New observations on shatter cones in the Vredefort impact structure, South Africa, and evaluation of current hypotheses for shatter cone formation

Abstract

available online at http://meteoritics.org New observations on shatter cones in the Vredefort impact structure, South Africa, and evaluation of current hypotheses for shatter cone formation Abstract-Shatter cones have been described from many meteorite impact structures and are widely regarded as a diagnostic macroscopic recognition feature for impact. However, the origin of this meso-to macroscopic striated fracture phenomenon has not yet been satisfactorily resolved, and the timing of shatter cone formation in the cratering process still remains enigmatic. Here, previous results from studies of shatter cones from the Vredefort impact structure and other impact structures are discussed in the light of new field observations made in the Vredefort Dome. Contrary to earlier claims, Vredefort cone fractures do not show uniform apex orientations at any given outcrop, nor do small cones show a pattern consistent with the previously postulated "master cone" concept. Simple back-rotation of impact-rotated strata to a horizontal pre-impact position also does not lead to a uniform centripetal-upward orientation of the cone apices. Striation patterns on the cone surfaces are variable, ranging from the typically diverging pattern branching off the cone apex to subparallel-toparallel patterns on almost flat surfaces. Striation angles on shatter cones do not increase with distance from the center of the dome, as alleged in the literature. Instead, a range of striation angles is measured on individual shatter cones from a specific outcrop. New observations on small-scale structures in the collar around the Vredefort Dome confirm the relationship of shatter cones with subparallel sets of curviplanar fractures (so-called multipli-striated joint sets, MSJS). Pervasive, meter-scale tensile fractures cross-cut shatter cones and appear to have formed after the closely spaced MSJ-type fractures.The results of this study indicate that none of the existing hypotheses for the formation of shatter cones are currently able to adequately explain all characteristics of this fracturing phenomenon. Therefore, we favor a combination of aspects of different hypotheses that includes the interaction of elastic waves, as supported by numerical modeling results and which reasonably explains the variety of shatter cone shapes, the range of striation geometries and angles, and the relationship of closely spaced fracture systems with the striated surfaces. In the light of the currently available theoretical basis for the formation of shatter cones, the results of this investigation lead to the conclusion that shatter cones are tensile fractures and might have formed during shock unloading, after the passage of the shock wave through the target rocks.

Impact-pressure controlled orientation of shatter cone magnetizations in Sierra Madera, Texas, USA

“…This observation agrees with Wieland et al (2006) that a front wave of an impact affect homogeneously throughout the bed rock (early stage) while the drop in the pressure caused localized melting along the fractures and could reset the magnetization vector of shatter cone material.…”

“…Sagy et al (2002Sagy et al ( , 2004 explained the mechanism of shatter cone formation was due to tensile fracturing in the early compression stage of a shock event. A contrasting account has been proposed by Wieland et al (2006) and Nicolaysen and Reimold (1999), particularly for the Vredefort shatter cones that they were late stage formation and shatter cones occurrence varies depending on the rock types. Another hypothesis based on a numerical modeling by Baratoux and Melosh (2003) suggested that shatter cones were tensional fractures formed at a local heterogeneity of rocks.…”

“…Another hypothesis based on a numerical modeling by Baratoux and Melosh (2003) suggested that shatter cones were tensional fractures formed at a local heterogeneity of rocks. Well summarized account for the genesis of shatter cone formation has been described in Wieland et al (2006) that a front wave of an impact acts homogeneously throughout the bed rock (early stage), while the scattering and reflecting waves act locally (later stage) due to heterogeneity of rocks (e.g., porosity, mineralogy).…”

The first large meteorite impact structure discovered in the Middle East: Jebel Waqf as Suwwan, Jordan

“…For example, at 31°02′57′′Ν/36°48′42′′Ε a small limestone enclave occurs within chert, which contains shatter cones throughout its mass. Figure 3b shows examples of striated, variably plane or curved, even curviplanar, fracture surfaces that are very much like the striated joint phenomena known extensively, for example, from the central uplift of the Vredefort impact structure in South Africa (e.g., Manton 1962Manton , 1965Nicolaysen and Reimold 1999;Wieland et al 2006). Striated fracture surfaces at Jebel Waqf as Suwwan at some sites occur in multiple orientations.…”

Rietveld analysis of X‐ray powder diffraction patterns as a potential tool for the identification of impact‐deformed carbonate rocks

“…Therefore, shock features within these two quartzrich units do not accurately indicate the highest shock pressures to which these rocks or the adjacent limestones were exposed. Nevertheless, the presence of shatter cones within the Hess, Cathedral Mountain, and Word Formations indicate rocks from the central uplift of Sierra Madera were exposed to shock pressures in the range of 3-10 GPa; however the actual pressure range of formation is probably narrower (French 1998;Baratoux and Melosh 2003;Wieland et al 2006;Osinski 2007).…”