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In a geological context, crenulation or crenulation cleavage is a fabric formed in metamorphic rocks such as phyllite, schist and some gneiss by two or more stress directions causing the formation of the superimposed foliations.
Crenulations form when an early planar fabric is overprinted by a later planar fabric. Crenulations form by recrystallisation of mica minerals during metamorphism. Micaceous minerals form planar surfaces known as foliations perpendicular to the principal stress fields. If a rock is subjected to two separate deformations and the second deformation is at some other angle to the original, growth of new micas on the foliation planes will create a new foliation plane perpendicular to the plane of principal stress. The angular intersection of the two foliations causes a diagnostic texture called a crenulation, which may involve folding of the earlier mica foliations by the later foliation.
Recognising a crenulation in a rock may require inspecting the rock with a hand-lens or petrographic microscope in thin section. Crenulations may be very cryptic, and there may be several recorded within a rock and especially, entrained within porphyroblasts.
Crenulations may manifest as kinking of previous foliation, such that the original foliation appears to be lined or inscribed by a later foliation.
In more advanced states, the later foliation will tend to form distinct foliation planes cross-cutting the earlier foliation, resulting in breaking, warping, and micro-scale folding of the earlier foliation into the new foliation.
When the crenulation foliation begins to dominate it may totally or almost completely wipe out the original foliation. This process occurs at different rates in rocks and beds of different lithology and chemical composition so that it is usually valuable to look at a variety of outcrops to gain a better appreciation of the effect of crenulation or discover the orientation or presence of earlier foliations.
Crenulation may also be the incipient foliation plane which precipitates shearing. In this case, it is often likely that the crenulation acts as a shear plane and it may be difficult to reconstruct earlier foliations and rock units across the crenulation foliation.
Crenulations, because they are the result of a second (or more) foliation, preserve important information on not only the stresses which formed the crenulation foliation, but the orientation of previous foliations. Firstly, the crenulation must be analysed to determine the initial foliation, termed S1, and the overprinting subsequent foliation. The intersection of these two planes forms an intersection lineation. This intersection lineation, L1-2 may approximate the plunge of F2 interference folds.
The initial impact of a crenulation foliation may be cryptic, microscopic growth of new minerals at an angle to previous foliations. This may occur only in certain compositions of the rocks which favor growth of minerals under the P-T conditions at that time.
In more brittle conditions, especially in highly micaceous rocks, a crenulation may appear as 'kink bands, where S1 foliations are kinked by the S2 foliation so that the original minerals are broken or deformed. This may not result in new mineral growth.
Eventually, the crenulation foliation overprints the S1 foliation. In extreme cases, the S2 foliation will obliterate the previous foliation, especially in wet rocks which have compositions amenable to growth of minerals at that time. In this case, porphyroblasts may be the only means in which to observe earlier foliations, assuming they have inclusion trails of the S1 foliation.
Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation (strain) in the rocks, and ultimately, to understand the stress field that resulted in the observed strain and geometries. This understanding of the dynamics of the stress field can be linked to important events in the geologic past; a common goal is to understand the structural evolution of a particular area with respect to regionally widespread patterns of rock deformation due to plate tectonics.
Schist is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes or plates. This texture reflects a high content of platy minerals, such as micas, talc, chlorite, or graphite. These are often interleaved with more granular minerals, such as feldspar or quartz.
Metamorphism is the transformation of existing rock to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of 150 °C (300 °F), and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism is distinct from weathering or diagenesis, which are changes that take place at or just beneath Earth's surface.
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical composition. The minerals which compose the rocks are dissolved and new mineral formations are deposited in their place. Dissolution and deposition occur simultaneously and the rock remains solid.
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. In the lower crust and mantle, the extreme conditions of pressure and temperature make the rock ductile. That is, the rock is capable of slowly deforming without fracture, like hot metal being worked by a blacksmith. Here the shear zone is a wider zone, in which the ductile rock has slowly flowed to accommodate the relative motion of the rock walls on either side.
Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a classification based on the textural appearance of the rock.
In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation. The hydraulic flow involved is usually due to hydrothermal circulation.
In geology, 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. Study of geological shear is related to the study of structural geology, rock microstructure or rock texture and fault mechanics.
A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained matrix. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape.
Foliation in geology refers to repetitive layering in metamorphic rocks. Each layer can be as thin as a sheet of paper, or over a meter in thickness. The word comes from the Latin folium, meaning "leaf", and refers to the sheet-like planar structure. It is caused by shearing forces, or differential pressure. The layers form parallel to the direction of the shear, or perpendicular to the direction of higher pressure. Nonfoliated metamorphic rocks are typically formed in the absence of significant differential pressure or shear. Foliation is common in rocks affected by the regional metamorphic compression typical of areas of mountain belt formation.
Lineations in structural geology are linear structural features within rocks. There are several types of lineations, intersection lineations, crenulation lineations, mineral lineations and stretching lineations being the most common. Lineation field measurements are recorded as map lines with a plunge angle and azimuth.
In geology, texture or rock microstructure refers to the relationship between the materials of which a rock is composed. The broadest textural classes are crystalline, fragmental, aphanitic, and glassy. The geometric aspects and relations amongst the component particles or crystals are referred to as the crystallographic texture or preferred orientation. Textures can be quantified in many ways. The most common parameter is the crystal size distribution. This creates the physical appearance or character of a rock, such as grain size, shape, arrangement, and other properties, at both the visible and microscopic scale.
A porphyroclast is a clast or mineral fragment in a metamorphic rock, surrounded by a groundmass of finer grained crystals. Porphyroclasts are fragments of the original rock before dynamic recrystallisation or cataclasis produced the groundmass. This means they are older than the groundmass. They were stronger pieces of the original rock, that could not as easily deform and were therefore not or hardly affected by recrystallisation. They may have been phenocrysts or porphyroblasts in the original rock.
Augen are large, lenticular eye-shaped mineral grains or mineral aggregates visible in some foliated metamorphic rocks. In cross section they have the shape of an eye.
Tectonites are metamorphic or tectonically deformed rocks whose fabric reflects the history of their deformation, or rocks with fabric that clearly displays coordinated geometric features that indicate continuous solid (ductile) flow during formation. Planar foliation results from a parallel orientation of platey mineral phases such as the phyllosilicates or graphite. Slender prismatic crystals such as amphibole produce a lineation in which these prisms or columnar crystals become aligned. Tectonites are rocks with minerals that have been affected by natural forces of the earth, which allowed their orientations to change. This usually includes recrystallization of minerals, and the foliation formation. Tectonites are studied through structural analysis and allows for the determination of two things:
Cleavage, in structural geology and petrology, describes a type of planar rock feature that develops as a result of deformation and metamorphism. The degree of deformation and metamorphism along with rock type determines the kind of cleavage feature that develops. Generally, these structures are formed in fine grained rocks composed of minerals affected by pressure solution.
This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.
The Thiviers-Payzac Unit is a metasedimentary succession of late Neoproterozoic and Cambrian age outcropping in the southern Limousin in France. The unit geologically forms part of the Variscan basement of the northwestern Massif Central.
In geology oblique foliation, steady state foliation or oblique fabric is a special type of a tectonically produced foliation or fabric, most commonly in quartz-rich layers. The microtectonic structure can be used to determine the shear sense in shear zones and their associated rocks, usually mylonites.
The Merlis Serpentinites are an aligned group of small serpentinite outcrops in the northwestern French Massif Central. Their parent rocks were peridotites from the upper mantle.
