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The brittle-ductile transition zone (hereafter the "transition zone") is the zone of the
Earth's crust
Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...
that marks the transition from the upper, more brittle crust to the lower, more ductile crust.
For
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
and
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
-rich rocks in
continental crust
Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called ''sial'' be ...
, the transition zone occurs at an approximate depth of 20 km,
at temperatures of 250–400 °C. At this depth, rock becomes less likely to
fracture
Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
, and more likely to deform ductilely by
creep because the
brittle strength of a material increases with
confining pressure, while its
ductile strength decreases with increasing temperature.
Depth of the Transition Zone
The transition zone occurs at the depth in the Earth's
lithosphere
A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...
where the downward-increasing
brittle strength equals the upward-increasing ductile strength, giving a characteristic "saw-tooth" crustal strength profile. The transition zone is, therefore, the strongest part of the crust and the depth at which most
shallow earthquakes occur. Its depth depends on both
strain rate
In materials science, strain rate is the change in strain (deformation) of a material with respect to time.
The strain rate at some point within the material measures the rate at which the distances of adjacent parcels of the material change wi ...
and temperature gradient; it is shallower for slow deformation and/or high
heat flow
Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
and deeper for fast deformation and/or low heat flow. Crustal composition and age also affect the depth: it is shallower (~10–20 km) in warm, young crust and deeper (~20–30 km) in cool, old crust.
Changes in Physical Properties
The transition zone also marks a shift in the
electrical conductivity
Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allow ...
of the crust. The upper region of the Earth's crust, which is about 10–15 km thick, is highly conductive due to electronic-conducting structures which are commonly distributed throughout this region. In contrast, the lower region of the crust is highly resistive and its electrical conductivity is determined by physical factors such as depth and temperature.
Although the transition zone generally marks a shift from brittle rock to ductile rock, exceptions exist in certain conditions. If stress is applied rapidly, rock below the transition zone may fracture. Above the transition zone, the rock may deform ductilely if pore fluids are present and stress is applied gradually.
Examples exposed on land
Sections of
fault zones once active in the transition zone, and now exposed at the surface, typically have a complex overprinting of brittle and ductile rock types.
Cataclasite
Cataclasite is a cohesive granular fault rock. Comminution, also known as cataclasis, is an important process in forming cataclasites. They fall into the category of cataclastic rocks which are formed through faulting or fracturing in the upper ...
s or
pseudotachylite
Pseudotachylyte (sometimes written as pseudotachylite) is an extremely fine-grained to glassy, dark, cohesive rock occurring as veinsTrouw, R.A.J., C.W. Passchier, and D.J. Wiersma (2010) ''Atlas of Mylonites- and related microstructures.'' Sprin ...
breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...
s with
mylonite
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 cl ...
clasts
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...
are common, as are ductilely deformed cataclasites and pseudotachylites. These sections become exposed in geologically active regions where the transition zone is located the
seismic zone
In seismology, a seismic zone or seismic belt is an area of seismicity potentially sharing a common cause. It may also be a region on a map for which a common areal rate of seismicity is assumed for the purpose of calculating probabilistic ground ...
, where most shallow earthquakes occur. A major example of this phenomenon is the Salzach‐Ennstal‐Mariazell‐Puchberg (SEMP) fault system in the
Austrian Alps
The Central Eastern Alps (german: Zentralalpen or Zentrale Ostalpen), also referred to as Austrian Central Alps (german: Österreichische Zentralalpen) or just Central Alps, comprise the main chain of the Eastern Alps in Austria and the adjacent ...
. Along this fault line, researchers have directly observed changes in structure and strength profiles in transition zone.
See also
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References
Further reading
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{{DEFAULTSORT:Brittle-ductile transition zone
Structure of the Earth
Seismology
Earth's crust