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In geology, a shear zone is a thin zone within the Earth's crust or upper mantle that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shear zone takes the form of a fracture called a fault .
A shear zone is a tabular to sheetlike, planar or curviplanar zone composed of rocks that are more highly strained than rocks adjacent to the zone. 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.
The geologic subdiscipline of stratigraphy is primarily concerned with depositional contacts, [4] while faults and shear zones are of particular interest in structural geology. Faults and shear zones can be regarded as a form of secondary structure in the rock beds. [5]
They initiate at depth in zones of intracrustal flow, where mylonitic gneisses form. Shear along the fault is ductile at mid to lower crustal depths, but brittle at shallower depths. The footwall can transport mylonitic gneisses from lower crustal levels to upper crustal levels, where they become chlorititic and brecciated. [5]
Transtension is the state in which a rock mass or area of the Earth's crust experiences both extensive and transtensive shear. As such, transtensional regions are characterised by both extensional structures (normal faults, grabens) and wrench structures (strike-slip faults). In general, many tectonic regimes that were previously defined as ...
The pebbles thus record important information on the orientation of the shear zone (subvertical) and the direction of movement of the shear zone, and the overall change in pebble shape from originally sub-spherical to presently elongate cigar-shaped, allows one to quantify the strain experienced by the rock mass in the geologic past.
In geology, the terms sinistral and dextral refer to the horizontal component of the movement of blocks on either side of a fault or the sense of movement within a shear zone. These are terms of relative direction, as the movement of the blocks is described relative to each other when viewed from above.
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