| Résume | To understand arithmetic properties of Shimura varieties, one studies their reduction over finite fields. One of the principal tools to investigate the special fiber is the Newton stratification. There is a unique closed Newton stratum, the so-called basic locus. In certain cases it is possible to understand the geometric structure of the basic locus very explicitly, as a union of classical Deligne-Lusztig varieties with a description of the closure relations between them in terms of a Bruhat-Tits building. We will present a group-theoretic approach in terms of affine Deligne-Lusztig varieties which gives a conceptual understanding in which cases one can hope for such a simple description and how it should look like. |
