Shock Metamorphism

Abstract

A requirement for the recognition and confirmation of meteorite impact structures is the presence of shock metamorphic indicators, either megascopic (e.g. shatter cones) or microscopic (e.g. planar deformation features in minerals), or high-pressure polymorphs (e.g. coesite and stishovite) and/or siderophile element (e.g. iridium) or isotopic (osmium) anomalies in specific geological settings. Crater morphology is not a sufficient argument, because a variety of circular features can be formed by completely different geological processes (e.g. volcanism or salt diapirism). The terms 'shock effects' or 'shock-metamorphic effects' cover all types of shock-induced changes, such as the formation of planar microstructures and phase transformations. Impact metamorphism is essentially the same as shock metamorphism, except that it also encompasses the melting, decomposition and vaporization of target rocks (Stöffler and Grieve, 2007). These irreversible changes are produced when rocks are subjected to shock pressures above their Hugoniot elastic limit (HEL). This limit is defined as 'the critical shock pressure at which a solid yields under the uniaxial strain of a plane shock wave' (Stöffler, 1972). The HEL of quartz is in the range of 5-8 GPa and ranges from approximately 1 to 10 GPa for most geological materials (e.g. Stöffler, 1972;Stöffler and Langenhorst, 1994). In nature, at the surface of the Earth, only hypervelocity impacts can generate such high shock pressures.A great diversity of shock effects in minerals are known and have been abundantly described in the literature during the last 40 years mostly for quartz (e.g.

Microspectroscopic and Petrographic Comparison of Experimentally Shocked Albite, Andesine, and Bytownite

“…While the work presented here focuses exclusively on plagioclase feldspars, it is worth noting that many of the models and ideas of how shock affects feldspar are based heavily on analogy with the well‐studied shock effects in quartz. Detailed reviews of shock metamorphic effects in framework silicates can be found in Alexopoulos et al (), Chao (), Ferrière and Osinski (), Grieve et al (), and von Engelhardt and Stoffler ().…”

Gale crater and impact processes – Curiosity’s first 364 Sols on Mars

“…15A are reminiscent of shatter cones. Shatter cones are best developed in fine-grained sedimentary rocks in terrestrial craters, but are also found in fine-grained igneous or metamorphic basement rocks commonly exposed in central uplifts (Ferrière and Osinski, 2013). Shatter cones are distinguished by divergent striations that appear to penetrate the rock, in comparison with slickensides with parallel striations, or ventifacts, where surficial textures are caused by aeolian erosion (Ferrière and Osinski, 2013).…”

“…Shatter cones are best developed in fine-grained sedimentary rocks in terrestrial craters, but are also found in fine-grained igneous or metamorphic basement rocks commonly exposed in central uplifts (Ferrière and Osinski, 2013). Shatter cones are distinguished by divergent striations that appear to penetrate the rock, in comparison with slickensides with parallel striations, or ventifacts, where surficial textures are caused by aeolian erosion (Ferrière and Osinski, 2013). Although well-developed shatter cones are very distinctive visually, less developed shatter cones and shatter textured rocks (rocks with subparallel lineations) are common at terrestrial craters but are mainly recognized due to their proximity to other evidence of impact.…”

“…15B shows an example of a rock with striations that could be related to shock, but the image resolution is not sufficient to confirm the nature of the striations, in particular to distinguish them from aeolian abrasion features. The limited number of candidates is not surprising, because shatter cones are usually associated with bedrock from beneath the transient cavity of relatively large craters, including the central uplift (Ferrière and Osinski, 2013). Such materials would not be expected in the locations visited by Curiosity unless transported from the crater rim as distal ejecta.…”

Petrographic Investigation of Target Rock Transformation under High Shock Pressures from the Colônia Impact Crater, Brazil

“…However, it is important to note that adiabatic decompression melting (i.e., magma generating process) is different to impact melting where melting of the target lithology occurs under different degrees of shock metamorphism, causing phase changes and variable degrees of mechanical mixing of the original target rock (Grieve & Therriault, 2004;). An important aspect of impact lithologies is that they may contain a series of distinctive microscopic deformation features (i.e., impact indicators) from simple fracturing to complete melting, such as planar deformation features in quartz, mechanical twins in pyroxene, kink-band deformation in mica, granular texture in zircon, diapletic glass, and ballen silica (French, 1998;French & Koeberl, 2010;Ferrière & Osinski, 2012). Consequently, impact melt-bearing impactites provide valuable information to better understand how the complex impact crater formation process operates.…”