Nappe Tectonics in the Southeastern Part of the Rheinisches Schiefergebirge

Abstract

Nappe tectonic concepts for the southeastern Rheinisches Schiefergebirge, already proposed by Kossmat (1927), are revived on the basis of new stratigraphic and tectonic findings. The Gießen Greywacke, a turbidite sequence in the northeastern Lahn Syncline, rests unconformably on Lower through Upper Devonian rocks. Conodonts retrieved from carbonate-cemented greywackes, together with data from the literature, suggest that most (if not all) of the greywacke is Frasnian in age. As the rock units surrounding and underlying the Gießen Greywacke comprise Frasnian sediments in various carbonate and shale facies without intercalation of coarse clastic rocks, the greywackes must be interpreted as a nappe. The greywacke unit is composed of recumbent, locally overturned folds. Its base is marked by a well-developed shear-zone with mylonites. A number of exotic rock units, now found at the margins of the greywacke area, are interpreted as allochthonous rafts at the base of the nappe. These are: phyllonitic sediments (much stronger metamorphosed than the surrounding units, with older K/Ar ages of mica), strongly sheared tectonic lenses of spilite (with the trace-element configuration of ocean-ridge basalt), and (near Gießen) slabs of Ordovician, Silurian, Lower Devonian and Lower Carboniferous sedimentary rocks, which do not occur in the surrounding, clearly autochthonous areas. The Gießen Nappe is probably rooted in the important shear zone which seperates the Mid German Crystalline Rise from the Northern Phyllite Zone. The “Werra-Grauwackengebirge”, an isolated outcrop of Frasnian/Famennian greywacke, associated with a unit of phyllitic slates, lies to the northeast of the Gießen Greywacke and most likely represents a part of the Gießen Nappe. These greywackes can be regarded as equivalents of the Devonian Südharz- and Selke Greywackes, along the strike in the Southern Harz Mountains, which already since some time have been recognized as nappes. A nappe concept is also proposed for the Kehna Greywacke and the Hörre/Kellerwald Zone of the Rheinisches Schiefergebirge, its equivalents in the Acker, Tanne and Sieber areas of the Harz Mountains, and the Flechtingen complex to the east of the Harz. All these units are chiefly composed of Upper Devonian through Lower Carboniferous clastic sediments which are seperated from their source area (the Mid German Crystalline Rise) by a belt of time-equivalent limestone/shale facies rocks. This arrangement is best explained by a nappe-tectonic emplacement of the clastic units. The Hörre/Acker greywackes, together with the Gießen/Werra/Südharz/Selke greywackes, probably represent the infill of a Devonian flysch trough, which was originally situated at the northern margin of the Mid-German Crystalline Rise and was later overthrust on the more external parts of the Rhenohercynian Zone.