Pseudotachylite
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Pseudotachylyte (sometimes written as pseudotachylite) is an extremely fine-grained to glassy, dark, cohesive rock occurring as
veins Veins () are blood vessels in the circulatory system of humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and fetal c ...
Trouw, R.A.J., C.W. Passchier, and D.J. Wiersma (2010) ''Atlas of Mylonites- and related microstructures.'' Springer-Verlag, Berlin, Germany. 322 pp. that form through frictional melting and subsequent
quenching In materials science, quenching is the rapid cooling of a workpiece in water, gas, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, suc ...
during
earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
s, large-scale
landslide Landslides, also known as landslips, rockslips or rockslides, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, mudflows, shallow or deep-seated slope failures and debris flows. Landslides ...
s, and impacts events. Chemical composition of pseudotachylyte generally reflects the local bulk chemistry, though may skew to slightly more
mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
compositions due to the preferential incorporation of hydrous and ferro-magnesian minerals (
mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into fragile elastic plates. This characteristic is described as ''perfect basal cleavage''. Mica is co ...
and
amphibole Amphibole ( ) is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is ...
, respectively) into the melt phase. Pseudotachylyte was first documented by Shand in the
Vredefort Impact Structure The Vredefort impact structure is one of the largest impact structures on Earth. The crater, which has since been eroded away, has been estimated at across when it was formed, the latter estimate suggesting the initial crater was larger than ...
and was named due to its close resemblance to tachylyte, a
basaltic Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron ( mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% ...
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
. Though pseudotachylyte is reported to have a glassy appearance, they are extremely susceptible to alteration and are thus rarely found to be entirely composed of glass. Typically, they are completely devitrified into a very fine-grained material with quench textures such as chilled margins, radial and concentric clusters of microcrystalites ( spherulites) or as radial overgrowths of microcrystalites on clasts, as well as skeletal and spinifex microcrystalites.


Formation


Seismic faulting

Pseudotachylytes have been referred to as "fossil earthquakes" as they represent definitive evidence of seismic slip. During seismic faulting (earthquakes), pseudotachylyte forms through an extreme concentration of frictional sliding onto a thin surface of a fault. The friction creates heat, and because rocks are insulators, the temperature increases on this surface allowing the rock to melt. This generates a "fault vein" which are often accompanied by "injection veins" that open from the fault vein as opening mode cracks. A melt origin for pseudotachylyte was controversial for some time, with some researchers favouring extreme
comminution Comminution is the reduction of solid materials from one average particle size to a smaller average particle size, by crushing, grinding, cutting, vibrating, or other processes. Comminution is related to pulverization and grinding. All use m ...
for their generation (crush-origin). Ample evidence of direct crystallisation from a melt though, has more or less put this argument to rest with most researchers defining pseudotachylyte as having a melt origin. Laboratory experiments investigating how pseudotachylytes form have shown that the initial phase of formation involves the flash melting of
asperities In materials science, asperity, defined as "unevenness of surface, roughness, ruggedness" (from the Latin ''asper''—"rough"), has implications (for example) in physics and seismology. Smooth surfaces, even those polished to a mirror finish, ar ...
that eventually grow and join together into larger patches of a high
viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...
melt. The high viscosity of these melt patches raises the fault's
coefficient of friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...
, hindering sliding. As the patches of melt continue to grow and join together, they form a continuous melt layer with a lower viscosity, which reduces the fault's coefficient of friction, effectively lubricating the fault and allowing sliding to occur more easily. Once the melt layer has reached some critical thickness, frictional heat can no longer be generated, and the melt begins to quench and crystallise thus again increasing the melt's viscosity and begins acting as a viscous brake to sliding. Once sliding is stopped, the quenching of the melt layer welds the fault shut and restores its strength to that of the unfaulted surrounding rock.


Abundance of seismic pseudotachylyte in nature

There is an apparent lack of pseudotachylyte in the
geologic record The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata. That is, deposits laid down by volcanism or by deposition of sediment derived from weathering detritus (clays, sa ...
relative to the observed
seismicity Seismicity is a measure encompassing earthquake occurrences, mechanisms, and magnitude at a given geographical location. As such, it summarizes a region's seismic activity. The term was coined by Beno Gutenberg and Charles Francis Richter in 194 ...
of today, which brings into question if this is an issue of the rarity of its production, lack of recognition in the field, or its ability to be preserved. It was once thought that pseudotachylyte could only be produced in dry, crystalline rock, this however, has been shown to be incorrect. Therefore, its production is likely not as rare as originally thought. Pseudotachylyte is often closely associated with other extremely fine grained rocks (e.g.
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 ...
and
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 ...
), and is extremely prone to alteration that often renders it unrecognisable which supports arguments that pseudotachylyte production isn't rare, but rather is likely to go unrecognised, and thus unreported.


Landslides

Pseudotachylytes have been observed at the base of some large-scale
landslide Landslides, also known as landslips, rockslips or rockslides, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, mudflows, shallow or deep-seated slope failures and debris flows. Landslides ...
deposits. The formation of pseudotachylyte along the base of a landslide occurs due to the same processes as earthquake-generated pseudotachylyte - frictional heating during gliding along the base of the detachment melts the surrounding rock. They are similar in appearance to earthquake-generated pseudotachylyte. Some notable examples of landslide-generated pseudotachylyte in the geologic record is the
Arequipa Arequipa (; Aymara language, Aymara and ), also known by its nicknames of ''Ciudad Blanca'' (Spanish for "White City") and ''León del Sur'' (Spanish for "South's Lion"), is a city in Peru and the capital of the eponymous Arequipa (province), ...
volcanic landslide A volcanic landslide or volcanogenic landslide is a type of mass wasting that takes place at volcanoes. Occurrences All volcanic edifices are susceptible to landslides, particularly stratovolcanoes and shield volcanoes where landslides are impor ...
deposit in Peru from approximately 2.4 million years ago, and the
Langtang Langtang Valley () also known as Lamtang Valley is a Himalayan valley in the mountains of north-central Nepal, known for its trekking routes and natural environment. Administrative The Langtang Valley lies in Rasuwa District, Rasuwa dist ...
landslide deposit in Nepal which occurred between 30,000 - 25,000 years ago. Pseudotachylyte has also been found along the base of more modern landslides, such as the landslide generated by the 1999 Taiwan earthquake.


Impact structures

Pseudotachylyte has also been associated with
impact structure An impact structure is a generally circular or craterlike geologic structure of deformed bedrock or sediment produced by impact on a planetary surface, whatever the stage of erosion of the structure. In contrast, an impact crater is the surface e ...
s.Chapter 5 of the online book, French, B.M. 1998. Traces of Catastrophe, A handbook of shock-metamorphic effects in terrestrial meteorite impact structures, Lunar and Planetary Institute 120pp.
/ref> Pseudotachylyte in impact craters typically occurs as abundant irregular, anastomosing, and dike-like bodies that contain several large and small rounded inclusions of the impacted, or target, rock in a dense fine-grained to glassy black to greenish
matrix Matrix (: matrices or matrixes) or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the m ...
. Individual pseudotachylyte bodies within impact craters are not uniform over long distances, and may change in size and shape drastically within meters or tens of meters. The most extensive examples of impact related pseudotachylytes come from impact structures that have been deeply eroded below the floor of the crater, such as in case of the
Vredefort impact structure The Vredefort impact structure is one of the largest impact structures on Earth. The crater, which has since been eroded away, has been estimated at across when it was formed, the latter estimate suggesting the initial crater was larger than ...
in South Africa, and the Sudbury impact structure in Canada. Impact-generated pseudotachylytes are classified into two types depending on their method of formation. S-Type pseudotachylytes, also known as "shock veins", are found as small (<1 cm, typically <1 mm) glassy
veins Veins () are blood vessels in the circulatory system of humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and fetal c ...
that contain high-pressure mineral polymorphs like
coesite Coesite () is a form (polymorphism (materials science), polymorph) of silicon dioxide (silicon, Sioxide, O2) that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (), are applied to quartz. Coesite was first ...
and
stishovite Stishovite is an extremely hard, dense tetragonal form ( polymorph) of silicon dioxide. It is very rare on the Earth's surface; however, it may be a predominant form of silicon dioxide in the Earth, especially in the lower mantle. Stishovite w ...
. These shock veins are thought to form via frictional and shock melting due to the higher pressure compressive stages (%need to make it skip to formation section%) of the
shockwave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...
expansion. E-Type (endogenic) pseudotachylytes are formed via frictional melting of the target rock due to high-speed slip caused by the collapse of the crater margin.


Pseudotachylyte vs. impact melt in impact structures

Though pseudotachylyte and impact melt within impact structures are visually similar, both occurring as dike-like bodies, they are chemically different. Since pseudotachylyte is derived locally, it will reflect the composition of the wall-rock from which it formed. Impact melts are generated from a much larger volume of rock by instantaneous shock melting, so their chemical compositions will be more reflective of regional-scale mixing and homogenization during melting, particularly in heterogeneous
terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its d ...
s. In the Sudbury impact structure, researchers have been able to distinguish dikes of pseudotachylyte from dikes of impact melt based on their chemical compositions.


See also

*


References


External links

Wieland, F. (2006
''Chapter 4: Pseudotachylitic breccias, other breccias and veins.''''Structural analysis of impact-related deformation in the collar rocks of the Vredefort Dome, South Africa.''
unpublished PhD. dissertation. School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa. {{Rock type Petrology Structural geology Impact craters