Background and General Characteristics

The conical fracturing phenomenon known as shatter cones (Figs 16 and 17, 20 and 21) is widely considered shock (impact) diagnostic. Since the early pioneering work by Dietz (1947, 1959, 1961, 1963, 1968), Hargraves (1961), Manton (1962, 1965), Milton (1977), Milton et al. (1972, also 1996), and Roddy and Davis (1977), shatter cones have been described from many impact structures and have been, in many cases, the first indication for the presence of an impact structure.

French (1998) defined shatter cones as "distinctive curved, striated fractures that typically form partial to complete cones". It must, however, be emphasized that we ourselves have not come across many complete cones - though several allegedly complete specimens from Vredefort have been described in the literature (Albat and Mayer 1990; see also discussion by Nicolaysen and Reimold 1999). Sizes of shatter cones reportedly range from just a few millimeters in length to more than 1 m. Our own observations from Vredefort indicate that most features are a few to 15 cm

Fig. 16. Photograph of the front of a well-developed shatter cone specimen from the Steinheim Basin meteorite crater, southern Germany. The coin allows to compare the orientation of this sample with that of the same, reversed specimen in Fig. 17, illustrating that the apex orientations of cone fractures visible on both sides of this sample do not consistently point into the same direction. Also note several incomplete cone fractures that nevertheless still indicate highly diverse directions in which striae seemingly converge. Sample length ca. 15 cm.

Fig. 16. Photograph of the front of a well-developed shatter cone specimen from the Steinheim Basin meteorite crater, southern Germany. The coin allows to compare the orientation of this sample with that of the same, reversed specimen in Fig. 17, illustrating that the apex orientations of cone fractures visible on both sides of this sample do not consistently point into the same direction. Also note several incomplete cone fractures that nevertheless still indicate highly diverse directions in which striae seemingly converge. Sample length ca. 15 cm.

Fig. 17. Photograph of the back of the same specimen from Steinheim shown in Fig. 16.

in length, and the largest ones observed have lengths of up to 50 cm and diameters of up to 50-60 cm. Partial cones may represent segments of cones not larger than 10°-20°, or may subscribe up to >270° segments. Workers are urged to investigate whether such significant proportions of cones really represent single geometries, or whether they - as favored by Nicolaysen and Reimold (1999) - represent combinations of smaller cone segments on intersecting curviplanar joints that have combined to apparent larger cone segments.

Shatter cones have generally been described from rocks of central uplifts of complex impact structures, but isolated fragments have been observed in impact breccia units as well (e.g., in clasts in Sudbury Breccia of South Range exposures). Because of this, it is generally thought that shatter cones form during the early shock compression stage, but some evidence of shatter cones post-dating other impact deformation (such as fault gouge or pseudotachylitic breccia - Simpson 1981 and Reimold and Colliston 1994, respectively) has, for example, been presented from Vredefort.

Siljan Shale

Fig. 18. Several sets of closely-spaced joints - termed multipli-striated joint sets by Nicolaysen and Reimold (1997) - in Hospital Hill Quartzite from the inner collar of the Vredefort Dome. Locality: Smilin Thru resort, north of Parys. Short white lines illustrate the orientations of important joint sets. Gum packet, for scale, 7 cm long.

Fig. 18. Several sets of closely-spaced joints - termed multipli-striated joint sets by Nicolaysen and Reimold (1997) - in Hospital Hill Quartzite from the inner collar of the Vredefort Dome. Locality: Smilin Thru resort, north of Parys. Short white lines illustrate the orientations of important joint sets. Gum packet, for scale, 7 cm long.

Shatter cones may occur in the form of individual cone features or as groups of many, then often partially overlapping, features. Striations on a specific cone segment may come together at a well defined apex or may emanate from a several millimeter to >1 cm wide apex area. They diverge distinctly, so that the base of a cone segment is generally at least 2-3 times as wide as the apex area. Striations are generally not straight but are curved and often meandering, and the microtextural analysis by Nicolaysen and Reimold (1999) has shown that striae may indeed meander around grain boundaries. It should be noted, however, that this is not the rule, as striae cross-cutting and possibly displacing mineral grains have also been observed. Near the base of a cone segment, striae may diverge even more into a pattern that has been described as "horse-tailing". French (1998) remarked that "parasitic cones commonly occur on the surfaces of both complete and partial shatter cones, forming a "nested" texture". Apices of many cones may, at a given locality, point into a uniform direction, which has led many workers to adhere to a theory that the apex

Fig. 19. Another example of MSJS, from the same locality as the example shown in Fig. 18.

direction can be related to the shock wave propagation direction. In this "master cone" hypothesis (see Nicolaysen and Reimold 1999 - for a detailed but critical discussion and references of earlier work), it is proposed that cone apices, after rotation of the host strata into the presumed pre-impact orientation, should point into the direction from which the shock wave originated. Nicolaysen and Reimold (1999), however, indicated that while apices at Vredefort may be preferentially aligned in a direction that was predetermined by a strong anisotropy (namely the bedding-plane) in the supracrustals of the Vredefort dome, other cone fractures may have opposite or strongly divergent orientations. Specimens from Steinheim Basin (southern Germany) demonstrate how varied the directions of cone apices may be on the multi-facets of a single specimen (Fig. 16, 17, and 21).

Sagy et al. (2002) presented some results on Vredefort shatter cones and proposed that the angle between outer striae on parasitic cones followed a distinct relationship with distance of sampling site from the center of an impact structure. They claimed that this angle decreased according to a definite relation with increasing distance of sampling sites from the center of the Vredefort dome. This hypothesis was further investigated by Wieland and Reimold (2003) and Wieland et al. (2003), who concluded that (a) no specific "striation angle" could be measured on any given shatter cone specimen and (b) both the hypothesis and trend promoted by Sagy et al. (2002) could not be reproduced.

Nicolaysen and Reimold (1999) drew further attention to the fact that shatter cones at Vredefort - and Sudbury - are intimately related to a pervasive system of curviplanar fractures occurring - in the case of Vredefort - in up to 12 orientations at a given site. These authors termed these fracture sets "MSJS" - multipli-striated joint sets (Figs 18 and 19). These curviplanar joints/microjoints are spaced at several mm to 1 cm, thus forming an intense and pervasive fracture system affecting large rock volumes. Cone segments and their striae were directly related to intersections and orientations of such joints (MSJ) by Nicolaysen and Reimold (1999). This observation was also already reported by Manton (1962). Nicolaysen and Reimold (1999) also illustrated the presence of MSJS in the Sudbury Structure.

Shatter cones can be observed in many different rock types, whereby finer-grained lithologies (e.g., limestone, siltstone) generally exhibit better examples of this deformation phenomenon than relatively coarser-grained materials. However, even at 0.5 cm grain size, as in some granitoids of the Vredefort basement, admittedly crude cone fractures have been observed. Nevertheless, the beautiful examples in limestone from Houghton Dome,

Siljan Shale
Fig. 20. A circa 40 cm long shatter cone from the Booysens Shale locality (Gibson and Reimold 2001) in the northwestern collar of the Vredefort Dome. Note the pronounced horsetailing of striae on one side of the sample. Scale bar in centimeters.

Steinheim, or Ries, and from the fine-grained metapelites and sandstones of the Vredefort and Sudbury structures are legendary.

Some workers maintain that it is not necessary to rely on shock pressures to generate shatter cone-like fractures in rock. However, from shock deformation scaling in impact structures and some shock experimentation, it has been established that low shock pressures, typically 2-10 GPa (perhaps as high as 30 GPa), can lead to shatter cone formation. It is also important to note that shatter cones from a number of structures have been shown to have surficial melt products (e.g., Gay 1976; Gay et al. 1978; Gibson and Spray 1998; Nicolaysen and Reimold 1999). Various workers (e.g., Martini 1991) have related this to friction melting and, thus, occurrences of pseudotachylitic breccia in impact structures. Nicolaysen and Reimold (1999) showed that distinct displacements can be observed in thin sections on MSJS, and that thin melt films may occur on such microjoints, as well as in intersections of microjoints of different orientations (see also below - section 4.4 on Microscopic Textures Related to Shatter Cones).

Multiply Striated Joint Sets Msjs
Fig. 21. Another shatter cone specimen from the Booysens Shale locality. Scale bar in centimeters. Note the different orientations of apices of shatter cones in this example.

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