Shear (geology)Figure 1.
Boudinaged Quartz vein(with strain fringe) showing sinistral shear sense. Starlight Pit, Fortnum Gold Mine, Western Australia.
Rocks typical of shear zones include mylonite, cataclasite, S-tectonite and L-tectonite, pseudotachylite, certain breccias and highly foliated versions of the wall rocks.
Asymmetric shear in basalt, Labouchere mine, Glengarry Basin, WA. Shear asymmetry is dextral, pen for scale.
A shear zone or shear is a wide zone of distributed shearing in rock. Typically this is a type of fault but it may be difficult to place a distinct fault plane into the shear zone. Shear zones may form zones of much more intense foliation, deformation, and folding.
Many shear zones host ore deposits as they are a focus for hydrothermal flow through orogenic belts. They may often show some form of retrograde metamorphism from a peak metamorphic assemblage and are commonly metasomatised.
Shear zones can be only inches wide, or up to several kilometres wide. Often, due to their structural control and presence at the edges of tectonic blocks, shear zones are mappable units and form important discontinuities to separate terranes. As such, many large and long shear zones are named, similar to fault systems.
Study of geological shear is related to the study of structural geology, rock microstructure or rock texture and fault mechanics.
Shear is the response of a rock to deformation usually by compressive stress and forms particular textures. Shear can be homogeneous or non-homogeneous, and may be pure shear or simple shear.
The process of shearing generally occurs within brittle-ductile and ductile rocks. Within purely brittle rocks, compressive stress results in fracturing and simple faulting.
1 Rocks
2 Shear zone
3 Mechanisms of shearing
4 Microstructures of shear zones
5 Ductile shear microstructures
6 Transpression
7 Transtension
8 See also
9 References
The process of shearing generally occurs within brittle-ductile and ductile rocks. Within purely brittle rocks, compressive stress results in fracturing and simple faulting.
1 Rocks
2 Shear zone
3 Mechanisms of shearing
4 Microstructures of shear zones
5 Ductile shear microstructures
6 Transpression
7 Transtension
8 See also
9 References
Rocks typical of shear zones include mylonite, cataclasite, S-tectonite and L-tectonite, pseudotachylite, certain breccias and highly foliated versions of the wall rocks.
Asymmetric shear in basalt, Labouchere mine, Glengarry Basin, WA. Shear asymmetry is dextral, pen for scale.
A shear zone or shear is a wide zone of distributed shearing in rock. Typically this is a type of fault but it may be difficult to place a distinct fault plane into the shear zone. Shear zones may form zones of much more intense foliation, deformation, and folding.
Many shear zones host ore deposits as they are a focus for hydrothermal flow through orogenic belts. They may often show some form of retrograde metamorphism from a peak metamorphic assemblage and are commonly metasomatised.
Shear zones can be only inches wide, or up to several kilometres wide. Often, due to their structural control and presence at the edges of tectonic blocks, shear zones are mappable units and form important discontinuities to separate terranes. As such, many large and long shear zones are named, similar to fault systems.
Mechanisms of shearing
The mechanisms of shearing depend on the pressure and temperature of the rock and on the rate of shear which the rock is subjected to. The response of the rock to these conditions determines how it accommodates the deformation.
The mechanisms of shearing depend on the pressure and temperature of the rock and on the rate of shear which the rock is subjected to. The response of the rock to these conditions determines how it accommodates the deformation.
Shear zones which occur in more brittle rheological conditions (cooler, less confining pressure) or at high rates of strain, tend to fail by brittle failure; breaking of minerals, which are ground up into a breccia with a milled texture.
Shear zones which occur under brittle-ductile conditions can accommodate much deformation by enacting a series of mechanisms which rely less on fracture of the rock and occur within the minerals and the mineral lattices themselves. Shear zones accommodate compressive stress by movement on foliation planes.
Shearing at ductile conditions may occur by , and dislocation creep within minerals, by fracturing of minerals and growth of sub-grain boundaries, as well as by lattice glide, particularly on platy minerals, especially micas.
Mylonites are essentially ductile shear zones.
Microstructures of shear zones
Typical example of dextral shear foliation in an L-S tectonite, with pencil pointing in direction of shear sense. Note the sinusoidal nature of the shear foliation.
Microstructures of shear zones
Typical example of dextral shear foliation in an L-S tectonite, with pencil pointing in direction of shear sense. Note the sinusoidal nature of the shear foliation.
During the initiation of shearing, a penetrative planar foliation is first formed within the rock mass. This manifests as realignment of textural features, growth and realignment of micas and growth of new minerals.
The incipient shear foliation typically forms normal to the direction of principal shortening, and is diagnostic of the direction of shortening. In symmetric shortening, objects flatten on this shear foliation much the same way that a round ball of treacle flattens with gravity.
Within assymmetric shear zones, the behavior of an object undergoing shortening is analogous to the ball of treacle being smeared as it flattens, generally into an ellipse. Within shear zones with pronounced displacements a shear foliation may form at a shallow angle to the gross plane of the shear zone. This foliation ideally manifests as a sinusoidal set of foliations formed at a shallow angle to the main shear foliation, and which curve into the main shear foliation. Such rocks are known as L-S tectonites.
If the rock mass begins to undergo large degrees of lateral movement, the strain ellipse lengthens into a cigar shaped volume. At this point shear foliaions begin to break down into a rodding lineation or a stretch lineation. Such rocks are known as L-tectonites.
Stretched pebble conglomerate L-tectonite illustrating a stretch lineation within a shear zone, Glengarry Basin, Australia. Pronounced assymmetric shearing has stretched the conglomerate pebbles into elongate cigar shaped rods.
Ductile shear microstructures
Very distinctive textures form as a consequence of ductile shear. An important group of microstructures observed in ductile shear zones are S-planes, C-planes and C' planes.
S-planes or schistosité planes are generally defined by a planar fabric caused by the alignment of micas or platy minerals. Define the flattened long-axis of the strain ellipse.
C-planes or cisaillement planes form parallel to the shear zone boundary. The angle between the C and S planes is always acute, and defines the shear sense. Generally, the lower the C-S angle the greater the strain.
The C' planes, also known as shear bands and secondary shear fabrics, are commonly observed in strongly foliated mylonites especially phyllonites, and form at an angle of about 20 degrees to the S-plane.
The sense of shear shown by both S-C and S-C' structures matches that of the shear zone in which they are found.
Other microstructures which can give sense of shear include:
sigmoidal veins
The incipient shear foliation typically forms normal to the direction of principal shortening, and is diagnostic of the direction of shortening. In symmetric shortening, objects flatten on this shear foliation much the same way that a round ball of treacle flattens with gravity.
Within assymmetric shear zones, the behavior of an object undergoing shortening is analogous to the ball of treacle being smeared as it flattens, generally into an ellipse. Within shear zones with pronounced displacements a shear foliation may form at a shallow angle to the gross plane of the shear zone. This foliation ideally manifests as a sinusoidal set of foliations formed at a shallow angle to the main shear foliation, and which curve into the main shear foliation. Such rocks are known as L-S tectonites.
If the rock mass begins to undergo large degrees of lateral movement, the strain ellipse lengthens into a cigar shaped volume. At this point shear foliaions begin to break down into a rodding lineation or a stretch lineation. Such rocks are known as L-tectonites.
Stretched pebble conglomerate L-tectonite illustrating a stretch lineation within a shear zone, Glengarry Basin, Australia. Pronounced assymmetric shearing has stretched the conglomerate pebbles into elongate cigar shaped rods.
Ductile shear microstructures
Very distinctive textures form as a consequence of ductile shear. An important group of microstructures observed in ductile shear zones are S-planes, C-planes and C' planes.
S-planes or schistosité planes are generally defined by a planar fabric caused by the alignment of micas or platy minerals. Define the flattened long-axis of the strain ellipse.
C-planes or cisaillement planes form parallel to the shear zone boundary. The angle between the C and S planes is always acute, and defines the shear sense. Generally, the lower the C-S angle the greater the strain.
The C' planes, also known as shear bands and secondary shear fabrics, are commonly observed in strongly foliated mylonites especially phyllonites, and form at an angle of about 20 degrees to the S-plane.
The sense of shear shown by both S-C and S-C' structures matches that of the shear zone in which they are found.
Other microstructures which can give sense of shear include:
sigmoidal veins
Transpression
Transpression regimes are formed during oblique collision of tectonic plates and during non-orthogonal subduction. Typically a mixture of oblique-slip thrust faults and strike-slip or transform faults are formed. Microstructural evidence of transpressional regimes can be rodding lineations, mylonites, augen-structured gneisses, mica fish and so on.
A typical example of a transpression regime is the Alpine Fault zone of New Zealand, where the oblique subduction of the Pacific Plate under the Indo-Australian Plate is converted to oblique strike-slip movement. Here, the orogenic belt attains a trapezoidal shape dominated by oblique splay faults, steeply-dipping recumbent nappes and fault-bend folds.
The Alpine Schist of New Zealand is characterised by heavily crenulated and sheared phyllite. It s being pushed up at the rate of 8 to 10 mm per year, and the area is prone to large earthquakes with a south block up and west oblique sense of movement.
Transtension
Transtension regimes are oblique tensional environments. Oblique, normal geologic fault and detachment faults in rift zones are the typical structural manifestations of transtension conditions. Microstructural evidence of transtension includes rodding or stretching lineations, stretched porphyroblasts, mylonites, etc.
See also
Convergent boundary
Crenulation
Fault (geology)
Foliation (geology)
Rock microstructure
Sense of shear indicators: dextral and sinistral
Transpression regimes are formed during oblique collision of tectonic plates and during non-orthogonal subduction. Typically a mixture of oblique-slip thrust faults and strike-slip or transform faults are formed. Microstructural evidence of transpressional regimes can be rodding lineations, mylonites, augen-structured gneisses, mica fish and so on.
A typical example of a transpression regime is the Alpine Fault zone of New Zealand, where the oblique subduction of the Pacific Plate under the Indo-Australian Plate is converted to oblique strike-slip movement. Here, the orogenic belt attains a trapezoidal shape dominated by oblique splay faults, steeply-dipping recumbent nappes and fault-bend folds.
The Alpine Schist of New Zealand is characterised by heavily crenulated and sheared phyllite. It s being pushed up at the rate of 8 to 10 mm per year, and the area is prone to large earthquakes with a south block up and west oblique sense of movement.
Transtension
Transtension regimes are oblique tensional environments. Oblique, normal geologic fault and detachment faults in rift zones are the typical structural manifestations of transtension conditions. Microstructural evidence of transtension includes rodding or stretching lineations, stretched porphyroblasts, mylonites, etc.
See also
Convergent boundary
Crenulation
Fault (geology)
Foliation (geology)
Rock microstructure
Sense of shear indicators: dextral and sinistral
Ductile Shear Zones, Textures and Transposition
Imagine a cold and wet day in northern Scotland, which may not be a far stretch of the imagination if you've ever visited the area. As you are mapping a part of the Scottish Highlands you are struck by the presence of highly deformed rocks that overlie relatively undeformed, flat-lying, fossiliferous sediments. This relationship is even more odd because the overlying unit has experienced much higher grade metamorphism than the underlying sediments, and it contains no fossils. When you arrive at the contact between these two characteristic rock suites, you notice that they are separated by a distinctive layer of particularly fine-grained rock. The regional relationships of these two suites and their superposition already suggest that the contact is a low-angle reverse (i.e., thrust) fault. So, what is the distinctive fine-grained rock at the contact? In your mind you envision the incredible forces associated with the emplacement of one unit over the other and you surmise that the rock at the contact was crushed and milled, like what happens when you rub two bricks against each other. Using your classes in ancient Greek you decide coin the name mylonite for this fine-grained rock unit, because 'mylos' is Greek for milling. Something like this happened over a hundred years ago in Scotland where the late Precambrian Moine Series ('crystalline basement') overlie a Cambro-Ordovician quartzite and limestone ('platform') sequence along a Middle Paleozoic low-angle reverse fault zone, called the Moine Thrust. This area was mapped by Sir Charles Lapworth of the British Geological Survey in the late 19th century. Anecdote has it that Lapworth became convinced that the Moine Thrust was an active fault and that it would ultimately destroy his nearby cottage and maybe take his life; his life ended in great emotional distress. In many areas you will find similar zones in which the deformation is markedly concentrated. The deformation in these zones is manifested by a variety of structures, which may include isoclinal folds, disrupted layering, well-developed foliations and lineations, and other deformation features. These zones, called ductile shear zones, may contain some of the most important information about the deformation history of an area, so let us begin this chapter with their definition. A ductile shear zone is a tabular band of definable width in which there is considerably higher strain than in the surrounding rock. The total strain within a shear zone typically has a large component of simple shear, and as a consequence, rocks on one side of the zone are displaced relative to those on the other side. In its most ideal form, a shear zone is bounded by two parallel boundaries outside of which there is no strain. In real examples, however, shear zone boundaries are typically gradational. The adjective ductile is used because the strain accumulated by ductile processes that may range from cataclastic flow to crystal-plastic processes and diffusional flow (Chapter 9).
Imagine a cold and wet day in northern Scotland, which may not be a far stretch of the imagination if you've ever visited the area. As you are mapping a part of the Scottish Highlands you are struck by the presence of highly deformed rocks that overlie relatively undeformed, flat-lying, fossiliferous sediments. This relationship is even more odd because the overlying unit has experienced much higher grade metamorphism than the underlying sediments, and it contains no fossils. When you arrive at the contact between these two characteristic rock suites, you notice that they are separated by a distinctive layer of particularly fine-grained rock. The regional relationships of these two suites and their superposition already suggest that the contact is a low-angle reverse (i.e., thrust) fault. So, what is the distinctive fine-grained rock at the contact? In your mind you envision the incredible forces associated with the emplacement of one unit over the other and you surmise that the rock at the contact was crushed and milled, like what happens when you rub two bricks against each other. Using your classes in ancient Greek you decide coin the name mylonite for this fine-grained rock unit, because 'mylos' is Greek for milling. Something like this happened over a hundred years ago in Scotland where the late Precambrian Moine Series ('crystalline basement') overlie a Cambro-Ordovician quartzite and limestone ('platform') sequence along a Middle Paleozoic low-angle reverse fault zone, called the Moine Thrust. This area was mapped by Sir Charles Lapworth of the British Geological Survey in the late 19th century. Anecdote has it that Lapworth became convinced that the Moine Thrust was an active fault and that it would ultimately destroy his nearby cottage and maybe take his life; his life ended in great emotional distress. In many areas you will find similar zones in which the deformation is markedly concentrated. The deformation in these zones is manifested by a variety of structures, which may include isoclinal folds, disrupted layering, well-developed foliations and lineations, and other deformation features. These zones, called ductile shear zones, may contain some of the most important information about the deformation history of an area, so let us begin this chapter with their definition. A ductile shear zone is a tabular band of definable width in which there is considerably higher strain than in the surrounding rock. The total strain within a shear zone typically has a large component of simple shear, and as a consequence, rocks on one side of the zone are displaced relative to those on the other side. In its most ideal form, a shear zone is bounded by two parallel boundaries outside of which there is no strain. In real examples, however, shear zone boundaries are typically gradational. The adjective ductile is used because the strain accumulated by ductile processes that may range from cataclastic flow to crystal-plastic processes and diffusional flow (Chapter 9).
So, a shear zone is like a fault in the sense that it accumulates relativedisplacement of rock bodies, but unlike a fault, displacement in a ductile shear zone occurs by ductile deformation mechanisms and no throughgoing fracture is formed. The absence of a single fracture is largely a consequence of movement under relatively high temperature conditions or low strain rates. Consider a major discontinuity that cuts through the crust and breaches the surface. In the first few km, brittle processes will occur along the discontinuity, which result in earthquakes if the frictional resistance on discreet fracture planes is overcome abruptly. Displacement may also occur by the movement on many small fractures, a ductile process called cataclastic flow (Chapter 9).
In either case, frictional processes dominate the deformation at the higher crustal levels of the discontinuity and this crustal segment is therefore called the frictional regime. With depth, crystal plastic and diffusional processes, such as recrystallization and superplastic creep, become increasing important mainly because temperature increases. Where these mechanisms are dominant, typically below 10-15 km depth for normal geothermal gradients (20-30 degrees C/km) in quartz-dominated rocks, we say that displacement on the discontinuity occurs in the plastic regime. Not surprisingly, the transitional zone between a dominantly frictional and dominantly plastic regime is called the frictional-plastic transition, but more commonly we call this the brittle-plastic transition . Note that technically it is not correct to call this the brittle-ductile transition, because ductile processes (such as cataclasis) may occur in the frictional regime. So, a crustal discontinuity that is a brittle fault at the surface, is a ductile shear zone with depth. Associated with this contrast in deformation processes, we predict that the discontinuity changes from a relatively narrow fault zone to a broader ductile shear zone with increasing depth because the host rock weakens (i.e., a reduction in differential stress).
Mylonites are dominated by the activity of crystal-plastic processes, which produces yet another characteristic element of deformed rocks: crystallographic-preferred fabrics or textures. The topic of textures will both be introduced and applied in this chapter, although some of the theory logically follows the material presented in Chapter 9.
Mylonites are dominated by the activity of crystal-plastic processes, which produces yet another characteristic element of deformed rocks: crystallographic-preferred fabrics or textures. The topic of textures will both be introduced and applied in this chapter, although some of the theory logically follows the material presented in Chapter 9.
Secondly, rocks within ductile shear zones typically are intensely folded and the original layering is transposed into a tectonic foliation. Transposition will close the chapter, but we emphasize that it is common but not unique to ductile shear zones. In our chapter you will see that shear zones may have more than one foliation, a strong lineaton and that shear zone rocks commonly contain rotated fabric elements, grain-shape fabrics, and in particular a grain size that is less than that of the host rock. Arguably, ductile shear zones are the most varied structural feature, and perhaps the most informative.
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