Mullion structures in the Ardennes and Eifel occur in a lower Devonian, shallow marine siliciclastic sequence, S and SE of the Stavelot-Venn Massif (Fig. 2), roughly between Dedenborn (Eifel) and SW of Bastogne (Ardennes). Mullion structures in the area are actually quite common. The most spectacular outcrops are well known from many publications.
The rocks here were buried to 8 to 12 km depth in a passive continental margin in late Devonian times. During this burial phase the rocks underwent very low grade metamorphism (from 350¾C in the NE, up to 450-500¾C and 200 MPa in the SW (Lielitz and Mansy, 1999; Kramm et al., 1985). This phase was accompanied by the development of close-to-lithostatic fluid pressure conditions (Sintubin et al., 2000; Hilgers et al., 2000).
In the late Devonian and Carboniferous, this sequence was inverted and thrust in a NW-direction to form a foreland fold- and thrust belt, which is now part of the Rhenohercynian zone of the Mid-European Variscides (Fig. 3). The important differences in sediment thickness and the involvement of basement massifs in the thrust sequence are the main factors influencing the typical outcrop pattern of the region (Adams and Vandenberghe, 1999; Fielitz, 1992; Hollman and von Winterfeld, 1992; Hollmann and Walter, 1995; Hance et al., 1999).