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Broken or deformed: What are the amazing changes in rocks under shear stress?
In geology, shear stress is the reaction of rock when it deforms, usually caused by compressive stress, and creates specific structural features. Shear may be uniform or non-uniform, and may be pure shear or simple shear. The study of geological shear is closely related to the study of structural geology, rock microstructure or rock texture, and fault mechanics. Rocks change under the action of shear stress in different states of brittleness, brittle-plasticity and plasticity, showing amazing diversity. In perfectly brittle rocks, the effects of compressive stress often lead to cracking and simple faulting phenomena.
A shear zone is a flat or curved band of rock that is more strongly strained and usually has a higher degree of deformation than the surrounding rock.
The formation of shear zones is of geological significance and can range from a few inches to several kilometers wide. Due to their structural control, shear zones often form important fault planes at the edges of structural blocks, resulting in geological separation between rock blocks. Large shear zones are often named the same as fault systems. When the horizontal displacement of these faults can be calculated as tens to hundreds of kilometers, such faults are called "super-large shear zones" or megashares. Ultra-large shear zones often represent the boundaries of ancient tectonic plates, providing valuable insights into Earth's history.
The mechanism of shear depends on the pressure and temperature of the rock, as well as the rate of shear the rock is subjected to. How the rock responds to these conditions determines how it adapts to deformation. When occurring under more brittle rheological conditions (such as colder or lower pressures) or at high strain rates, shear bands tend to fail through brittle fracture; this causes the mineral to fracture and generate grinding bands. Textured breccia. Under brittle-plastic conditions, shear bands can withstand large amounts of deformation through multiple mechanisms within the mineral and its crystal lattice itself.
The microstructure of the shear zone is initially formed during the rock deformation process, including the growth of planar wafers and new minerals.
During the initial stages of shearing, the first penetrable planar laminae that appear are caused by the rearrangement of grains within the rock. This structure is usually perpendicular to the main direction of shortening and is diagnostic of the direction of shortening. In the case of symmetric shortening, the object will be compressed by gravity and flatten like a sphere; in the asymmetric fault zone, the change will be similar to the extension of the sphere, usually becoming an ellipse. When the rock undergoes large deformation during lateral movement, the strain ellipse will become longer and form a shape similar to a cigar. At this time, the shear surface begins to evolve into a rod-like or tensile-like structure. This type of rock is called It is the L-ridge.
During plastic shearing, very characteristic microstructures are formed, such as S-planes, C-planes and C' planes. These planes are caused by the arrangement of metals or flat minerals and define the long axis of the strain ellipse. The C-plane is parallel to the boundary of the shear zone, and the angle between the two is always an obtuse angle, indicating the direction of the shear. In strongly metamorphic rocks, the C' plane also often appears, forming some secondary shear structures. The shear direction perceived by these microstructures is consistent with the shear zone.
When crustal plates interact in non-orthogonal collisions or subduction processes, complex situations of compression and tension develop, which are expressed through faults, fractures, and geological deformation. For example, the Alpine Fault Zone in New Zealand is a classic example of this phenomenon. Here, the oblique subduction motion of the Pacific and Indo-Australian plates transforms into oblique strike-slip motion, creating a remarkable tectonic story and emphasizing the complexity of crustal movement.
So, when geological structures are constantly changing in their own ways, how do we understand the importance of these changes to the planet and the environment in which we live?
