Large-scale Variscan shearing at the southeastern margin of the eastern Rhenohercynian belt: a reinterpretation of chaotic rock fabrics in the Harz Mountains, Germany

For the past 50 years, large parts of the Harz Mountains (eastern Rhenohercynian Zone, Germany) have been considered to consist of olistostromes of Early Carboniferous age, which were later deformed by Variscan tectonics. The olistostrome model was associated with the assumption that sliding of coherent nappe units occurred contemporaneously with the submarine mass-flow processes. A sedimentary origin of olistostromes was derived mainly from the block-in-matrix fabric of the rocks, assumed soft-rock deformation fabrics, and the composition and age of blocks. Based on available biostratigraphic data, block composition and fabric studies of outcrops and drilling cores, it can be shown that the fabric of chaotic rocks is mainly caused by Variscan tectonic deformation at Late Carboniferous, without any indication of soft-rock deformation. Processes and structures indicating a tectonic origin are boudinage, extensive cataclasis and transposition of folded rocks, commonly associated with intense mineralization, as well as the regular spatial distribution and orientation patterns of blocks. The chaotic rock fabric mainly evolved in the brittle–ductile transitional zone late-to-post-kinematic relative to the synmetamorphic first cleavage. Widespread chaotic rocks are localized in the allochthonous domain which itself is characterized by generally strong tectonic rock deformation and thrust faulting. Our results suggest that significant mass-flow deposits do not occur. The previously proposed olistostromes are mélanges and broken formations of tectonic origin which form a large-scale network of anastomosing shear zones that penetrate and juxtapose more or less intact coherent rock units of different stratigraphy and provenance. Due to the tectonic origin the entire early Paleozoic sequences may exhibit chaotic rock fabrics.