Geological shear

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.

Rocks

Rocks typical of shear zones include mylonite, cataclastite, S-tectonite and L-tectonite, pseudotachylyte, certain breccias and highly foliated versions of the wall rocks.

Shear Zones

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 can be only inches wide, or up to several kilometres wide.

Shear zones may form zones of much more intense foliation, deformation, and folding.

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 deformaion.

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

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.

Ductile Shear Microstructures

Very distinctive textures form as a consequence of ductile shear. The microstructures of ductile shear zones are S-planes, C-planes and C' planes.

S-planes or sissalement planes are parallel with the shear direction and are generally defined by micas or platy minerals. Define the flattened long-axis of the strain ellipse.
C-planes or cissalement planes form oblique to the shear plane. 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 are rarely observed except in ultradeformed mylonites, and form nearly perpendicular to the S-plane.

Other microstructures which can give sense of shear include:

sigmoidal veins
mica fish
rotated porphyroblasts

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.

References

Diagrams and definitions of shear

Structural geology | Geological processes

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