Fracture Deformations of Impact Rock Bodies

Apart from the block faulting of the Popigai crater, numerous smaller faults and fractures, which cut separate bodies of impact rocks, were discovered during large-scale mapping of selected areas within the crater; these fractures are well-exposed on aerial photographs of areas composed of different impact lithologies, for example, allogenic megabreccia or suevites (Fig. 5, 6). The photogeologic guides of fractures and faults are lineaments of drainage pattern (Fig. 7); zones of abundant vegetation that are indicated by dark colors;

Radial dirrjclions (RD) vfe. Fault orientalon-s (FQ) FO= 161,93 - OrOan5 ' PD Comalaüon; r= -O.OQ43

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Radial dirrjclions (RD) vfe. Fault orientalon-s (FQ) FO= 161,93 - OrOan5 ' PD Comalaüon; r= -O.OQ43

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175 18C 165 190 Radial directions (degrees)

Fig. 10. Correlation plot of fault orientation vs. radial direction for all fault deformations from the Chordu-Daldyn area. The distribution of points shows a chaotic orientation of fractures.

175 18C 165 190 Radial directions (degrees)

Fig. 10. Correlation plot of fault orientation vs. radial direction for all fault deformations from the Chordu-Daldyn area. The distribution of points shows a chaotic orientation of fractures.

boundaries between lithologies differing in color and specific appearance. As a whole, near-vertical tear faults predominate, usually with minor displacements; however, sometimes heaves more than 10-20 m are observed. The brecciated zones vary in thickness from some centimeters to 20 m, and from several meters to 5 km in length.

The spatial distribution of deformation patterns is uneven; they occur mostly within areas where lithic megabreccias are of maximum thickness, such as the annular trough. As a typical area where the fracturing of impactites is widespread, the Chordu-Daldyn River area in the southern part of the crater (Fig. 1) is considered. This area is located in the midstream of the Chordu-Daldyn River; its general geology is given by Mashchak and Selivanovskaya (1988). It embraces both the annular trough and the outer deformation zone and is composed of diverse impact lithologies (Fig. 8). In the northern part of the area, a thick blanket (more than 400 m) of suevites and fine-grained lithic breccia, with numerous lens-like tagamite bodies with up to 50 m thickness,

Fig. 11. Geologic map of the Tongulakh area. All faults obtained from interpretation of aerial photographs are shown.

exists. The southern part comprises a radial trench filled by lithic allogenic megabreccia (100-200 m thick) with some tagamite bodies. The northern and southern parts are divided by the narrow zone where a chain of klippen of Archean crystalline rocks emerges through the impact lithologies, possibly fixing the deformed structural rim of the crater. Fractures range from 0.5 to

5.5 km in length. They cut through all impact lithologies; thus, they have formed after the emplacement of these rocks. Some fractures are also widespread within klippen and target rocks of the rim, and they may extend into host allogenic breccias.

The faults exhibit a wide variation in orientation and length throughout the Chordu-Daldyn area (Fig. 9). At this distance from the impact center (22-45 km) those fractures dominate that are oriented obliquely to the radial and concentric directions. A regular orientation pattern of fractures and faults have not been established, and no correlation exists between fracture orientation and radial direction from the crater center (Fig. 10). At the same time, some near-concentric conjugate fracture systems could be determined (Fig. 8).

For comparison with the Chordu-Daldyn area, the fracture pattern of impact rocks in the Tongulakh area is presented. This area is located in the northeastern part of the crater (Fig. 1). The crater rim is mainly composed of deformed Cambrian carbonate rocks, with a cover of polymict allogenic megabreccia (up to 200 thick). In the area, blocks and klippen of carbonate rocks, quartz arenites of Proterozoic age, and shocked Archean crystalline r4— 1S

Fig. 12. Orientation distribution data for all 58 fault deformations from the Tongulakh area. Data are displayed as a rose plot. Areas of radial and concentric orientations are shown.

Fig. 13. Aerial photograph of the Profilny area, showing the abundance of topographic and drainage lineaments, which trace fracture deformations. Massive impact melt rocks (tagamites) and suevites are indicated by a homogenous light-gray shade, whereas fine-grained lithic breccia is revealed by the specific patterns formed by alluvial fans.

0 1 km rocks are common. On top of the breccia, suevites form separate blankets (up to 5 km2 in area and up to 70 m thick). In contrast with the Chordu-Daldyn basin, fractures within the Tongulakh area are rare and concentrated along some discontinuous radial bands up to 3 km wide (Fig. 11), which possibly correspond to axes of radial trenches. Fractures are from 0.4 to 2.3 km in length (0.8 km on the average). Although fracture deformations vary widely in orientation, they do have preferred radial or concentric orientations (Fig. 12).

For a detailed consideration of the fracture pattern of individual impact melt bodies, mapping at a 1:16,000 scale of a local area (the so-called "Profilny" area) with an extent of 10 km2 in the southeastern sector of the crater, where fractures are prominent and widespread, has been carried out. In this area, a complex sheet-like impact melt body extends in a meridional direction within a fine-grained lithic breccia field. From both aerial photograph (Fig. 13) and geological map (Fig. 14), the impact melt body is broken into several blocks

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Fig. 14. Geologic map of the Profilny area (mapped by M.S. Mashchak and V.A. Maslov). Shaded line outlines the area covered by the aerial photograph in

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Fig. 14. Geologic map of the Profilny area (mapped by M.S. Mashchak and V.A. Maslov). Shaded line outlines the area covered by the aerial photograph in

with both vertical and lateral displacements; it is apparent that many fractures extend into the enclosing fine-grained lithic breccia. Most geological boundaries are controlled by faults. Fractures are expressed by negative topographic lineaments and are usually traced by narrow lines of larches among the treeless tundra. Locally, fault zones in tagamites and suevites up to 3 m thick with throws up to 6 m are observed in outcrops.

In a large and continuous outcrop of impact rocks on the Chordu-Daldyn River (see Fig. 3), tagamites are cut by several clastic dikes, which are from

0.6 to 3 m thick and up to 30 m in length (Fig. 15). They are composed of re-washed psammitic-aleuritic material, which represents the product of disintegration and redeposition of fine-grained lithic breccias (Mashchak and Fedorova 1987).

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