What Should Be Called Pseudotachylite

Allaby and Allaby (1996), in their Oxford Concise Dictionary of Earth Sciences, are succinct: "Pseudotachylite [is a] rare glassy rock produced by frictional melting during extreme dynamic metamorphism in a fault and thrust zone". This definition is different from that provided by Bates and Jackson (1987). These contrasting nomenclatures and the generally loose usage of the term "pseudotachylite" are the reasons for widespread misunderstandings and neverending discussion. In the context of this discussion of "pseudotachylites" in impact structures, these definitions are entirely ill-suited, for example, for the case of the small (2.5 km diameter) Roter Kamm impact crater, the rocks of which are only poorly exposed in the dune sands of the southern Namib Desert. A major occurrence of pseudotachylite-like breccia in an exposed area of the crater rim was described by Reimold and Miller (1989). Admittedly, evidence for melting in this material was rare, but glassy patches were described, and - in the absence of definite evidence of shock metamorphism -it was concluded that this material represented pseudotachylite. Later work (Degenhardt et al. 1996) established beyond doubt, however, that this material represents an extremely fine-grained, partially cataclastic schist of the target geology. The absence of evidence for faulting (impact-induced or not) at this locality makes the most probable cause of this breccia impact-induced cataclasis involving mainly brittle fracture, but possibly with a component of shock melting caused by local shock pressure excursions.

Several problems exist with the definition of Allaby and Allaby (1996). For one, it is apparent that pseudotachylitic breccias are not a rare phenomenon: That it is inappropriate to consider tectonically formed pseudotachylite a "rare" phenomenon has already been discussed above, and the breccias at the Vredefort type locality and in the Sudbury Structure are certainly not rare. It is also clear from the literature that most workers do not regard a glassy, or even aphanitic, matrix as a prerequisite either (many pseudotachylites are at least partially crystallized, and microlites are frequently observed; just a few examples: Clarke (1990); Austrheim and Boundy (1994); Hetzel et al. (1996); Müller et al. (2002). Many of the more voluminous breccias in the Vredefort Structure have well-crystallized igneous textures or, at least, microlites in a glassy or cryptocrystalline mesostasis (e.g., Reimold and Colliston 1994). Second, this statement "produced by frictional melting": this is the view of a tectonic worker who is not concerned with impact-produced breccias. However, the type locality for "pseudotachylite" after Shand (1916) is the world's currently largest known impact structure (Vredefort) and the literature is awash with descriptions/reports of such breccias from other impact structures. In addition, the possibly planetary importance of the high-temperature processes that may lead to the formation of large-scale breccias of this/these type(s) has also been speculated about (Fiske et al. 1995a). One benefit one can take from the Allaby and Allaby (1996) definition is that it makes it very clear what tectonic workers/structural geologists consider as "pseudotachylite": only bona fide friction melt!

The definition then refers to "dynamic metamorphism" as the environment in which such breccias form. This statement does not refer to any specific metamorphic regimes in terms of pressure and temperature, as well as strain rate. To refer to "dynamic metamorphism" implies that there is no external heat source to cause changes which are driven by differential stress. Thus, this statement does not recognize the impact process/shock metamorphism. Impact breccias form at much higher strain rates than those attained under "normal" tectonic conditions that have been termed "pseudotachylites". The review by Spray (1998) has demonstrated this in detail. And the tectonic literature provides ample documentation that pseudotachylites have been formed over the entire lithospheric pressure-temperature regime, from lower greenschist to upper granulite and even eclogite facies conditions.

Fig. 4. (a top) Width of the image is ca. 1 m. Typical only a cm wide, veinlet of pseudotachylitic breccia in Hospital Hill quartzite on farm Parsons Rest, NW collar of the Vredefort Dome. Note the subparallel, NNW trending joint set that has displaced the veinlet locally. (b bottom) The pen used for scale is ca 13 cm long. Two veins of pseudotachylitic breccia in Hospital Hill quartzite near the Smilin Thru Resort in the northern collar of the Vredefort Dome. The ca. 7 cm wide, vertical vein is seemingly cut by the thinner, NW-SE trending vein. Pen for scale: ca. 12 cm long. Displacements, however, are not observed. Without detailed analysis of the wider outcrop, it is not possible to ascertain what the actual relationship between these two veins could be: distinct generations emplaced consecutively, or both veins being part of a complex vein system that may not necessarily reflect temporally separate deformation events.

Fig. 4. (a top) Width of the image is ca. 1 m. Typical only a cm wide, veinlet of pseudotachylitic breccia in Hospital Hill quartzite on farm Parsons Rest, NW collar of the Vredefort Dome. Note the subparallel, NNW trending joint set that has displaced the veinlet locally. (b bottom) The pen used for scale is ca 13 cm long. Two veins of pseudotachylitic breccia in Hospital Hill quartzite near the Smilin Thru Resort in the northern collar of the Vredefort Dome. The ca. 7 cm wide, vertical vein is seemingly cut by the thinner, NW-SE trending vein. Pen for scale: ca. 12 cm long. Displacements, however, are not observed. Without detailed analysis of the wider outcrop, it is not possible to ascertain what the actual relationship between these two veins could be: distinct generations emplaced consecutively, or both veins being part of a complex vein system that may not necessarily reflect temporally separate deformation events.

Fig. 4 c. En echelon veinlets of pseudotachylitic breccia, also from the Smilin Thru quartzite exposure. The width of this image is about 1.2 m. These veins do not indicate motion direction, but it is clear that they were formed under tension.

Finally, to restrict "pseudotachylite" formation to "thrust and fault zone" environments is not correct either. Contrary to the tectonic occurrences of friction melt, pseudotachylite-like breccias in impact settings may or may not be related to such structural elements. Again, the Vredefort impact structure provides a case in point: In the Vredefort Dome, the deeply eroded root of the central uplift of this impact structure, apparently fault-related brecciation is observed at many sites in the collar of supracrustal strata, but this is not the rule. Inevitably, these fault-related (i.e., displacements along a linear disruption of the host rock fabric are noticeable) occurrences are thin (examples of pseudotachylitic breccia development in collar strata are shown in Fig. 4a-c) and generally (with just a handful of known exceptions - mostly in the hinge zones of large-scale, i.e., tens or even hundreds of meters, fold structures and, in rare cases, at contacts between mafic intrusions and shale or quartzite) not wider than 1-5 cm. This applies, in comparison to the absolute bulk of breccia observed in the granitoids of the core, to a relatively minor breccia proportion. The overwhelming amount of pseudotachylitic breccia observed in the dome occurs in bodies of generally irregular geometries that -even if they are up to 2.6 km long - do not exhibit a distinct alignment to a recognizable structural element. At a handful of places in the core granitoids,apparent movement on possible faults has also been observed. For example, B.O. Dressler noted a dike-like occurrence of pseudotachylitic breccia in the Kudu Quarry immediately north of the town of Parys, which occurs parallel to a mafic dike or transposed large inclusion of mafic rock that shows signs of tectonic movement.

In contrast, in the Witwatersrand Basin in the environs of the Vredefort Dome, the bulk of breccia observed occurs along bedding-plane parallel and normal fault zones. All three types of occurrences have been linked through detailed field observation (reviewed by Reimold and Colliston 1994 and Dressler and Reimold 2004) as well as absolute chronological data (Trieloff et al. 1994; Spray et al. 1995; Kamo et al. 1996; Moser 1997; Friese et al. 2003) to the Vredefort impact event.

In the discussion of apparent relationships between breccia occurrences and apparently linear structures, the work by Brandl and Reimold (1990) and subsequent field work by these authors on pseudotachylite (friction melt) from the Sand River Gneiss, Limpopo Province of South Africa, is relevant. These workers have shown that veins that may on surface appear perfectly straight, even planar, can just below surface turn - as determined by drilling of such veins - by any angle oblique to the surface exposure, making a mockery of strike trend measurements. Similar observations have been made by us on apparently bedding-plane parallel (Fig. 5a), impact-produced veins of breccia in the collar strata of the Vredefort Dome (also F. Wieland, Univ. of the Witwatersrand, Johannesburg, pers. commun.). Dressler and Reimold (2004) report this effect from the Esperanza Quarry in the southwestern part of the Dome, in core granitoid.

Breccias of pseudotachylite-like appearance from diverse geological (fault/shear or impact) settings may macroscopically and microscopically closely resemble each other (see also below). Two examples shall suffice to emphasize this point: Massive dark-matrix breccia has recently been mapped on the northern anticlinal margin of the Witwatersrand Basin. First field observations resulted in emphasis of close resemblance to Vredefort breccia, with a dark, microcrystalline to aphanitic appearing matrix and sub-millimeter to centimeter size inclusions of granitic country rock. In contrast, subsequent microscopic analysis revealed that the matrix is neither glassy nor aphanitic, and instead represents fine-grained cataclastic material with ample product of secondary alteration. What is more, this breccia closely resembles the so-called Gardnos Breccia from the Gardnos impact structure in Norway (French et al. 1997; Gilmour et al. 2003). It has been demonstrated by French et al. (1997) that Gardnos Breccia represents definite impact breccia of both clastic (lithic impact breccia) and melt-bearing varieties. Macroscopically, it is not possible to distinguish the cataclasite from the Witwatersrand Basin from the Gardnos Brrccia. The breccia from the northern margin of the Witwatersrand Basin could have been generated as a consequence of the Vredefort impact event or, alternatively, in one of the tectonic events that have affected this region over nearly 3 billion years.

Clearly, the descriptive criteria for pseudotachylite are insufficient to separate breccias of such appearance but of different origin. Thus, the normally applied definitions are insufficient. In view of the overwhelming number of pseudotachylite occurrences being located in tectonic - and not in impact - settings, and as the formation of pseudotachylite (friction melt) by cataclasis and frictional melting has been demonstrated by Spray (1992, 1995), we propose that the term "pseudotachylite" be reserved for bona fide friction melt breccia.

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