Pseudotachylite is a fault rock that has the appearance of the basaltic glass, tachylyte. It is dark in color and has a glassy appearance. However, the glass has normally been completely devitrified into very fine-grained material with radial and concentric clusters of crystals. It may contain clasts of the country rock and occasionally crystals with quench textures that began to crystallize from the melt.[1]
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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.[2] This has caused them to be termed "fossil earthquakes".[3] 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.[3]
Pseudotachylite has been found at the base of some large landslides involving the movement of large coherent blocks,[4] 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.[5] 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.[6] 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.