The olistostromes of the Central Harz Mountains north-west of Bad Lauterberg - a result of accretionary processes

Based on the results of extensive geological mapping, the geology of the ar ea between the Sieber, Oder, and Sperrlutter rivers in the southwestern Har t Mountains was newly examined. Within this part of the mountain range, no older marine sediments such as chert, siliceous shale, and 'Wetzschiefer' o f the Upper Devonian to the Lower Carboniferous (do I-IV: ?cu) were deposit ed. All older sediments belonging to the time ranging from the Silurian up to the Upper Middle Devonian were redeposited and occur as components (olis toliths) in olistostromes. Besides minor mudflow sediments there are three main olistostromes: the Lauterberg, Scharzfeld, and Herzberg olistostromes. They can be correlated viiith the much greater Harzgerode and Huttenrode o listostromes of the Middle and Lower Hart Mountains. Within the research ar ea, these olistostromes doubtlessly overlie the graywacke formations of the Tanne system consisting of 'Nehden-Hemberg' graywackes (do II beta-doIII, ?IV) as well as sandstones of the 'graywacke-slate formation' (do ?IV; doV- cu II). The source area of the graywacke formations certainly has been the province of the later Mid-German Crystalline High which acted as a magmatic are during the Upper Devonian. The material of the olistostromes derived f rom the internal margin of the Rhenohercynian Ocean. During the subduction of the Rhenohercynian oceanic crust under the continental lithosphere of th e Mid-German crystalline terrane, an accretionary prism developed. Within t his accretionary complex at first younger and then successively older sedim ents and volcanics were exhumed. This material gravitationally slid in a NW direction forming mud flows which, by accumulation generated the olistostr ome bodies. Within the studied area between the Sieber, Oder, and Sperrlutt er rivers, a stack of sedimentary wedges formed during the Variscan orogeny , consisting of at least four duplex-like structures. These hinterland-dipp ing units situated in front of the Eastern Hart Nappe display the mentioned olistostromes at their bases which acted as detachment and shear horizons.