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Evidence from Lonar Crater, India P JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, B12402, doi:10.1029/2005JB003662, 2005 Structural effects of meteorite impact on basalt: Evidence from Lonar crater, India P. Senthil Kumar National Geophysical Research Institute, Hyderabad, India Received 31 January 2005; revised 17 August 2005; accepted 26 August 2005; published 8 December 2005. [1] Lonar crater is a simple, bowl-shaped, near-circular impact crater in the 65 Myr old Deccan Volcanic Province in India. As Lonar crater is a rare terrestrial crater formed entirely in basalt, it provides an excellent opportunity to study the impact deformation in target basalt, which is common on the surfaces of other terrestrial planets and their satellites. The present study aims at documenting the impact deformational structures in the massive basalt well exposed on the upper crater wall, where the basalt shows upward turning of the flow sequence, resulting in a circular deformation pattern. Three fracture systems (radial, concentric, and conical fractures) are exposed on the inner crater wall. On the fracture planes, plumose structures are common. Uplift and tilting of the basalt sequence and formation of the fractures inside the crater are clearly related to the impact event and are different from the preimpact structures such as cooling-related columnar joints and fractures of possible tectonic origin, which are observed outside the crater. Slumping is common throughout the inner wall, and listric faulting displaces the flows in the northeastern inner wall. The impact structures of Lonar crater are broadly similar to those at other simple terrestrial craters in granites and clastic sedimentary rocks and even small-scale experimental craters formed in gabbro targets. As Lonar crater is similar to the strength-controlled laboratory craters, impact parameters could be modeled for this crater, provided maximum depth of fracture formation would be known. Citation: Kumar, P. S. (2005), Structural effects of meteorite impact on basalt: Evidence from Lonar crater, India, J. Geophys. Res., 110, B12402, doi:10.1029/2005JB003662. 1. Introduction impact cratering is important, as it addresses how the outer layer of Earth has been modified due to impacts, [2] Impact cratering is one of the important geological and also its effect on the physical, chemical and biolog- processes that have modified the surfaces of all planets and ical systems. The hazard of near-Earth asteroid or comet satellites of our Solar System [Melosh, 1989]. The impact impacts on Earth is to be assessed very carefully [e.g., event produces craters of various sizes and shapes; based on Chapman, 2004]. Impact cratering processes are under- geometry, the craters are classified into simple and complex stood by integrating impact geological information, dril- craters. Simple craters are circular, bowl-shaped depressions ling results, geophysical investigations, laboratory with raised rims and quasi-parabolic profiles, whereas experiments and numerical modeling studies. Of these, complex craters exhibit central structural uplifts, rim syn- numerical modeling has been found to be a powerful tool clines and outer concentric zones of normal faulting. An in understanding the various stages of formation of impact-cratering event may be indicated by geomorphology impact craters [e.g., O’Keefe and Ahrens, 1993, 1999; (crater shape) but is recognized on the basis of shock Melosh and Ivanov, 1999; Wu¨nnemann and Ivanov, 2003; metamorphic signatures in the target rocks [e.g., Melosh, Collins et al., 2004]. Collins et al. [2004] modeled the 1989; Koeberl, 1997] and characteristics of the impact melt impact deformation around a 10-km-diameter crater, products [e.g., Dressler and Reimold, 2001] and the ejecta whose target composition was assumed to be granite. deposits [e.g., Melosh, 1989]. They showed that the stresses, strains, and strain rates [3] Earth has witnessed impact events throughout its geo- are all highest near the impact site and decrease with logical history [e.g., Frey, 1980; Glikson, 1993; Montanari et radial distance. However, as pointed out by these authors, al., 1998]. More than 170 impact structures have been it is very important to validate numerical models based recognized so far, and are listed in the ‘‘Earth Impact on the observational evidence, especially using the quan- Database’’ of Planetary and Space Science Centre, University titative analysis of damage and deformation in the target of New Brunswick, Canada (http://www.unb.ca/passc/ surrounding a natural impact crater. Therefore structural ImpactDatabase/essay.html). An understanding of terrestrial geological studies, which include identification of various impact deformational features, their geometries, kinemat- Copyright 2005 by the American Geophysical Union. ics, and quantification of strain, are required for modeling 0148-0227/05/2005JB003662$09.00 the impact cratering processes. B12402 1of10 B12402 KUMAR: IMPACT STRUCTURES IN LONAR CRATER B12402 Figure 1. (a) Geological sketch map of Lonar crater. Boundary of the ejecta is drawn on the basis of the work by Ghosh [2003]. A massive basalt flow is well exposed on the upper crater wall, and the underlying flows are covered by basalt debris, soil, and vegetation. Impact melt fragments and spherules are found in the clayey alteration products of the ejecta materials, and one such occurrence is shown in the map. (b) A sketch map of normal faulting in the northeastern part of the inner wall. [4] Although impact structures have been studied exten- [1980]. Lonar crater is a simple, bowl-shaped crater with sively on Earth [e.g., Ackermann et al., 1975; Grieve and a rim-to-rim diameter of 1.8 km, and a rim-to-floor depth Robertson, 1976; Brandt and Reimold, 1995; Dressler and of 150 m (apparent depth). A continuous blanket of ejecta Sharpton, 1997; Reimold et al., 1998a; Bjørnerud, 1998; extends outward to a distance of 1350 m from the rim crest, Christeson et al., 2001; Lemieux et al., 2003; Sagy et al., and contains basalt fragments of various sizes and shapes, 2004], there is no detailed study available on the structural clays, and glassy impact melt fragments and spherules at a effects of meteorite impact on basalt, forming a simple number of places. The slope of the ejecta blanket is 2°–6° impact crater, like Lonar (Figure 1). As Lonar crater is one away from the crater. Maximum elevation of the crater rim is of the two known rare natural craters formed entirely in 590–600 m above mean sea level, and the elevation of the basalt, the study of its structural geology would throw light central crater floor is 470 m. A shallow alkaline lake on the impact deformation processes that takes place on occupies the crater floor. Drilling carried out in the crater terrestrial planets and their satellites. Therefore, in the floor revealed a 100 m thick lensoidal deposit of crater present study, the structural geology of basalts exposed sediments underlain by intensely fractured basaltic rocks both inside and outside Lonar crater is documented, in [Fredriksson et al., 1973]. The depth of the true crater floor order to resolve the precratering, syncratering, and post- may be >400 m below the rim crest, as calculated from the cratering structures. drilling results. Basalt breccia recovered in the drill cores shows evidence for postimpact hydrothermal alteration [e.g., Hagerty and Newsom, 2003]. 2. Geology [6] Five individual flows of massive, vesicular and [5] Lonar crater was formed in Deccan basalts (Figure 1), amygdaloidal basalts are exposed on the inner wall. The at 19°580N, 76°310E, near Lonar village in Buldhana district basalts are quartz-normative tholeiites of high total iron and of Maharashtra State in India. The 65 Myr old Deccan CaO, and lower Al2O3 and MgO, similar to the composi- basalts overlie the Precambrian rock formations and tions of basaltic Martian meteorites [Hagerty and Newsom, Paleozoic-Mesozoic sedimentary rocks of the southern 2003]. Although Lonar crater was initially thought to be Indian Shield. The regional geology of the Deccan basalts volcanic in origin, La Fond and Dietz [1964] demonstrated has been reviewed by Subbarao [1999], and the geological its impact origin based on geomorphology and documenting setting of Lonar crater was discussed by Fudali et al. shock effects [see also Nayak, 1972; Fredriksson et al., 2of10 B12402 KUMAR: IMPACT STRUCTURES IN LONAR CRATER B12402 dip steeply and have multiple orientations, resulting in circular pole density contours (Figure 2b). The deeply altered underlying flows are rich in laterites and clays, in which columnar joints are absent. [8] Massive basalt without columnar joints is also present but is comparatively rarer. It shows fractures, which are related to tectonic deformation (Figures 2e, 2f, and 2g). Most of them are subvertical and are oriented in NW-SE to NNW-SSE directions, and a few other fractures strike in NE-SW direction. Some fractures occur as conjugate pairs. Although the exact origin of these fractures is unknown as of now, they are broadly similar to the major fracture systems found in other parts of the Deccan Volcanic Province [e.g., Widdowson and Mitchell, 1999]. [9] Thickness and nature of the basaltic sequence in Lonar is unknown. Drilling studies at the 1993 Latur earthquake site, which is located 150 km south of Lonar, suggested that the thickness of the basalt sequence is 350 m, and is underlain by Archaean granites and gneisses [Gupta et al., 1999], which are the common rock types in the southern Indian Shield. In Lonar, the Figure 2. Preimpact structures observed in massive basalt basalt sequence may be thicker, as several studies have outside of Lonar crater. (a) Equal-area plots of poles to the indicated a northward increase of its thickness [e.g., orientation of columnar joints. (b) Pole density contours of Mitchell and Widdowson, 1991]. More geophysical inves- columnar joint orientations, (c) Length-weighted rose tigations and drilling studies are required at Lonar and in diagram for columnar joint orientations.
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