The probable direction of impact at Dhala impact structure, India deciphered from microfracture intensity and X-ray diffractometry: a new potential impact direction indicator

Abstract

The most widely used method of determining impact direction employs asymmetric ejecta distribution around the crater. However, the active terrestrial landscape seldom preserves the pristine ejecta blanket, making it challenging for this analysis to be carried out. The deeply eroded Dhala impact structure, formed during the Proterozoic, is devoid of an ejecta blanket. We, therefore, utilize the variation in the full width at half maxima (FWHM) of the quartz (100) peak in X-ray diffraction (XRD) spectra and the P₁₀ microfracture intensity in the monomict breccia to estimate the probable downrange direction of the Dhala impact structure. The monomict breccia rocks of the Dhala impact structure have experienced low shock pressures (< 10 GPa) and are highly fractured, making them the ideal target lithology for our study. Previous studies have used XRD extensively for strain analysis in synthetic materials and rocks. Microfracture intensity acts as an indicator for the degree of fracturing or brittle damage in the rocks, with the maximum shock-induced damage being concentrated in the downrange direction. The results from the XRD are consistent with the microfracture intensity analyses and indicate that the probable direction of impact was from southwest to northeast, with northeast being the downrange direction. Furthermore, we suggest that the degree of fracturing and X-ray diffractometry can be used to identify the downrange direction of an impact crater.

Graphical Abstract

Introduction

For planetary impacts, the obliquity and direction of impact play a pivotal role in defining the shape of the crater and the central uplift, the distribution of ejecta, the size of the crater, the final state of the projectile, the decay of shock pressure, the amount of shock melting and the ejection of matter from planetary surfaces to name a few (Gault and Wedekind 1978; Pierazzo and Melosh 2000a; Kenkmann et al. 2020; Sugandhi and Agarwal 2022; Sugandhi et al. 2024). Hence, characterizing the direction of impact is a crucial aspect of the study of impact craters. Theoretical studies have established that oblique impacts are more likely than vertical ones (Gilbert 1893; Pierazzo and Melosh 2000a).

Actual progress in understanding the effect of obliquity, i.e., changes in parameters with respect to uprange and downrange direction, has been made possible with recent advancements in numerical and 3D simulations and with the feasibility of carrying out hypervelocity impact experiments (Burcheil and Mackay 1998; Pierazzo and Melosh 1999, 2000a, b; Dahl and Schultz 2001; Heineck et al. 2002; Anderson et al. 2003, 2004; Elbeshausen et al. 2009; Davison et al. 2011; Shuvalov 2011; Michikami et al. 2017; Collins et al. 2020; Davison and Collins 2022). The easiest identifier of the direction of impact is the distribution of ejecta, for example, the bi-lateral symmetric ejecta distribution around a crater, with ejecta extending further in the downrange direction in low-oblique impacts, and concentrated crossrange in highly oblique impacts (Gault and Wedekind 1978). However, the active geologic landscape of the Earth poses a major challenge to the identification of terrestrial oblique impact craters as the ejecta blanket gets eroded over time. The scope of identifying the direction of impact by the crater shape is low, as despite oblique impacts being more likely, most of the documented planetary craters are circular in nature (Pierazzo and Melosh 2000a; Elbeshausen et al. 2013). Only for highly oblique impacts (impact angle < 15°), the crater is elongated in the direction of impact (Gault and Wedekind 1978; Bottke et al. 2000). This necessitates other tools.

During the initial contact of a projectile with the ground, shock waves are generated which move outward from the point of impact. The propagation of shock waves through a medium causes an irreversible deformation known as shock metamorphism (Ahrens and Rosenberg 1968; Stöffler et al. 1975; Langenhorst et al. 1992; Stöffler and Langenhorst 1994). Fractures associated with the impact form at different stages during the propagation of the shock waves (Kenkmann et al. 2014). These can generally be classified into (i) radial, (ii) concentric, and (iii) spall fractures (Field 1971; Ahrens and Rubin 1993). While the radial fractures develop during the compressive phase of the shock wave, the concentric fractures develop during the release phase (Agarwal et al. 2015, 2016, 2017). Numerical simulations carried out to study oblique impacts indicate that most of the shock pressure is concentrated in the downrange direction (Pierazzo and Melosh 2000a). These results are backed up by cratering experiments which show that the magnitude of peak stress and the shock-induced damage is higher in the downrange as compared to the uprange direction (Schultz and Anderson 1996; Dahl and Schultz 2001; Ai and Ahrens 2005).

X-ray diffractometry (XRD) is extensively used to determine material properties like crystal structure, crystallite size, and lattice strain. The nature of the lattice strain is determined by the shifting of a peak, characterized by a change in the 2θ angle (peak position), and the broadening of a peak, determined by variation in the full width at half maxima (FWHM) (Williamson and Hall 1953; Ungár 2004). FWHM is generally used as an indicator of strain with higher FWHM indicating higher strain in rocks (Williamson and Hall 1953; Nasiri-Tabrizi 2014). Peak shifting indicates uniform strain whereas peak broadening indicates non-uniform strain. In synthetic materials, XRD has been extensively used not just to study peak shifting and broadening but also for the calculation of particle size and strain (Khorsand Zak et al. 2012; Thandavan et al. 2015; Kibasomba et al. 2018; Wu et al. 2019). Though rare, XRD has been used to carry out similar studies in natural rocks and minerals as well (Reznik et al. 2016; Agarwal and Alva-Valdivia 2019; Kumar et al. 2023). Kumar et al. (2023) reported an increase in the FWHM of the quartz (011) peak with increasing strain rate and Reznik et al. (2016) documented an increase in FWHM in magnetite with increasing shock pressure, at low shock pressures.

The Dhala impact structure is more than 1.7 Ga old and is a deeply eroded structure (Pati et al. 2008). The lack of a preserved ejecta blanket and the presence of syn- to postimpact sediments over a significant part of the structure makes it difficult to employ the conventional methods of identification of oblique impact craters. Hence, we turn to the two aforementioned laboratory techniques (degree of fracturing and XRD) to look for evidence of an oblique impact and to estimate the probable direction of the impact.

Geologic setting

Located nearly 50 km west of Jhansi city, the Dhala impact structure (Fig. 1) is almost 11 km in diameter (Fig. 1) and occupies an area of nearly 64 km² (Pati et al. 2008, 2019). Initially thought of as a “crypto-volcanic explosion” structure (Basu 1986), the presence of shock planar deformation features (Pati et al. 2008) proved the meteoritic impact origin of the Dhala structure. The Bundelkhand craton forms the target basement of the impact structure. The Bundelkhand craton is one of the five Archean cratons of the Indian subcontinent. With an estimated areal extent of almost 29,000 km², the Bundelkhand craton is dominantly composed of the older tonalite–trondhjemite–granodiorite (TTG) gneisses intruded by younger granitoids and granodiorites (Pati et al. 2019; Deb and Bhattacharyya 2022). In addition, meta-sediments of Banded Iron Formations, felsic volcanics, calc–silicate rocks, amphibolites, corundum-bearing schists, and quartzites are also present (Mondal et al. 2002; Malviya et al. 2006; Pati et al. 2010; Saha et al. 2011). Collectively, they are known as the Bundelkhand Granitoid Complex. Three tectonic events between 3.3 and 2.4 Ga have deformed the TTG gneisses and the meta-sediments. The younger granitoids are further classified as medium-to-fine-grained and coarse-grained. The darker medium-to-fine-grained granitoids are composed of alkali feldspar, quartz, plagioclase, muscovite, hornblende, biotite, zircon, magnetite, chlorite, and epidote. The porphyritic coarse-grained granitoids are dominantly composed of quartz, plagioclase, alkali feldspar, hornblende, biotite, and chlorite. NNE–SSW trending giant quartz veins crosscut by NW–SE trending tholeiitic mafic dike swarms are also present throughout the craton (Pati et al. 2010).

Fig. 1

Geologic map of the Dhala impact structure showing major lithologies and sample locations (modified after Singh et al. 2021)

At Dhala, the impactite lithologies consist of suevites, pseudotachylitic breccias, monomict breccias, and impact melt rocks. The bottommost layer, suevite is composed of lithic and melt clasts imbedded in a fine-grained matrix composed of shocked lithic and mineral clasts and glassy or crystalline impact melt (Pati et al. 2019). The pseudotachylite breccias (PTBs) occur as veins in the host granitoids. Within the PTBs, the lithic clasts are more abundant than the mineral clasts and the matrix of the PTBs can be extremely fine-grained, rich in phyllosilicates (chlorite, biotite, and sericite), or may contain melt components. The PTBs are further classified into the light grey colored cataclastic PTBs and the dark grey to black colored melt-bearing PTBs. The clasts of cataclastic PTBs are highly angular and do not show significant alteration whereas the melt-bearing PTBs are highly altered (Pati et al. 2015). Impact melt rocks are reddish orange in color with elliptical vesicles and amygdales on their surface. They are composed of clasts containing feldspars, quartz, biotite, magnetite, ilmenite, and zircons and indicate significant post-impact hydrothermal alterations (Pati et al. 2010, 2019; Joshi et al. 2023). The presence of planar deformation features and feather features in shocked quartz grains indicate shock pressures of ~ 20–25 GPa, while the presence of zircon grains and diaplectic glass indicate shock pressures as high as 60 GPa (Pati et al. 2019). Thus, the impact melt rocks likely experienced shock pressures between 20 and 60 GPa.

Monomict breccia is generally autochthonous and forms during the excavation stage of crater formation in the proximal ejecta blanket close to the wall of the transient cavity or the fractured basement rocks of the crater (Kenkmann et al. 2014). In Dhala, the reddish-brown, whaleback-like elevated outcrops of monomict breccia consist of extensively fractured and brecciated coarse-to-medium grained granitoid and form the outermost ring of the Dhala structure. They are generally composed of clasts of K-feldspar and quartz in a feldspar-rich matrix; the clasts are pervaded by trans-granular microfractures filled with impact melt. Pati et al. (2019) reported a shatter cone in the northeastern outcrops of monomict breccia indicating shock pressure less than 10 GPa.

The impactite lithologies were overlain by rocks of the Dhala formation, which are stratigraphically equivalent to the Semri Group of the Vindhyan supergroup (Pati et al. 2019). The Dhala formation is a layer of post-impactite sediments composed of sandstones, siltstones, shales, and conglomerates. These sediments are dominantly composed of poorly sorted, angular clasts of quartz, feldspars, biotite, and sericite in a feldspar and sericite-rich matrix (Agarwal et al. 2020). The central elevated area (CEA) is a ~ 418 m tall mesa-like structure that unconformably overlies the rocks of the Dhala formation. It is almost 5 km² in area and is composed of Sumen sandstone of the Kaimur group in the Vindhyan supergroup. The presence of a scarce amount of shocked quartz grains in the CEA indicates the reworking and deposition of the ejecta blanket by postimpact processes to form the CEA (Agarwal et al. 2020). The age of the Vindhyan supergroup has been calculated as 1.7 ± 0.11 Ga., by the Pb–Pb whole-rock technique (Sarangi et al. 2004). This also serves as the minimum age limit for the Dhala impact event.

Methodology

For this study, we collected thirty hand samples of monomict breccia oriented in the field with a magnetic compass. Oriented thin sections were prepared from each hand sample for X-ray diffraction (XRD) analysis and microfracture intensity study. The thin sections were analyzed and photographed under plane and cross-polarized light using an optical scanning microscope, Leica DM4. For each of these thin sections, the P₁₀ microfracture intensity was calculated. P₁₀ is one of the methods within the P_ij system of microfracture intensity measurements (Dershowitz and Herda 1993), commonly used by the Discrete Fracture Network modeling community (Rogers et al. 2017; Tonkins and Coggan 2017; Lei et al. 2017). P₁₀ is a linear measure of fracture intensity. It is given as the number of fractures per unit length of scan lines. We calculated the number of intersections between the microfractures and scan lines and the total length of the scan lines within the clasts (Supp. Table 1). The intensity was calculated as

$${\text{P}}10\left( {{\text{mm}}^{{ - 1}} } \right) = \frac{{Number{\mkern 1mu} \,of{\mkern 1mu} \,inter\,sections{\mkern 1mu} \,between{\mkern 1mu} \,microfractures{\mkern 1mu} \,and{\mkern 1mu} \,scan - lines}}{{Total{\mkern 1mu} \,length{\mkern 1mu} \,of{\mkern 1mu} \,the\,scan - lines{\mkern 1mu} \,within{\mkern 1mu} \,the{\mkern 1mu} \,clasts(mm)}} $$

The scan lines required in this method were constructed in the ArcGIS Pro desktop software. A mesh of two mutually perpendicular sets of scan lines, with 1 mm line spacing, was constructed over the entire thin section image (Supp. Figure 1). The microfracture intensity was calculated for all the thin sections and the variation in the microfracture intensity values with the azimuth was plotted. Only fractures present within the clasts were considered.

The XRD analysis of the samples was carried out with a PANalytical X’Pert Pro diffractometer housed in the Advanced Center for Material Sciences at the Indian Institute of Technology, Kanpur. The 2θ angle was varied between 5° and 70° at an angular speed of 0.1°/sec to generate the XRD spectra, which were then analyzed in the Origin Pro software. To calculate the full width at half maxima (FWHM) and the peak position (2θ) a Gaussian curve (Williamson and Hall 1953) was fitted on each peak in the XRD spectra (Supp. Figure 2). With the help of the X’Pert Highscore Plus 3.0 software, the 2θ values were used to identify the lattice planes of the minerals.

The application of the proposed method warrants certain precautions. First, it is only effective in impactite lithologies that have experienced shock pressures < 20 GPa. At shock pressures over 20 GPa, the formation of fractures is no longer the preferred mechanism of deformation, and localized amorphization of rocks takes place (Kenkmann et al. 2014). Low shock pressure regions can be identified in the field by the presence of shatter cones (~ 1–10 GPa), and under a microscope by the presence of planar fractures (~ 5–10 GPa), feather features, and planar deformation features (~ 5–35 GPa) within the mineral grains (Kenkmann et al. 2014). Second, the collected rock samples should be almost equidistant from the estimated point of impact. With increasing distance from the point of impact, the amount of brittle deformation (Buhl et al. 2013b, a, 2014) as well as the FWHM of the peaks in the XRD spectra (Agarwal and Alva-Valdivia 2019) decreases. Third, the rock should be autochthonous or parautochthonous. Rocks that have suffered significant displacement may not be true indicators of the direction and distance with respect to the estimated point of impact. Fourth, the pre- and post-impact tectonic activity in the study area should be well-characterized. Tectonic activity can introduce new fractures as well as strain in the samples leading to an overestimation in the fracturing intensity and FWHM values and fifth, the samples should be evenly distributed around the estimated point of impact. Missing samples would lead to gaps in our data which might lead to errors in the estimation of the direction of impact.

To appreciate the variation in microfracture intensity and XRD data around the impact structure, the bearing of the sample locations with respect to a reference point was needed. We fitted a circle through the maximum outer extents of the monomict breccia (Supp. Figure 3). The center of this circle was regarded as the probable point of impact. The angle subtended from the North (in degrees), by each sample location on this estimated point of impact was calculated.

Results

Petrographic analysis

The monomict breccia are highly fractured, with angular clasts, which are surrounded by a very fine-grained matrix or by glassy textured impact melt (Fig. 2d). The impact melt also fills some of the microfractures (Fig. 2e). Feldspars, namely plagioclase and orthoclase and quartz are the dominant minerals both within the clasts and the fine-grained matrix. The size of the quartz grains can range from very fine-grained to almost 1500 µm, while the feldspar grains can be as large as 1800 µm. The modal percentage ratio of feldspars to quartz within the clasts varies between 60:30 and 40:50, while minerals such as chlorite, biotite, zircon, apatite, rutile, and opaque minerals make up the remaining 10% of the composition (Fig. 2). Within the clasts, most of the mineral grains have a euhedral-to-subhedral shape. Pre-impact tectonic deformation at high P–T conditions is realized as undulose extinction and serrated grain boundary in quartz grains (Fig. 2g). There are three stages of pre-impact deformation reported from the area (Prasad et al. 1999; Bhatt and Singh 2011; Deb and Bhattacharyya 2022). Post-impact hydrothermal alteration in some samples (Pati et al. 2008; Singh et al. 2021) is evidenced by sericitization of the plagioclase feldspar grains (Fig. 2h).

Fig. 2

a Oriented block of monomict breccia (H28) collected from the field. b, c Outcrops of monomict breccia in the field. A thin section under plane-polarized light showing d different clasts embedded in the finer matrix and e melt filling the space between clasts and f multiple microfractures within the clasts. A thin section under cross-polarized light showing g pre-impact undulose extinction and serrated grain boundaries in quartz and h sericitization in K-feldspar. i Cross-cutting relationships showing the concentric fractures terminating against the radial fractures. c–g White arrow within the black inset at the top-right points towards the estimated point of impact

Microfracture intensity

Most of the clasts are pervaded by high aspect-ratio opening-mode microfractures having lengths between 1 and 10 mm and apertures between 0.01 and 0.04 mm. The traces of the microfractures are generally curvilinear or straight and they either trend radially outwards from the point of impact, parallel to the crater rim, i.e., concentric, or have a random orientation. Based on the orientation of the thin sections, most of the microfractures can be classified as radial or concentric fractures. The cross-cutting relationships indicate that the radial fractures formed earlier than the concentric fractures (Fig. 2i). In the investigated fifteen samples, the microfracture intensity varies between 1.23 and 0.17 mm⁻¹ (Fig. 3a). It averages at 0.64 mm⁻¹ with and standard deviation of 0.29 mm⁻¹. The intensity is lowest in sample no. N12 and highest in sample no. N45 (Fig. 3a, Supp. table 1). In general, the microfracture intensity is highest in samples from ENE and lower in samples collected from other parts of the impact structure (Fig. 3a).

Fig. 3

Variation in the a P₁₀ microfracture intensity, b FWHM of quartz (100) peak, c FWHM of quartz (101) peak around the estimated point of impact. The x-axis shows the bearing of the sample locations from the estimated point of impact

XRD analysis

The X-ray diffraction spectra of the samples are dominated by signatures of the lattice planes of quartz (Supp. Table 2). Multiple lattice planes of quartz including (100), (101), (211), (212), (110), (102), (200), (201), (112), and (202) have been identified in the samples. Not all these planes have a high intensity or can be identified relatively easily in the XRD plots of all thirty samples (Supp. Figure 4). Thus, the variation in FWHM of only quartz (100) peak, identified in 21 samples, and quartz (101) peak, identified in 27 samples were considered.

The FWHM value of quartz (100) peak varies between 0.02° and 0.14° and it averages at 0.07° with and standard deviation of 0.03°. The intensity is lowest in sample no N40 and highest in sample no. N47 (Fig. 3b, Supp. Table 2). The FWHM value of quartz (101) peak varies between 0.03° and 0.13° (Fig. 3c). It averages at 0.05° with and standard deviation of 0.02°. The intensity is lowest in sample no. N27 and highest in sample no. N47 (Fig. 3c, Supp. Table 2). For the quartz (100), peak the highest FWHM values are observed in samples collected from ENE (~ 070°), while the FWHM of the quartz (101) peak does not show any trend (Fig. 3b, c).

Discussion

The highest intensity fracturing, P₁₀ values, are recorded in samples from the ENE, ~ 060° (Fig. 3a). The brittle deformation at the microscopic scale is, therefore, highest in the ENE. On moving away from this direction, the intensities of fracturing and, thus, the brittle deformation decreases. More intense deformation in ENE may have been caused by a stronger shock in this direction. Concurring trends are observed in the XRD spectra with higher FWHM values of Quartz (100) peak towards the ENE (~ 70°) from the probable point of impact.

However, we do not see any discernible trend in the variation of the FWHM values of quartz (101) peak (Fig. 3c). It is possible that the deformation intensity or mechanism varies across the different lattice planes of quartz as previous studies have indicated that planar deformation features (PDFs) and planar fractures in shocked quartz have a preferential distribution along certain lattice planes (Goltrant et al. 1992; Stöffler et al. 2017). This has been observed in the Dhala impact structure as well (Pati et al. 2019).

The Bundelkhand granitoid and the Dhala impact structure have a significant post-impact geologic history, including the emplacement of the Dhala formation and Sumen sandstone, three distinct phases of hydrothermal activity, and the intrusion of mafic dykes (Pati et al. 2015, 2019; Agarwal et al. 2020; Singh et al. 2021; Joshi et al. 2023). Furthermore, evidence of post-impact fracturing and fluvial and glacial activity has also been reported from the area (Singh et al. 2021). While this may have affected our estimation of the P₁₀ fracturing intensity, most of the measured microfractures could be classified as radial and concentric, thus indicating their impact origin (Fig. 2). Furthermore, although it has been established with relative certainty that higher degrees of fracturing can cause enhanced rates of weathering (Molnar et al. 2007; Roy et al. 2015, 2016; Duvall et al. 2020), the converse is not so well-established. Thus, it is not possible to separate out the effect of weathering and hydrothermal activity from our estimated fracture intensity values. In addition, necessary precautions were taken during sampling to avoid collecting weathered samples.

Notably, a trend similar to that of the fracture intensity values is also indicated by the FWHM values of Quartz (100) peaks. Quartz is highly resistant to both mechanical and physical weathering (Gerrard 1988; Nesse 2017), thus its XRD peaks are unlikely to be significantly affected by weathering processes. While quartz is still susceptible to hydrothermal alterations (Monecke et al. 2002), the samples selected for this study were not collected from the alteration halo, which, at Dhala is located in the immediate proximity of the Giant Quartz Veins. Thus, we argue that the effect of hydrothermal alteration on the fracture intensity and quartz FWHM was minimal. This argument is supported by the presence of unaltered grains of quartz and magnetite (highly susceptible to alteration) in some of the collected samples (Supp. Figure 6).

Several authors have previously attempted to evaluate the direction as well as the angle of impact in terrestrial craters from the crater morphology by integrating experimental, field, and remote sensing observations (Schultz and Anderson 1996; Schultz and D’Hondt 1996; Shoemaker and Shoemaker 1996; Ekholm and Melosh 2001; Stöffler et al. 2002; Herrick and Forsberg-Taylor 2003; Kenkmann et al. 2005; Lindström et al. 2005; Tsikalas 2005; Wallis et al. 2005; Poelchau and Kenkmann 2008). These methods are, however, difficult to apply to the deeply eroded Dhala impact structure whose morphology is not preserved and is also overlain by post-impact sedimentary layers. In this study, we propose a new method which integrates field and micro-scale observations from the impactite lithologies with previous experimental studies to estimate the direction of impact. The accuracy of this method cannot be commented upon as of right now as there are no other studies estimating the direction of impact at the Dhala impact structure. However, we propose that this method, which is yet to be verified, can be applied to various pristine and eroded craters meeting the criteria laid down in the Methodology section.

3-D numerical models for oblique impacts indicate that although the propagation of shock waves in the target rock is symmetric, the strength of the shock waves is asymmetric with the strongest waves concentrated in the downrange direction (Pierazzo and Melosh 1999, 2000a). These results have been backed up by cratering experiments which report higher shock-induced damage beneath craters in the downrange as compared to the uprange direction (Ai and Ahrens 2005). Furthermore, experiments measuring the stress wave asymmetry in oblique impacts indicate that in target rocks downrange, the peak stress is almost twice the peak stress in the uprange direction (Dahl and Schultz 2001). This higher peak stress also suggests a higher damage in the downrange direction.

Conclusions

In this study, we have calculated the FWHM of quartz and microfracture intensity in the monomict breccia of the Dhala impact structure. While the FWHM is an indicator of lattice strain, the microfracture intensity has a positive correlation with the shock-induced brittle damage. Thus, we propose that fracturing intensity and FWHM of a mineral peak from XRD data can be used as a viable tool to understand the distribution of damage around an impact crater. Since, in inclined impacts, higher damage is focused in the downrange, we suggest that fracturing intensity and FWHM can be used to identify the downrange direction of a crater. In our study, the monomict breccia outcrops located towards the northeast from the calculated point of impact show the highest values of fracturing intensity as well as FWHM of quartz. Hence, we suggest that northeast is the probable downrange and the direction of impact at the Dhala impact structure was possibly from southwest to northeast.