The Setting of Pseudotachylitic Breccia in Impact Craters

Veins of dark-matrix breccia resembling tectonic pseudotachylite have been described from many impact structures - generally from crater floor rocks, but occasionally from crater rim settings. Such reports are very rare for small, simple bowl-shape craters, and it must be questioned whether any bona fide pseudotachylite (friction melt) has ever been described from these. For example, the only breccia ever recovered from the crater rim of the Tswaing (Pretoria Saltpan) meteorite crater that macroscopically resembled a pseudotachylite turned out to be a manganese/iron-oxide cemented monomict granitic cataclasite. The case of the Roter Kamm breccia has already been discussed above. Furthermore, narrow veinlets of pseudotachylitic breccia have also been described from clasts in impact melt breccia (Dressler and Sharpton 1997).

As reviewed in detail by Dressler and Reimold (2004), significant amounts of pseudotachylitic breccia have been observed only in very large impact structures. Even medium-sized structures like the Ries Crater (24 km diameter), Rochechouart (23 km), and Lappajarvi (17 km diameter) have only yielded centimeter-wide pseudotachylitic breccia veins. Even from the 100 km diameter Manicouagan structure in Canada, only centimeter-wide veinlets of pseudotachylitic breccia have been described to date. However, in such comparisons it is important to consider that these structures are exposed at different erosion levels and that the formation of pseudotachylitic breccia in the floors of impact structures may be mostly a feature of the uppermost floor level. The apparent increase in abundance of such breccia from the Vredefort Structure to the Sudbury Structure is likely a result of the relatively much increased degree of erosion in both case (e.g., Grieve and Therriault 2000; Gibson and Reimold 2001a).

Fig. 6. Two images of the recently identified massive breccia at De Pan, on the Rand Anticline at the northern margin of the Witwatersrand Basin. Leucogranite clasts of mostly - but not exclusively - angular to subangular shapes occur in a dark-grey matrix. Clasts are nearly all derived from the adjacent country rock (leucogranite), and the few apparent exotics may well be derived from mafic xenoliths or bands in the granitic country rock. Knife for scale, ca. 9 cm long.

Fig. 6. Two images of the recently identified massive breccia at De Pan, on the Rand Anticline at the northern margin of the Witwatersrand Basin. Leucogranite clasts of mostly - but not exclusively - angular to subangular shapes occur in a dark-grey matrix. Clasts are nearly all derived from the adjacent country rock (leucogranite), and the few apparent exotics may well be derived from mafic xenoliths or bands in the granitic country rock. Knife for scale, ca. 9 cm long.

Pseudotachylitic

Fig. 7. (a - upper, b - lower, c - right) Photographs show millimeter to sub-millimeter wide veinlets of pseudotachylitic breccia that would represent the so-called "A-Type pseudotachylite" of Martini (1991). Note the undulating geometry of these veins. In (b), a network of such veinlets is actually observed. Country rock: Government Reef quartzite, from northwest of the Smilin Thru Resort in the northern collar of the Vredefort Dome. Hammer for scale: ca. 25 cm long. (c) Drill core specimen from the Inlandsee borehole ca. 4 km south of the geographic center of the Vredefort Dome. This sample consists of pseudotachylitic breccia, of mottled appearance, that does not display a distinct boundary to the gneissic component in this image (at right and left sides). Rather the dark breccia appears to grade into the country rock. The latter only locally displays the originally strong gneiss fabric. Clearly the entire assembly of dark breccia and lighter gneiss seems to have been plasticised. Length of drill core segment ca. 15 cm.

Fig. 7. (a - upper, b - lower, c - right) Photographs show millimeter to sub-millimeter wide veinlets of pseudotachylitic breccia that would represent the so-called "A-Type pseudotachylite" of Martini (1991). Note the undulating geometry of these veins. In (b), a network of such veinlets is actually observed. Country rock: Government Reef quartzite, from northwest of the Smilin Thru Resort in the northern collar of the Vredefort Dome. Hammer for scale: ca. 25 cm long. (c) Drill core specimen from the Inlandsee borehole ca. 4 km south of the geographic center of the Vredefort Dome. This sample consists of pseudotachylitic breccia, of mottled appearance, that does not display a distinct boundary to the gneissic component in this image (at right and left sides). Rather the dark breccia appears to grade into the country rock. The latter only locally displays the originally strong gneiss fabric. Clearly the entire assembly of dark breccia and lighter gneiss seems to have been plasticised. Length of drill core segment ca. 15 cm.

The setting of the pseudotachylitic breccias in Vredefort has already been discussed above. It shall only be added here that some very narrow veinlets of strongly altered and thus not classifiable breccia were described (Fletcher and Reimold 1989) from the northeastern edge of the Witwatersrand Basin (Northcliff Koppie, Johannesburg; Zwartkops inlier of Witwatersrand strata in the western Johannesburg Dome). Whether these occurrences are the result of faulting due to the Vredefort impact event, can be speculated on, but to date, no time marker has been identified for this "tectonic" event. In addition, an occurrence of massive breccia in Archean granite was recently discovered by WUR, at a site called De Pan on the Rand Anticline (Fig. 6); as discussed above, this breccia represents a cataclasite, the matrix of which is strongly altered. It is also not clear whether this is a Vredefort-related breccia, although the massive formation is suggestive of this association.

The massive Sudbury Breccia occurs, apparently somewhat enriched in annular zones (Thompson and Spray 1994; Spray and Thompson 1995), within the Archean gneiss of the basement to the Sudbury Igneous Complex. Preferential enrichment of pseudotachylitic breccia around the Vredefort Dome has also been discussed, but is likely a result of limited knowledge of spatial distribution of such breccia in the region of the Witwatersrand Basin -due to the fact that most relevant information has come from the gold-mining districts that straddle a semi-annular zone around the Vredefort Dome where mining is possible at acceptable depths and where, thus, intersections of major, basin-wide fault zones (Fletcher and Reimold 1989; Killick and Reimold 1990; Killick et al. 1988; Killick 1993) could be investigated.

Hilke (1991), in his detailed discussion of breccia dikes from the 39 km diameter Carswell impact structure in Saskatchewan (Canada), chose to introduce still another term - "Reibungsschmelzbreccia " (which translates to "friction melt breccia"), instead of subscribing to "pseudotachylite", that he explicitly defines as a genetic term. This author stated that he feels that the term "pseudotachylite" is applied to "both impact-induced as well as impact-independent, tectonically formed dikes and an exact definition has not been fixed" (Hilke 1991). He then distinguished, allegedly on textural grounds and discordant contacts, between "early" and "late" veins. "Early veins" were ~200-300 ^m wide and composed of glass and its alteration products, in "network-like geometries." Note that this geometric observation does not correspond to Martini's (1991) definition of early "Type A pseudotachylite". "Late" dikes were macroscopically distinguishable because of their blackness, they could be up to several centimeters wide, and transitions from crystalline to clastic matrix could be observed (according to Hilke). Significantly, this author emphasized that "the matrix can not be resolved by optical microscopy and consists of secondary phyllosilicates". This statement represents a clear contradiction to Hilke's claim that crystalline and clastic matrix types could both be observed at Carswell. It also leads to the serious question whether these veins can be classified as "friction melt breccia" (a term that we would consider equivalent to pseudotachylite = friction melt), if it is not possible to positively resolve the nature of matrix. It is also interesting that Hilke (1991) observed very small ("gering") and significant displacements along "early" and "late" veins, respectively. He declares that both "generations" of "Reibungsschmelzbreccie" were formed during the compression to early excavation phase of cratering; thus, Hilke's "late" generation can not be equated with Martini's (1991) "B-Type pseudotachylite" either, which Martini linked to the later modification stage of cratering.

From the 30 km diameter Azuara structure (Spain) of still controversial origin sub-millimeter veinlets of "glass-bearing dike breccia" were classified as pseudotachylite (Ernstson and Fiebag 1992). In view of the debate about the origin of Azuara, it is certainly premature to attempt a genetic discussion of this occurrence, and, anyway, information presented on this alleged breccia occurrence is insufficient to claim an origin of this material as friction melt.

Besides the studies mentioned above, not much very detailed work has been carried out on pseudotachylitic breccias. Veins are known from a number of other structures, such as Morokweng and Puchezh-Katunki, where they have been described from drill core only - thus not providing any information on structural controls that perhaps could be used to investigate the time during the cratering event when these veins were formed. The drill core Hattberg BH5 from the Siljan impact structure is also said to contain numerous pseudotachylites; however, the true nature of these occurrences has not been published yet (T. Kenkmann, pers. commun., 2003). Kenkmann also reports that a number of outcrops in the Siljan structure, such as the Trollberget location, showed such breccia occurrences.

Elandsrand Gold Mine

Fig. 8. (Previous page ) Four examples of pseudotachylitic breccia in, or associated with, Ventersdorp Contact reef (small-pebble conglomerate) from Elandsrand Gold Mine, ca. 70 km north-northwest of the center of the Vredefort impact structure. (a- upper left) Two cross-cutting veinlets of dark-matrix breccia that do not display obvious displacement. (b upper right) Several veinlets of pseudotachylitic breccia, whereby the NE-SW trending veinlet has displaced the longer, N-S trending one by several millimeters. (c lower left) Two generations of breccia: the lighter-colored mylonite in the top half of the specimen has been transected by an apparently younger (younger by how much - microseconds or million years - is of course not known!) and very thin dark-matrix breccia, and in the upper right corner of the sample, it is obvious that the banded mylonite has been displaced by at least 3 cm. (d lower right) A complex sample with two - lighter and dark colored - breccia types that occur in intimate association with each other. Nevertheless, this sample gives the impression that the darker phase post-dates the lighter material, which is found in the form of very small inclusions in the swirl of dark breccia in the central part of the image. Note that the sample from Elandsrand discussed by Killick and Reimold (1990) comprised three distinct generations of breccia - based on cross-cutting relationships and inclusion of clasts of an older in relatively younger breccia. Scales on all four images: 50 mm wide.

In the Champagnac Quarry in the Rochechouart impact structure of France, several veins of up to 10 centimeter width have been observed, all at intermediate to high angles to the quarry floor and dipping towards the crater center. One could argue that such geometry could indicate formation of friction melt during collapse of the central uplift - however, we do not even know whether the Rochechouart breccias are the result of friction melting or shock melting. Indeed, some of the observations of such melt breccia by Reimold et al. (1987) and Bischoff and Oskierski (1987) may be suggestive of an origin by localized shock melting. These reports include observations of anastomozing veinlets and microscopic melt "pods". Also, these occurrences are closely related to hydrothermal quartz veins and impregnations with secondary quartz, which may suggest melt formation under extensional conditions - such as at crater modification. Accordingly, these authors concluded that melt formation took place late with regard to crater evolution, due to friction melting as a consequence of block movement in the crater floor during the modification stage of cratering. Kenkmann et al. (2000b) reported pseudotachylite at Champagnac that occured on fault surfaces belonging to a low-angle normal fault system that they interpreted to have accommodated inward movement during crater collapse.

The Champagnac Quarry represents only a small window into the Rochechouart crater floor. Far more extensive information is required before a comprehensive model for the 3D occurrence, distribution, and orientation of pseudotachylitic breccias can be established.

One of the latest additions to the terrestrial impact crater record, the Woodleigh Structure of Australia, was said to have a "penetrative network of pseudotachylite", intersected in a drill core (Mory et al. 2000a). Diagnostic shock deformation has been described (Mory et al., 2000b; Koeberl et al. 2001; Reimold et al. 2003b) from drill core, confirming Woodleigh's origin by impact. However, authors are divided with regard to the diameter of this structure, for which values of 120 km (Mory et al., 2000a,b; Uysal et al. 2001, 2002) or 60 km (Reimold and Koeberl 2000; Reimold et al. 2003b; Renne et al. 2002) have been favored. Regarding the alleged "pseudotachylite", Reimold et al. (2003b) have demonstrated that individual narrow (not wider than 1 mm) veinlets with a dark matrix are indeed found in a few drill core samples. They concluded that to use the term "penetrative" represents exaggeration. As to the nature of this material, all veins studied by Reimold et al. were found to be thoroughly altered and do not allow identification of a bona fide pseudotachylite. A range of other breccia types (such as cataclasite -of tectonic or impact origin, mylonite of pre-impact origin) could be considered as candidates as well.

This scenario for Woodleigh is very similar to what is known from Morokweng, a 70-80 km wide impact structure in the North West Province of South Africa (Reimold et al. 1999a, 2002). Early workers (Andreoli et al. 1995; Hart et al. 1997) reported "pseudotachylite" development in the crater floor as seen in drill core, but Reimold et al. (1999a) and Koeberl and Reimold (2003) have shown that there are occurrences of both impact melt injections and cataclasite in this drill core section, as well as some other breccia veins, the filling of which is so thoroughly altered that assignation of a breccia type is not possible anymore. As at Woodleigh, dark-matrix breccia, similar to pseudotachylite = friction melt, occurs in the crater floor of Morokweng, but -as discussed above - several types of breccia are represented. Both Woodleigh and Morokweng demonstrate that caution is advised to avoid such indiscriminate categorization of dark-matrix breccias as "pseudotachylite".

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