Tectonic phase
Encyclopedia
A tectonic phase or deformation phase is in structural geology
and petrology
a phase in which tectonic movement
or metamorphism
took place. Tectonic phases can be extensional
or compressional in nature.
When numerous subsequent compressional tectonic phases share the same geodynamic
cause (usually some plate tectonic
mechanism) this is called an orogeny
. During an orogeny tectonic phases lead to mountain building, which causes deformation and metamorphism of rocks. In most Phanerozoic
orogenies many tectonic phases are distinguished.
principle of original horizontality
, sedimentary rock
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. Any angular unconformity is prove 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
in North America
(during the Carboniferous
period) 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
.
s and microstructure
s (petrography
) in metamorphic rock
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
the absolute age of the different phases of mineral growth can be determined, showing when the rock was under certain pressure
-temperature
conditions. The obtained age is not necessarily the age of a tectonic phase, but can show when the tectonic phase ended and crystallisation of new minerals occurred.
Structural geology
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 in the rocks, and...
and petrology
Petrology
Petrology is the branch of geology that studies rocks, and the conditions in which rocks form....
a phase in which tectonic movement
Tectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...
or metamorphism
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
took place. Tectonic phases can be extensional
Extensional tectonics
Extensional tectonics is concerned with the structures formed, and the tectonic processes associated with, the stretching of the crust or lithosphere.-Deformation styles:...
or compressional in nature.
When numerous subsequent compressional tectonic phases share the same geodynamic
Geodynamics
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, mountain building, volcanoes, earthquakes, faulting and...
cause (usually some plate tectonic
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
mechanism) this is called an orogeny
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
. During an orogeny tectonic phases lead to mountain building, which causes deformation and metamorphism of rocks. In most Phanerozoic
Phanerozoic
The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...
orogenies many tectonic phases are distinguished.
Tectonic phases in basin geology and sedimentary rocks
According to Steno'sNicolas Steno
Nicolas Steno |Latinized]] to Nicolaus Steno -gen. Nicolai Stenonis-, Italian Niccolo' Stenone) was a Danish pioneer in both anatomy and geology. Already in 1659 he decided not to accept anything simply written in a book, instead resolving to do research himself. He is considered the father of...
principle of original horizontality
Principle of original horizontality
The Principle of Original Horizontality was proposed by the Danish geological pioneer Nicholas Steno . This principle states that layers of sediment are originally deposited horizontally under the action of gravity...
, sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
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. Any angular unconformity is prove 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
Alleghenian orogeny
The Alleghenian 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 1957....
in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
(during the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
period) 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
Basin (geology)
A structural basin is a large-scale structural formation of rock strata formed by tectonic warping of previously flat lying strata. Structural basins are geological depressions, and are the inverse of domes. Some elongated structural basins are also known as synclines...
.
Tectonic phases in metamorphic rocks
In petrology, metamorphic phases can be distinguished by analysis of mineralMineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s and microstructure
Microstructure
Microstructure is defined as the structure of a prepared surface or thin foil of material as revealed by a microscope above 25× magnification...
s (petrography
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. Petrographic descriptions start with the field notes at the...
) in metamorphic rock
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
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
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
the absolute age of the different phases of mineral growth can be determined, showing when the rock was under certain pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
-temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
conditions. The obtained age is not necessarily the age of a tectonic phase, but can show when the tectonic phase ended and crystallisation of new minerals occurred.