Pseudotachylyte (sometimes written as pseudotachylite) is an extremely fine-grained to glassy, dark, cohesive rock occurring as veins[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 during ,
Pseudotachylyte was first documented by Shand in the Vredefort Impact Structure and was named due to its close resemblance to tachylyte, a basaltic glass.
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 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 that eventually grow and join together into larger patches of a high viscosity melt.
Abundance of seismic pseudotachylyte in nature
There is an apparent lack of pseudotachylyte in the
geologic record relative to the observed
seismicity 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 and
cataclasite),
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 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 fault 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 volcanic landslide deposit in Peru from approximately 2.4 million years ago,
and the
Langtang 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
.
[ 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.] Pseudotachylyte in impact craters typically occurs as abundant irregular,
Anastomosis, 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.
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 in South Africa, and the
Sudbury Basin in Canada.
Impact-generated pseudotachylytes are classified into two types depending on their method of formation.
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
.
In the
Sudbury Basin, researchers have been able to distinguish dikes of pseudotachylyte from dikes of impact melt based on their chemical compositions.
See also
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.
