Shatter Cone Formation

Different mechanisms have been proposed for the formation of shatter cones. This includes the idea by Johnson and Talbot (1969) that the elastic precursor of a shock front is scattered by a heterogeneity in a rock (see also Sharpton et al. 1996). Gash (1971) proposed that shatter cones were produced from the interaction of an incident shock wave with a tensile wave reflected from a reflective source such as a local heterogeneity in a rock (for example a relatively large grain, pore space, or fracture). Milton (1977) suggested that shatter cones formed during relaxation after peak compression. And Baratoux and Melosh (2003) have just recently advocated that while cone initiation takes place at heterogeneities in a rock, this would occur after "the passage of the main plastic compression pulse" (ibid). It is obvious that more work, combining geological (i.e., field work) and mineralogical investigations with theoretical advance, is required before a comprehensive model for the formation of this impact phenomenon can be established. Nicolaysen and Reimold (1999) concluded from their macroscopic to microscopic analysis of shatter cones and joint phenomena at Vredefort that "formation of shatter cones appears to be invariably linked with the occurrence of the MSJS micro-jointing phenomenon". One serious impediment related to the investigation of the pervasive joint structures in the Vredefort Dome is the lack of temporal constraint on the number of generations of joints and the timing of joint formation with respect to the short-term impact process.

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