__NOTOC__ The brittle-ductile transition zone (hereafter the "transition zone") is the zone of the

Earth's crust Earth's crust is its thick outer shell of rock, referring to less than one percent of the planet's radius and volume. It is the top component of the lithosphere, a solidified division of Earth's layers that includes the crust and the upper ...

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 Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

and

feldspar Feldspar ( ; sometimes spelled felspar) is 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 ''plagiocl ...

-rich rocks in

continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary 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 '' si ...

, 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 appearance of a crack or complete separation of an object or material into two or more pieces under the action of stress (mechanics), stress. The fracture of a solid usually occurs due to the development of certain displacemen ...

, 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 and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time ...

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 mechanics and materials science, strain rate is the time derivative of strain of a material. Strain rate has dimension of inverse time and SI units of inverse second, s−1 (or its multiples). The strain rate at some point within the mat ...

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 volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...

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 (geology)#Fault rock, 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 faultin ...

s or pseudotachylite

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

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 ...

clasts 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 can be referred to as an earthquake belt as well. It may also be a region on a map for which a common areal rate of seismicity is assume ...

, 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 (), also referred to as Austrian Central Alps () or just Central Alps, comprise the Main chain of the Alps, main chain of the Eastern Alps in Austria and the adjacent regions of Switzerland, Liechtenstein, Italy and Slov ...

. Along this fault line, researchers have directly observed changes in structure and strength profiles in transition zone.

Depth of the Transition Zone

Changes in Physical Properties

Examples exposed on land

See also

References

Further reading