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).

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).

Friedel, C.-H.¹, Cunäus, E.L.², Kreitz, J.³, Leiss, B.⁴, Stipp, M.⁵

1) Karl-Marx-Str. 56, 04158 Leipzig, chfriedel@gmx.de; https://orcid.org/0000-0002-3380-5193
2) Baugrunduntersuchung Naumburg GmbH, Wilhelm-Franke-Str. 11, 06618 Naumburg, info@baugrunduntersuchung-naumburg.de
3) Smart Asphalt Solutions GmbH, Goethestraße 2, 37120 Bovenden, j.kreitz@smart-asphalt- solutions.de
4) Geowissenschaftliches Zentrum der Universität Göttingen, Strukturgeologie und Geodynamik,     Goldschmidtstr. 3, 37077 Göttingen, bleiss1@gwdg.de
5) Institut für Geowissenschaften und Geographie, Martin-Luther-Universität Halle-Wittenberg, Von‑Seckendorff‑Platz 3, 06120 Halle,  michael.stipp@geo.uni-halle.de

 

The distinction between sedimentary and tectonic processes in the formation of chaotic rock units (mélanges, broken formation) is especially difficult in ancient orogenic belts, where sedimentary structures are usually overprinted by tectonic deformation (e.g. Fiesta et al. 2019). This also applies to the chaotic rock units, which are widespread in the allochthonous domain of the Harz Mountains, an exposed part of the Eastern Rhenohercynian Belt in Germany. For these units, it has been previously assumed that their chaotic rock fabric was initially sedimentary in origin and was merely tectonically overprinted by subsequent Variscan deformation. In contrast, it could be shown, that tectonic deformation is crucial for the formation of the "chaotic" texture (Friedel et al. 2019). This is particularly evident in the structural characteristics of Devonian limestone blocks.

Within the allochthonous domain of the Harz Mountains, blocks of predominantly hemipelagic, condensed limestone of different ages and up to several tens of metres in size are widespread incorporated in a slaty clayey matrix. So far, the blocks were mostly regarded as olistholites and thus considered as clear evidence for a sedimentary origin of the chaotic rock units (olistostromes). However, our investigations show that the limestone blocks are fault-bounded, folded and internally imbricated stacks of limestone strata, i.e. tectonically sheared blocks formed during Variscan collisional deformation whose final fragmentation and isolation occurred subsequently to folding.  Like rootless folds, also blocks of fault-bounded imbricate stacks of rock strata are a diagnostic feature to identify a strong tectonic overprint or even a tectonic origin of chaotic rock fabrics, provided that the tectonic character of folding and faulting is sufficiently proven (Blanc et al. 2010, Friedel et al. 2022). Since such blocks of imbricated limestone stacks are regionally widespread, they support, together with other criteria, a predominantly tectonic origin of the chaotic rock units in the Harz Mountains.

References:
Blanc et al. 2010, Geogazeta, 48, 187-190,
Fiesta et al. 2019, Gondwana Research, 74, 7-30
Friedel et al. 2019, Intern. Journal of Earth Science, 108, 2295-2323
Friedel et al. 2022, Hallesches Jahrb. f. Geowissenschaften, Beiheft 51, 47-53