Post Impact Tectonic Structure of the Popigai Crater

During its 35 Myr-long history, the Popigai crater has been slightly modified by denudation and deposition. The maximum depth of erosion is estimated to vary from about 100 m in the inner depression to 250-500 m at the southwestern rim of the crater. According to Plotnikova (1990), four periods of the late modification can be distinguished: (1) Miocene - Early Pliocene: denuda-

Fig. 7. Aerial photograph of a suevite field, showing well-exposed linear drainage pattern caused by faulting, in the southern part of the crater. The location of the area is shown in Fig. 1.

tion of impact rocks, (2) Pliocene-Early Quaternary: deposition of lake sediments (sands with gravel intercalations), (3) Middle and Late Quaternary: Sartan Glaciation and deposition of sands, aleurites, gravels, and clays up to 77 m thick in lake basins; (4) Holocene: formation of river valleys and alluvial deposits (up to 20 m thick). The present distribution of Pliocene and Quaternary deposits is partly controlled by block faulting (Plotnikova 1990).

The post-impact tectonic structure of the Popigai crater involves a series of fault blocks differing in extent and direction of vertical displacements (Fig. 2). The fault boundaries of blocks are traced from linear patterns of river valleys, continuous linear bluffs of impactites (Fig. 3), and linear zones where postimpact sediments are of the increased thickness. Some faults are proposed from sharp changes of the geological structure and sudden changes in thickness of impactites in nearby areas. For example, crystalline rocks of the peak ring form a chain of table-shaped hills, divided by narrow radiate-orientated depressions where the post-impact deposits are rather thick (15-40 m). Long axes of separate ranges deviate from concentric orientations fixing rotational movements of blocks in addition to translational ones.

Two groups of boundary faults can be distinguished: (1) radial and concentric pre-impact (partly revived during the cratering) and syn-impact faults, which have caused the present radial-concentric drainage pattern within the Popigai depression, and (2) northwestward and southwestward orientated postimpact faults that have been caused by regional tectonics (a rise of the Anabar Shield) during the Neogene-Quaternary period. Radial faults are sub-vertical and record predominantly strike slip movements, whereas concentric ones are high-angle faults with both center-down and center-up displacements.

Uneven erosion of different blocks causes a considerable geological difference between separated areas. Inferred from a proposed original impact stratigraphy, subsided and uplifted blocks could be distinguished, and magnitudes of displacements could be evaluated as well. The southern and western parts of the crater, which are adjacent to the Anabar Shield, as well as the western part of peak ring projections, belong to the most elevated blocks; there, impact melt rocks and authigenic breccia are exposed. The central depression and some areas of the annular trough where the thickness of lithic breccias is at a maximum, represent the most subsided blocks. The eastern and northern parts of the crater, which are composed mainly of breccia after carbonate-terrigenous rocks and dolerites, represent blocks of an intermediate position.

Fig. 8. Geologic map of the Chordu-Daldyn area (the southern part of the Popigai crater). All faults obtained from interpretation of aerial photographs are shown.

A typical radial fault zone is mapped by deep drilling in the southwestern sector of the crater (Balagan-Yuregue area, Fig. 4). There, the roof of the thick impact melt sheet is downfaulted with a vertical shift of above 100 m, so that

Fig. 9. Orientation distribution data for all 198 fault deformations from the Chordu-Daldyn area. Data are displayed as a rose plot. Areas of radial and concentric orientations are shown.

in the southeastern part of the area a continuous field of fine-grained lithic breccia was mapped. The major fault plane dips southeastward at high angle. The subsided block probably represents the uppermost part of the impact rock sequence that was removed by erosion elsewhere in the Balagan-Yuregue area.

A maximum amplitude of vertical displacements during late modification stage could be estimated from sudden variations in thickness and altitude of post-impact sediments. For example, the thickness of the Pliocene-Quaternary deposits reaches 160 m in the northwestern sector of the annular trough (Majachika Upland area), and the base is at an elevation of -100 m there. In the southern part of the crater this stratigraphic level is 220-300 m above sea level. This indicates that the block faulting continued at least up to the Holocene; the amount of Pliocene-Quaternary vertical displacements, thus, reaches several hundred meters.

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