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deformation of rocks Buried flat-lying sediments beneath a river plain can be elevated into a mountain belt by the processes of folding and faulting. Deep within the mountain belt, a metamorphic rock becomes a locus for movement, flowing so intensely from shear that the rock is changed. This change extends down to the scale of the crystal lattices of the minerals that constitute the rock. Later as the mountain belt collapses perhaps under its own weight, normal faults grow, fracturing the rocks and juxtaposing folded sediment against sheared metamorphic rock. These events are all examples of deformation: the process by which rocks move and alter in response to tectonic stress. Types of movement or kinematic behaviour include distortion, which changes the shape of rocks; rotation, which changes the orientation of rocks; and translation, which changes the position of rocks. A commonly used measure of distortion is strain, which is sometimes incorrectly assumed to represent the entire deformation of a rock.
Deformation leaves its imprint on geological structures at all scales. For example, large structures include the trace of the San Andreas fault through the countryside of California, the scarp faces of normal faults framing the rift valleys of East Africa, and individual folds which define bulbous linear mountains in the Zagros Mountains. Yet, despite producing structures that range from mountains to defects in mineral lattices, almost all deformation processes or mechanisms operate at the scale of mineral grains and their atomic lattices. The larger-scale structures are simply the cumulative result of these fine-scale processes operating over a large volume of rock for extended periods of time.
Deformation mechanisms involve either fracture or flow. Differentiating between these two types of processes can be difficult. A single structure may display both fracture and flow effects as either a function of position or time. For example, a large vertical fault contains fracture-related structures in the upper crust, but flow-related structures where it is expressed by a shear zone in the lower crust. Alternatively, rocks initially shortened by cleavage formation during flow may then shorten by fracture-related thrusting as deformation progresses. Another problem is that some processes are viewed as having elements of both. For example, cataclasis is a fracture-related process that changes a fault from a simple pair of moving surfaces to a zone containing deformed, poorly sorted, fractured, fine-grained material by converting country rock to fault rock. The conversion is by grain-size reduction, grain-boundary sliding, and microfracturing driven by the work done to overcome the frictional resistance to fault movement. These processes involve fracturing, but some research workers wish to view the product at regional rather than grain scale, and interpret cataclasis as a flow mechanism. Such disagreements will no doubt continue for a long time among structural geologists.
Fracture produces clean breaks or discontinuities where a rock loses cohesion. The common motions are rotation and translation across the discontinuities. The most common structures are joints, where the rock dilates across the fracture. Joints can provide pathways for fluids such as oil or groundwater. The most important structures formed by fracturing are faults, where the rock slips parallel to the walls of the new fracture. Large faults form mountain belts, create basins during crustal extension, form plate boundaries, and are sites of almost all major shallow earthquakes.
Although flow, like fracturing, may produce discontinuities, it preserves rock cohesion and deformation is continuous. Flow causes rotation and translation, but it can also be marked by tremendous distortions. Common structures that are the result of flow include folds, foliations, lineations, and shear zones. Large folds may form individual mountains such as in the Zagros, Appalachian, or Rocky mountain belts; or they can decorate the entire sides of mountains, as in the Helvetic nappes of the Swiss Alps. Folds can also create sites for hydrocarbon accumulation.
All flow mechanisms entail a change in some combination of grain size, mineralogy, and rock chemistry. The type of flow is a function of such parameters as temperature, pressure, strain rate, and fluid content. For example, pressure solution, where grains dissolve at grain contacts in response to tectonic stress, operates at low temperatures (commonly less than 250 °C). Fluids are prevalent in rocks, and the small-scale products of pressure-solution deformation include pitted grains and sutured solution surfaces. An important result of this process is rock cleavage and the formation of stylolites.
At intermediate temperatures (commonly 250–500 °C), fluids are much less abundant, and stress drives intragranular mechanisms that operate within mineral lattices. Three mechanisms are common under these conditions. Diffusion is the process in which atoms leave their lattice sites and migrate to other positions within the same lattice or other lattices. Twinning of crystals occurs where the lattice kinks and collapses like an accordion. Dislocation glide happens when lattice bonds break and heal during slip on crystallographic slip surfaces in the grains. Although slip occurs on a surface during glide, it is not a fracture process, because healing of broken bonds after translation prevents the formation of discontinuities and preserves lattice cohesion. These processes produce strained grains with folded or twisted atomic lattices, or even grains that are compartmentalized into regions (i.e. subgrains) of common lattice orientation. They also increase the number of defects or imperfections in the lattice as an artefact of the deformation.
At elevated temperatures (commonly more than 500 °C), intragranular processes such as grain-boundary formation and migration become active. Some grains become bigger, while others disappear, as a function of lattice orientations, stress orientations, accumulated defects, and other conditions. These processes produce recrystallization, replacing the original texture and even the composition of the rock during the formation of a schist, gneiss, or mylonite. All flow mechanisms may produce a shape fabric in which the mineral grains of the rock have parallel long axes. The intragranular processes may also produce a fabric in the rock where all the same minerals have commonly oriented mineral lattices, producing a crystallographic-preferred orientation. The production of these fabrics is a result of flow mechanisms operating pervasively through the rock, unlike fracture mechanisms, where deformation concentrates at discontinuities.
Deformation mechanisms compete. Changing conditions such as temperature, pressure, fluid pressure, or grain size determine which mechanism is most efficient, and hence, dominant during deformation. The dominant mechanism or mechanisms will dictate which structures develop in the rock.