Remobilisation of Overpressured Muds As a Mechanism for Block-in-matrix Fabric in the Harz Mountains, Germany

Chaotic units of Palaeozoic metasedimentary rocks in the Harz Mountains have been attributed to Early Carboniferous submarine mass-flow processes or to Variscan block-in-matrix deformation. This paper aims to explain block-in-matrix deformation in terms of the remobilisation of overpressured, undercompacted muds early during Variscan deformation. Burial of the Devonian and Carboniferous sediments trapped fluids within the clay-rich muds, causing overpressure and hindering mechanical compaction. The resulting centroid effect generated extremely high fluid pressures and mobilised the muds, leading to mud injectites and brecciation of lithified carbonates and greywackes. Remobilisation may have been influenced by clay mineralogy, particularly high smectite content. Fracturing enabled the fluids to escape, reduced pore pressure and allowed the muds to compact and lithify, before Variscan contraction and metamorphism occurred. The paper describes possible analogues that are not overprinted by later deformation, including mud breccias.