Textures of Shock Metamorphism

Shock metamorphism is the irreversible chemical, mineralogical and physical changes in the target materials that occur during the hypervelocity impact event due to sudden extreme escalation of pressure, temperature and strain rates. Shock metamorphism provides evidence for conditions associated with impact cratering (e.g., French and Short, 1968; Stöffler and Langenhorst, 1994; Grieve et al., 1996; Koeberl, 1997; Melosh, 1989; French, 1998, Deutsch, 1998and references therein) including high pressures, temperatures, and strain rates (106−108 s−1), which lead to characteristic structural and phase changes in minerals. To understand their evolution in space and time, it is important to correlate the shock deformation episodes with specific mineral assemblages. The mineral assemblages that stabilized at specific pressure – temperature (P-T) regime during successive shock deformation phases can be used for identifying the shock metamorphic conditions under which they have formed, and these relationships can also be used to establish the evolution of shock metamorphic conditions with progressive impact event mechanism. This can be achieved through detailed textural and microstructural analysis of the different mineral assemblages that stabilized in impactites with varying P-T conditions. The mineral chemistry of a particular phase in a specific textural domain that possibly represents equilibrium conditions needs to be determined to retrieve the peak shock metamorphic conditions under which these assemblages were stabilized. The unique conditions of shock-wave environments produce unique effects in the affected target rocks. The nature and intensity of these changes depend upon the shock pressures. Physical expressions of shock wave compression followed by immediate decompression are irreversible deformation effects, e.g., Planar Deformation Features (PDFs), Planar Fractures (PFs) and high pressure high temperature (HPHT) polymorphsin many rock-forming minerals. These deformation and transformation effects are collectively known as “shock (or impact) metamorphism” (after Grieve et al., 1996; French, 1998, Stöffler et al., 1991, Stöffler and Langenhorst, 1994,Reimold and Koeberl, 2014and references therein).

An understanding of terrestrial impact cratering is important as it addresses how the outer layer of the Earth has been modified due to impacts, and also its effect on the physical, chemical, and biological systems. Impact cratering processes can well be understood by integrating a host of multidisciplinary disciplines such as remote sensing, geological, geophysical (gravity, magnetic, seismic, and electric methods), petrographical, mineralogical, geochemical, geochronological, numerical modelling, and laboratory experimental studies. These studies are aimed at identification of possible new impact structures, verification of their origin, and detailed analysis of the geological structure and rock deformation in such crater structures. However, these are rare features on the surface of the Earth and only 190 structures are currently known and accepted as confirmed impact structures (Earth Impact database developed by Planetary and Space Science Centre, Canada; http://www.passc.net/EarthImpactDatabase/Worldmap.html), out of which three confirmed impact structures are from India (e.g. Lonar, Dhala and Ramgarh craters).

An attempt is made to categorize these shock metamorphic textures with the available data base form the Dhala, Lonar and Ramgarh craters of India. In the following section such different characteristic micro textures with photomicrographs have been illustrated for better understanding of impact crater mechanism from different geological milieu and provinces in India.

Rock fragments/clasts are observed in the cataclasite matrix / groundmass. Brecciatedgranitoids constitute the dominant clast population, and clast : matrix ratios may vary between 90:10 and 60:40. The matrix / groundmass (defined as the optically not resolvable material, apanetic) is either pink or purple to chocolate brown colored (mesoscopically), overall fine grained and ferruginous too.It contains quartz, opaque minerals, mica, and rock fragments. The clast population of this breccia is dominated by granitoids (> 95%) besides small amounts derived from the various xenolithic components, either mafic or phyllosilicate rock fragments presently altered to clay.

No evidence of possible melt components has been detected in these breccias. The dark colour of the matrix / groundmass is due to the presence of extremely fine-grained opaque minerals (mostly magnetite + ilmenite ± Fe-hydroxides), and minor amphibole ± biotite ± chlorite crystals throughout the matrix. Major clast types are sub-angular to angular in shape, partly digested, and show random orientation.

Planar fractures (PFs – fracturing akin to imperfect cleavage plane) have been described from low shock regime (< 5 GPa). The impact-diagnostic value of these phenomena is still unclear. For example, it is not impossible that single sets of PFs be formed under tectonic conditions, but multiple sets of PFs of different crystallographic orientations formed in a quartz host grain would be a typical for impact origin as suggested by Reimold and Koeberl (2014).

Planar Deformation Features (PDFs) formed in a range of important rock-forming minerals, including quartz and feldspars (for Dhala structure), are the most widely accepted recognition criterion for shock metamorphism. Texturally they are absolutely straight (planar), crystallographically controlled features of 1-2 μm width and 1-6 μm spacing (approx.) that may occur in parts of a crystal or traverse penetratively throughout the grain.