Introduction

Rock fragments (or so called "xenoliths") are usually found within dikes inside and outside of impact craters. They are different in lithology from the country rocks of the dike walls. These fragments may be named "wandering clasts", and are derived from different horizons of the target, and transported for a long distance during cratering, sometimes in opposite directions. The dimensions of clasts may vary from mm to tens of cm, they can be angular or rounded. Wandering clasts, although rare, are discernable by their peculiar lithologies, which allow to reconstruct their initial position in the vertical section of layered target rocks, and thus to estimate the trajectory and amplitude of their displacements. Various clast-bearing dikes have been distinguished in many impact craters (Wilshire et al. 1972, Halls and Grieve 1976, Grieve and Robertson 1976, Stöffler 1977, Lambert 1981, Mashchak and Ezersky 1982, Bischoff and Oskierski 1987, Stöffler et al. 1987, Müller-Mohr 1992, Rondot 1994, 1995, Hunton and Shoemaker 1995, Therriault et al. 1997, Sturkell and Ormö 1997, Warme and Kuehner 1998, Masaitis 1999, and others). Three main groups of dikes, that differ by their respective mode of origin and that bear wandering clasts, can be distinguished: dikes formed by injection, dikes formed by squeezing, and dikes formed by infilling (Fig. 1, Table 1). In impact craters and in their vicinity other groups of dikes exists (e.g., pseudotachylites and cataclasites, apophyses of thick impact melt sheets etc.), which formed variably during excavation and modification stages. As they show only small-scale displacements of clasts, they are not considered here.

The first group comprises clast-bearing dikes that formed during the excavation stage by downward injection of brecciated and melted material. Three varieties that differ in the composition of matrix, occurrence in the crater, and mode of transportation are distinguished. The second group is formed mainly during the compression and excavation stages, due to

Fig. 1. Schematic occurrence of different groups of dikes within impact craters and in their vicinity. A complex crater with a central uplift in the two-layered target is shown. Dikes formed by injection: DI: impactites, DM: mylolisthenites and polymict lithic breccias, DE: polymict lithic breccias. Dikes formed by squeezing: DS: monomict lithic breccias. Dikes formed by infilling: DF: sandstones, clays. Pseudotachylite dikes: DP.

Fig. 1. Schematic occurrence of different groups of dikes within impact craters and in their vicinity. A complex crater with a central uplift in the two-layered target is shown. Dikes formed by injection: DI: impactites, DM: mylolisthenites and polymict lithic breccias, DE: polymict lithic breccias. Dikes formed by squeezing: DS: monomict lithic breccias. Dikes formed by infilling: DF: sandstones, clays. Pseudotachylite dikes: DP.

increased pressure in the water-saturated target layers in the vicinity of a growing crater, and by squeezing out of these unconsolidated deposits. The third group of dikes forms at the late modification stage by infilling of open fissures in the apparent crater floor. The dikes are indicated with letters, according to the type of dike-forming rocks (impactite or impact melt rock - DI, mylolisthenite (see below) and polymict lithic breccia -DM), or mode of origin (ejection - DE, squeezing - DS, infilling - DF).

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