Anisotropy of magnetic susceptibility (AMS) of impact melt breccia and target rocks from the Dhala impact structure, India

Abstract

The ~11-km-wide, Paleoproterozoic Dhala impact structure in north-central India comprises voluminous exposures of impact melt breccia. These outcrops are discontinuously spread over a length of ~6 km in a semicircular pattern along the northern, inner limit of the monomict breccia ring around the central elevated area. This study of the magnetic fabrics of impact breccias and target rocks from the Dhala impact structure identified a weak preferred magnetic orientation for pre-impact crystalline target rocks. The pre- and synimpact rocks from Dhala have magnetite and ilmenite as common magnetic phases. The distributions of magnetic vectors are random for most impact melt breccia samples, but some do indicate a preferred orientation. Our anisotropy of magnetic susceptibility (AMS) data demonstrate that the shape of susceptibility ellipsoids for the target rocks varies from prolate to oblate, and most impact melt breccia samples display both shapes, with a slight bias toward the oblate geometry. The average value for the corrected degree of anisotropy of impact melt rock (P′ = 1.009) is lower than that for the target rocks (P′ = 1.091). The present study also shows that both impact melt breccia and target rock samples of the Dhala structure have undergone minor postimpact alteration, and have similar compositions in terms of magnetic phases and high viscosity. Fine-grained iron oxide or hydroxide is the main alteration phase in impact melt rocks. Impact melt rocks gave a narrow range of mean magnetic susceptibility (Km) and P′ values, in contrast to the target rock samples, which gave Km = 0.05–12.9 × 10−3 standard international units (SI) and P′ = 1.036–1.283. This suggests similar viscosity of the source magma, and limited difference in the degrees of recorded deformation. Between Pagra and Maniar villages, the Km value of impact melt breccias gradually decreases in a clockwise direction, with a maximum value observed near Pagra (Km = 1.67 × 10−3 SI). The poor grouping of magnetic fabrics for most impact melt rock samples implies local turbulence in rapidly cooled impact melt at the front of the melt flow immediately after the impact. The mean K1 for most impact melt samples suggests subhorizontal (<5°) flow in various directions. The average value of Km for the target rocks (4.41 × 10−3 SI) is much higher compared to the value for melt breccias (1.09 × 10−3 SI). The results of this study suggest that the melt breccias were likely part of a sheet-like body of sizeable extent. Our magnetic fabric data are also supported by earlier core drilling information from ~70 locations, with coring depths reaching to −500 m. Our extensive field observations combined with available widespread subsurface data imply that the impact melt sheet could have covered as much as 12 km2 in the Dhala structure, with an estimated minimum melt volume of ~2.4 km3.

On the Emplacement of the Impact Melt at the Dhala Impact Structure, India

“…In contrast, the K m value gradually decreases clockwise, starting from 1.862 at site 18.1 in the western sector to 0.682 at site 7 in the north‐eastern side (Table 1). The K m values of the impact melt rocks up to 2.5 × 10 −3 SI units in this study (Table S1 in Supporting Information ) are significantly higher than the K m of the unshocked granites, which have average values of 0.28 × 10 −3 near Bhaunti and 0.05 × 10 −3 near Maniar (Singh et al., 2021a). We suggest that the formation of Ti‐ magnetites due to impact resulted in an increase in magnetic susceptibility of the impact melt rocks.…”

“…Singh et al. (2021a) have examined the magnetic fabrics from the five‐impact melt rock outcrops, which correspond to our IM1, IM2, IM3, IM4, IM5, and IM6. They most probably considered IM4 and IM5 as one.…”

“…It is likely that a large clast population in their samples may have affected the magnetic fabrics. Unlike the present study, Singh et al (2021a) did not exclusively focus on clast poor impact melt rocks.…”

“…Although Singh et al. (2021a) investigated the flow directions in impact melt rocks, they did not find any consistent trend. Arif et al.…”

“…Magnetic fabrics investigations are a significant tool in interpreting the origin and emplacement of impact melt in different structural and tectonic settings (e.g., Cowan, 1999; Cowan et al., 1999; Eitel et al., 2016; Scott & Benn, 2001, 2002; Singh et al., 2021a; Urrutia‐Fucugauchi et al., 2012; Yokoyama et al., 2012). Moreover, magnetic fabrics studies are a very well‐established technique to determine the flow direction in volcanic and fluvial deposits (e.g., Alva‐Valdivia et al., 2017; Cañón‐Tapia et al., 1995, 1996, 1997; Guimarães et al., 2018; Liu et al., 2001; Papanna et al., 2014; Tarling & Hrouda, 1993).…”