

  1. Leeder, Mike, Perez-Arlucea, Marta, 2006, Physical Processes in Earth and Environmental Sciences, Blackwell Publishing Ltd.

 

 

 

 

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 11/05/2013

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading