Blocks of fault-bounded imbricate stacks of Devonian limestones, a diagnostic feature for a tectonic origin of chaotic rock fabrics in the Harz Mountains (Eastern Rhenohercynian Belt, Germany).

The distinction between sedimentary and tectonic processes in the formation of chaotic block-in-matrix fabrics is especially difficult in ancient mountain belts, where sedimentary structures are usually overprinted by orogenic deformation (e.g. Festa et al. 2019, GR, 74). This also applies to the chaotic rock units occurring widespread in the allochthonous domain of the Harz Mountains. For these units, it has previously been assumed that their rock fabric was sedimentary in origin and subsequently only tectonically overprinted by late Carboniferous Variscan deformation. In contrast, it could be shown, that tectonic deformation is crucial for the formation of the "chaotic" fabric (Friedel et al. 2019, Int.J.Earth.Sc., 108). This is particularly evident in the structural characteristics of blocks of (hemi)pelagic limestone of different Devonian ages, which are incorporated in a slaty clayey matrix. So far, the blocks were mostly regarded as olistoliths and thus considered as clear proof for a sedimentary origin of the chaotic units being olistostromes. However, our investigations show that these blocks are fault-bounded, tectonically folded and internally imbricated stacks of limestone strata, whose final fragmentation and isolation occurred after folding. Similar to rootless folds, these blocks of fault-bounded imbricate stacks are an outcrop-scale diagnostic feature to identify a strong fault-tectonic overprint or even a tectonic origin of chaotic rock fabrics, provided that the tectonic character of folding and faulting is sufficiently proven. The regionally widespread occurrence of such blocks, together with meso- to micro-scale faulting criteria, suggests a predominantly tectonic origin for the chaotic rock units in the Harz.