The Sense of Pseudotachylite

Modern textbook definitions of "pseudotachylite" are still basically descriptive but do sometimes recognize that such breccia appears to occur in two different geological settings - tectonic fault and shear zones on the one hand, and impact structures on the other. A typical example is found in Bates and Jackson (1987), who define pseudotachylite as "[a] dense rock produced in the compression and shear associated with intense fault movements, involving extreme mylonitization and/or partial melting. Similar rocks, such as the Sudbury Breccias, contain shock metamorphic effects and may be injection breccias emplaced in fractures formed during meteoric [sic] impact ... material carries fragmental enclosures and shows evidence of having been at high temperature...". This definition already highlights a specific problem concerning "pseudotachylites", namely the fairly commonly observed relationship between (ultra) mylonites, and/or (ultra) cataclasites and a melt phase (see Reimold 1995, 1998).

Another example, from Passchier et al. (1990), states that "Pseudotachylites are formed by fault-associated melting of the host rock in response to the heat produced by major seismic slip events, or possibly in some cases by intense cataclasis." Clearly, these authors have placed the emphasis only on fault/shear related processes. The tectonic worker consequently associates "pseudotachylite" with friction melting triggered by faulting/shearing. Much of the existing literature on pseudotachylites, accordingly, is dominated by discussion of tectonic breccias, but some groups have, in the recent past, worked extensively on impact-related, pseudotachylite-like breccias (Dressler and Reimold 2004, for an extensive review of this literature). Reimold (1995, 1998) and this work recommend that the term "pseudotachylite" should only be used as a synonym for bona fide friction melt, and that all other breccias of pseudotachylite-like appearance should be assigned the preliminary classification "pseudotachylitic breccia" until such time as the nature and origin of the material is identified, at which point other genetic nomenclature can be applied.

Tectonic pseudotachylite occurrences are generally limited to breccia veins with centimeter (rarely up to decimeter) width. One of us (WUR), recently, had opportunity to observe pseudotachylite occurrences typical for the

Fig. 3 a (upper) and b (lower). Two impressions of tectonically produced breccia veinlets from the Homestake Lake fault zone in Colorado (USA). The dark-matrix breccia either occurs in the form of centimeter (or less) wide veinlets and stringers (a), generally following the fabric trend in the fault zone, but occasionally oriented oblique to it. Clasts in and along such occurrences have both roundish or angular shapes. As shown in (b), occasionally wider developments of breccia can be seen, with some well-rounded clasts attesting to the highly dynamic nature of this system. However, despite the fact that this fault zone is of a kilometer scale, the breccia occurrences are seemingly sparse and cannot be compared in extent to the massive occurrences at Vredefort or Sudbury.

Fig. 3 a (upper) and b (lower). Two impressions of tectonically produced breccia veinlets from the Homestake Lake fault zone in Colorado (USA). The dark-matrix breccia either occurs in the form of centimeter (or less) wide veinlets and stringers (a), generally following the fabric trend in the fault zone, but occasionally oriented oblique to it. Clasts in and along such occurrences have both roundish or angular shapes. As shown in (b), occasionally wider developments of breccia can be seen, with some well-rounded clasts attesting to the highly dynamic nature of this system. However, despite the fact that this fault zone is of a kilometer scale, the breccia occurrences are seemingly sparse and cannot be compared in extent to the massive occurrences at Vredefort or Sudbury.

prominent and extensive Homestake Lake Fault Zone in Colorado (Fig. 3a and b). In comparison to the voluminous breccias at Vredefort, the generally several millimeter-wide occurrences of dark-matrix breccia that occur over a width of centimeters to perhaps several decimeters did not appear very impressive. However, one really significant tectonic pseudotachylite-bearing fault zone, up to 1000 meters wide and containing some 4% of pseudotachylite veining, has been described by Camacho et al. (1995) from the Woodroffe Thrust in the Musgrave Block of the interior of Australia. Up to 2-3 m wide extensional deformation zones with tectonically produced "pseudotachylyte" forming up to 30% of the total rock volume are also known from the Insubric Line bordering the Ivrea Zone of northern Italy (e.g., Techmer et al. 1992). Here, pseudotachylite formation has been related to transpressional strike-slip movements along the fault zone. In both cases, these pseudotachylites occur within crustal-scale fault zones that are several orders of magnitude larger than the actual zones with pseudotachylite development and that involve slip magnitudes of many kilometers.

Endogenic pseudotachylites are not rare. Numerous investigations of recent years have provided evidence that tectonically produced pseudotachylites are quite common in the brittle-to-ductile transition zone of the Earth's crust. Such occurrences occur in a variety of tectonic settings and at different crustal depths, including strike-slip fault zones (e.g., Insubric Line - Techmer et al. 1992), on thrust planes (e.g., Silvretta, Alps - Koch and Masch 1991), subduction zones at eclogite facies conditions (Norway - Austrheim and Boundy 1994), under granulite facies metamorphic conditions (Antarctica -Passchier et al. 1991; Clarke and Norman, 1993), and in landslides (Silvretta -Alps, Himalaya - e.g., Masch et al. 1985; Heuberger et al. 1984; Kofels - e.g., Preuss, 1971; Erismann and Abele 2001; Arequipa, Peru - Legros et al. 2000). Explanations for these occurrences range from stick-slip faulting processes to catastrophic volume reduction. Early-formed pseudotachylites within a fault zone may be obliterated or deformed during subsequent shearing or faulting events (e.g., Passchier et al. 1991).

The volumetrically most important occurrences of pseudotachylite-like breccias are, as stated, the Vredefort Structure in South Africa and the Sudbury Structure in Canada. After lengthy controversies, both structures are now widely accepted as the world's largest known impact structures (e.g., Gibson and Reimold 2001a; Grieve and Therriault 2000). In the Sudbury Structure, breccia dikes of up to 42 km length and, at least, 400 m width have been described, whereas at Vredefort several occurrences of kilometer length and up to a hundred meter width have been mapped. Dressler and Reimold (2004) report a dike-like occurrence of network breccia that could be mapped for 2.6 km at a width of up to 100 m. Smaller impact structures do not appear to exhibit either voluminous or extensive pseudotachylite development (although this statement is relative and its validity depends entirely on what is considered "pseudotachylite"). In the following, this issue and other problematics concerning impact-related "pseudotachylite-like" breccias are discussed. The discussion focuses largely on the so-called "pseudotachylite" in the Vredefort Structure, not only because this is the type locality for this material, which, itself, is a major reason for the difficulty regarding nomenclature and application of this term in relation to impact structures, but also because the excellent preservation of breccias in a range of impact-related environments allows evaluation of the relative contributions of shock and slip processes to breccia formation.

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