Shock pressure estimates in target basalts of a pristine crater: A case study in the Lonar crater, India

Abstract

The study of shock pressure indicators can provide important clues for understanding the cratering process, though the estimation of shock pressures in weakly shocked rocks is commonly difficult. In this study, we selected a very young and well-preserved impact structure, the Lonar crater in India. The crater, devoid of any tectonic overprint, can be assumed as pristine. We used a combination of rock magnetic and microfracture studies to estimate shock pressure in the crater rim. On the basis of present results, the magnetic fabrics are interpreted to be of magmatic origin related to the Deccan basalt emplacement. The high-coercivity component of the natural remnant magnetization in the crater rim basalt is similar to that in the unshocked basalt. The lack of any shock-related magnetic overprint on the crater rim basalt is, therefore, evident in the Lonar crater.In contrast, radial and concentric microfractures observed in basalts at the crater rim and farther away show symmetric distribution with respect to the crater. The concentric microfractures consistently overprint the radial microfractures. We infer that the radial and concentric microfractures were developed during propagation of the early compressional and the late decompressional shock wave components, respectively. The results of our rock magnetic and microfracture studies, when interpreted in light of published experimental and numerical simulation studies on the Lonar basalt, reveal that the shock pressure in the Lonar crater rim was less than 0.5 GPa but greater than 0.2 GPa. This shock pressure was high enough to produce fractures but too low to affect the magnetic fabrics. These results give new information on the relationship between shock pressure and resulting microfractures.

Variation in Magnetic Fabrics at Low Shock Pressure Due to Experimental Impact Cratering

“…AMS of magnetic fabric is already an established technique for understanding impact processes, especially at low peak shock pressures (0.5 to 3 GPa), at which other common shock indicators are rare (e.g., Agarwal et al, ; Agarwal, Kontny, Srivastava, & Greiling, ; Misra et al, ; Stöffler et al, ). The present study concludes that folding and kinking of biotite due to shock deformation may cause a significant reorientation of magnetic fabrics by passively changing the position of magnetite grains with respect to each other.…”

“…However, present results disagree with Nishioka et al (2007), who report alignment of K1 at 0.5-3 GPa in Ti-magnetite bearing andesitic basalt from Lonar crater, India. This disagreement may be due to the fact that these basalts possess both normal and inverse magnetic fabrics (Agarwal, Kontny, Srivastava, & Greiling, 2016). Gattacceca et al (2007) attribute alignment of K3 to fracturing and/or dislocations of the ferromagnetic grains.…”

“…Based on AMS data, Misra et al (2010) suggested an oblique impact at Lonar crater, India. Furthermore, AMS investigations have been used to reveal the peak shock pressures experienced by the target crystalline rocks of the Lockne impact structure, Sweden, and Lonar impact structure, India (Agarwal et al, 2015;Agarwal, Kontny, Srivastava, & Greiling, 2016). The necessity to understand carrier minerals is a general prerequisite for interpreting AMS fabrics and has been known long before these impact-related studies (e.g., Borradaile & Henry, 1997;Tarling & Hrouda, 1993).…”

Alternating augite‐plagioclase wedges in basement dolerites of Lockne impact structure, Sweden: A new shock wave‐induced deformation feature

“…; Agarwal et al. , ). Geometrically similar microstructures, such as the MDL and symplectite intergrowth, formed during the metamorphic and magmatic processes, inflict ambiguity in the identification of shock wave‐induced deformation features (French and Koeberl ).…”

“…Propagation of the shock waves commonly leads to the formation of planar and nonplanar fractures (Robertson et al 1968;Engelhardt and Bertsch 1969;Kieffer 1971;St€ offler and Langenhorst 1994;French et al 1997French et al , 2004Arakawa et al 2000;Agarwal et al 2015aAgarwal et al , 2015b. Geometrically similar microstructures, such as the MDL and symplectite intergrowth, formed during the metamorphic and magmatic processes, inflict ambiguity in the identification of shock wave-induced deformation features (French and Koeberl 2010).…”