In geology, a rock's fabric describes the spatial and geometric configuration of all the elements that make it up. [1] [2] In sedimentary rocks, the fabric developed depends on the depositional environment and can provide information on current directions at the time of deposition. In structural geology, fabrics may provide information on both the orientation and magnitude of the strains that have affected a particular piece of deformed rock.
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.
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.
Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has two related subfields: lithostratigraphy and biostratigraphy.
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Examples of lithologies in the second sense include sandstone, slate, basalt, or limestone.
In structural geology, a fold is a stack of originally planar surfaces, such as sedimentary strata, that are bent or curved during permanent deformation. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur as single isolated folds or in periodic sets. Synsedimentary folds are those formed during sedimentary deposition.
Paleomagnetism, or palaeomagnetism, is the study of the record of the Earth's magnetic field in rocks, sediment, or archeological materials. Magnetic minerals in rocks can lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of tectonic plates. The record of geomagnetic reversals preserved in volcanic and sedimentary rock sequences (magnetostratigraphy) provides a time-scale that is used as a geochronologic tool. Geophysicists who specialize in paleomagnetism are called paleomagnetists.
A shear zone is a very important structural discontinuity surface in the Earth's crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones. Intervening (crustal) blocks stay relatively unaffected by the deformation. Due to the shearing motion of the surrounding more rigid medium, a rotational, non co-axial component can be induced in the shear zone. Because the discontinuity surface usually passes through a wide depth-range, a great variety of different rock types with their characteristic structures are produced.
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, 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.
Rock microstructure includes the texture of a rock and the small-scale rock structures. The words texture and microstructure are interchangeable, with the latter preferred in modern geological literature. However, texture is still acceptable because it is a useful means of identifying the origin of rocks, how they formed, and their appearance.
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.
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.
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.
A deformation mechanism, in geotechnical engineering, is a process occurring at a microscopic scale that is responsible for changes in a material's internal structure, shape and volume. The process involves planar discontinuity and/or displacement of atoms from their original position within a crystal lattice structure. These small changes are preserved in various microstructures of materials such as rocks, metals and plastics, and can be studied in depth using optical or digital microscopy.
Skolithos is a common trace fossil ichnogenus that is, or was originally, an approximately vertical cylindrical burrow. It is produced by a variety of organisms in shallow marine environments globally and appear as lineated features in sedimentary rocks.
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.
Strain partitioning is commonly referred to as a deformation process in which the total strain experienced on a rock, area, or region, is heterogeneously distributed in terms of the strain intensity and strain type. This process is observed on a range of scales spanning from the grain – crystal scale to the plate – lithospheric scale, and occurs in both the brittle and plastic deformation regimes. The manner and intensity by which strain is distributed are controlled by a number of factors listed below.
Fault zone hydrogeology is the study of how brittlely deformed rocks alter fluid flows in different lithological settings, such as clastic, igneous and carbonate rocks. Fluid movements, that can be quantified as permeability, can be facilitated or impeded due to the existence of a fault zone. This is because different mechanisms that deform rocks can alter porosity and permeability within a fault zone. Fluids involved in a fault system generally are groundwater and hydrocarbons.
