A tectonic phase or deformation phase is in structural geology and petrology a phase in which tectonic movement or

metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ch ...

took place. Tectonic phases can be

extensional In any of several fields of study that treat the use of signs — for example, in linguistics Linguistics is the science, scientific study of human language. It is called a scientific study because it entails a comprehensive, systematic, obj ...

or compressional in nature. When numerous subsequent compressional tectonic phases share the same

geodynamic Geodynamics is a subfield of geophysics dealing with dynamics of the Earth. It applies physics, chemistry and mathematics to the understanding of how mantle convection leads to plate tectonics and geologic phenomena such as seafloor spreading, mo ...

cause (usually some

plate tectonic Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...

mechanism) this is called an orogeny. During an orogeny tectonic phases lead to

mountain building Mountain formation refers to the geological processes that underlie the formation of mountains. These processes are associated with large-scale movements of the Earth's crust (tectonic plates). Folding, faulting, volcanic activity, igneous int ...

, which causes

deformation Deformation can refer to: * Deformation (engineering), changes in an object's shape or form due to the application of a force or forces. ** Deformation (physics), such changes considered and analyzed as displacements of continuum bodies. * Defor ...

and metamorphism of rocks. In most Phanerozoic orogenies many tectonic phases are distinguished.

Tectonic phases in basin geology and sedimentary rocks

According to Steno's

principle of original horizontality The principle of original horizontality states that layers of sediment are originally deposited horizontally under the action of gravity. It is a relative dating technique. The principle is important to the analysis of folded and tilted strata. I ...

sedimentary rock 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 ...

s are normally deposited as horizontal layers. When tectonic movement occurs sedimentary layers can be tilted. under such circumstances, the sedimentary planes will have a dip (an angle with a horizontal reference plane). When new sedimentary layers are deposited on top of tilted ones, they will have an angle with the older ones, a structure which is called an

angular unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

. Any angular unconformity is proof that a tectonic phase took place between the deposition of the layers below and on top of it. It is important to know if the tectonic phase was a longer event or if it was local or regional. Tectonic phases can be important events that affected large areas. The

Alleghenian orogeny The Alleghanian orogeny or Appalachian orogeny is one of the geological mountain-forming events that formed the Appalachian Mountains and Allegheny Mountains. The term and spelling Alleghany orogeny was originally proposed by H.P. Woodward in 195 ...

in North America (during the Carboniferous

period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

) for example can be found as an angular unconformity between rock layers in large parts of that continent. When a tectonic phase occurred while sedimentation of new sediments continued, every new layer will have a slightly different dip from the one below. The result is a sequence of sediments that wedges out in one direction. This is usually the case on the margins of geologic basins.

Tectonic phases in metamorphic rocks

In petrology, metamorphic phases can be distinguished by analysis of

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

s and

microstructure Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material (such as metals, polymers ...

s (

petrography Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. The class ...

) in

metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...

s. By analysing the sequence in which the minerals and structures were formed, more than one phase can be found in most metamorphic rocks (this is called polymetamorphism). By

radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares ...

the absolute age of the different phases of mineral growth can be determined, showing when the rock was under certain

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

conditions. The obtained age is not necessarily the age of a tectonic phase, but can show when the tectonic phase ended and

crystallisation Crystallization is the process by which solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some ways by which crystals form are precipitating from a solution, freezing, or more rarely deposi ...

of new minerals occurred.

Tectonic phases in basin geology and sedimentary rocks

Tectonic phases in metamorphic rocks

See also