Pseudotachylite or Pseudotachylyte is a cohesive glassy or very fine-grained fault rock that is composed of an extremely fine-grained or glassy matrix that often contains inclusions of wall-rock fragments. Pseudotachylite characteristically occurs in veins; is dark in color; and is glassy in appearance. It often has the appearance of the basaltic glass, tachylyte. Typically, the glass has been completely devitrified into very fine-grained material with radial and concentric clusters of crystals. It occasionally contains crystals with quench textures that began to crystallize from the melt.
It is generally found either along fault surfaces, often as the matrix to a breccia, or as veins injected into the walls of the fault. In most cases there is good evidence that the pseudotachylite formed by frictional melting of the wall rocks during rapid fault movement associated with a seismic event. This has caused them to be termed "fossil earthquakes". The thickness of the pseudotachylite zone also gives geologists a general idea of the magnitude of the associated displacement and the general magnitude of the paleoseismic event. Some pseudotachylites have been interpreted as forming by comminution rather than melting. They have a similar occurrence to melt-derived pseudotachylites but lack clear indications of a melt origin.
Pseudotachylite has been found at the base of some large landslides involving the movement of large coherent blocks, such as the one that moved Heart Mountain in the U.S. state of Wyoming to its present location, the largest known landslide in history on land.
Pseudotachylite is also associated with impact structures such as that which formed the Vredefort crater, South Africa. In an impact event, the melting forms part of the shock metamorphic effects. The pseudotachylite veins associated with impacts are much larger than those associated with faults and are thought to have formed by frictional effects within the crater floor and below the crater during the initial compression phase of the impact and the subsequent formation of the central uplift. The most extensive examples of impact related pseudotachylites come from impact structures that have been deeply eroded to expose the floor of the crater, such as Vredefort crater, South Africa and the Sudbury Basin, Canada.
- Maddock, R.H. (1983) Melt origin of fault-generated pseudotachylites demonstrated by textures. Geology. 11(6):105–108.
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- Sibson, R.H., 1975. Generation of pseudotachylite by ancient seismic faulting. Geophysical Journal of the Royal Astronomical Society 43, 775– 794.
- Lin, A. (2007). Fossil earthquakes: the formation and preservation of Pseudotachylytes. Lecture notes in earth sciences 111. Springer. p. 348. ISBN 978-3-540-74235-7. Retrieved 2009-11-02.
- Legros, F., Cantagrel, J-M. & Devouard, B. 2000. Pseudotachylyte (Frictionite) at the Base of the Arequipa Volcanic Landslide Deposit (Peru): Implications for Emplacement Mechanisms. The Journal of Geology, volume 108, p. 601–611.
- Spray, J.G. 1998. Localized shock- and friction-induced melting in response to hypervelocity impact. Journal of the Geological Society, London, Special Publication, 140, 195-204.
- 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.
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.