Clastbearing Dikes Formed by Infilling

Another group of dikes forms during the relatively long-term late modification stage of cratering. These dikes caused by doming of the brittle crater fill, its adjustment to slow upward movements and the development of the tension fractures in the crater floor. Open fissures and cracks in the apparent crater floor are later infilled with debris due to gravitation. This debris consists of poorly cemented clast-bearing sandy and clayey material forming the fillings of such openings, and was derived from redeposited impactites and allochthonous lithic breccia. During redeposition at the bottom of the crater lake, or fissure walls destroying the clasts are transported downward into these open fractures together with water-saturated silt and sand, and are partly rounded. The lithology of fragments usually mirrors the whole target rock diversity; representatives of all target horizons may be found. Some fragments are shocked, and admixture of glass particles and melted material may take place. Formerly

Fig. 9. Kara impact structure, Russia. The repeatedly filled sandy dike contains fragments of various lithologies. The dike, that strikes parallel to the hammer handle, dissects suevite (upper right and lower left). Southeastern sector of the structure, Kara River. Hammer for scale.
■ *m

t/ax iiriI lieiyliL Of CtiUlnl upl ifL 2-*3 liy p hu irie Lriupl if L ! H UI 2-—sliidiyiaphiL upliTL |5U|

Fig. 10. Wandering clasts: amplitudes of displacement in an impact crater. X -positions of clast, derived from the uppermost layer (dark gray): a. initial position, b. downwards displacement, c. upwards displacement. The displacements measured in normal stratigraphic section are shown on the left sides of the schemes (2', 3').

t/ax iiriI lieiyliL Of CtiUlnl upl ifL 2-*3 liy p hu irie Lriupl if L ! H UI 2-—sliidiyiaphiL upliTL |5U|

Fig. 10. Wandering clasts: amplitudes of displacement in an impact crater. X -positions of clast, derived from the uppermost layer (dark gray): a. initial position, b. downwards displacement, c. upwards displacement. The displacements measured in normal stratigraphic section are shown on the left sides of the schemes (2', 3').

some of these dikes were described as "clastic dikes" (Mashchak and Ezersky 1980, 1982, Mashchak and Fedorova 1987). They are similar to neptunic dikes, known in some geological regions.

Dikes formed by infilling of debris are found in the Kara and Ust-Kara impact craters, where they dissect the suevite sequence, occur all around and close to the crater edge (Mashchak and Ezersky 1980, 1982). In these craters such dikes (up to several meters thick), have radial and circumferential arrangements (Mashchak 1990, Fig. 8). The dikes cut the

Fig. 11. The diagram shows estimated amplitudes of downward clasts (+ melt) displacement vs. diameter of transient crater. HD = hypsometric descent of wandering clasts, Dtc = diameter of transient crater.

host suevites and split and thinned out downward, and sometimes they show repeated infilling (Fig. 9). Individual dikes have been traced by drilling to a depth of about 400 m. This observation indicates, that secondary descent of clasts during the late modification stage may be significant. Such dikes also occur in the Popigai (Mashchak and Fedorova 1987, Masaitis et al. 1998), and Ries (Chao et al. 1978 ) craters. In the Puchezh-Katunki impact structure these dikes cross-cut suevites and allogenic breccias, and sometimes penetrate the underlying brecciated parautochthonous rocks of the true crater floor (Masaitis and Pevzner 1999).

This dike type formed by late infilling is possibly related to radial and concentric patterns on the floor of some lunar craters, so called "floor-fractured" craters, e.g. Humboldt (Wilhelms et al. 1987). These patterns caused by uplift of underlying layer due to viscous relaxation after the crater formation (Melosh 1989).

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