Linear to non-linear relationship between vein spacing and layer thickness in centimetre- to decimetre-scale siliciclastic multilayers from the High-Ardenne slate belt (Belgium, Germany)

Typical spacing distributions have frequently been described for fractures in thin sedimentary layers (<1 m). Regularly spaced fractures often result from saturation during fracture development. Spatial distribution of veins is less commonly studied although it also can show regular patterns. This study focuses on the spatial distribution of quartz veins in Lower Devonian siliciclastic multilayer sequences from the High-Ardenne slate belt (Belgium, Germany) and compares the observed vein spacing with published fracture spacing in order (i) to investigate the effect of the layer thickness to vein spacing and (ii) to understand the processes of early vein development during the late stages of burial in a sedimentary basin at the onset of orogeny. The results show that a quasi linear relationship between vein spacing and layer thickness in thin (<40 cm) competent sandstone layers, embedded in a pelitic matrix, becomes non-linear in thicker sandstone layers (>40 cm). Vein spacing tends to increase to a maximum value becoming more or less independent of layer thickness. The resemblance with fracture spacing suggests that in an unfractured rock vein saturation can occur. High fluid pressures are responsible for vein nucleation but the stress state around the initial veins controls the spacing pattern. Subsequently, in a vein-saturated rock, or the existing veins will thicken by the process of crack-sealing, or a new cross-cutting vein generation will develop in case the regional stress field changes relatively with respect to the existing veins.