Sedimentology and tectonic setting of Devonian-Carboniferous turbidites and debris flow deposits in the Variscan Vosges Mountains (Markstein Group, NE France)

Turbidites and debris-flow deposits are exposed in the Upper Devonian to late Lower Carboniferous Markstein Group of the Southern Vosges Mountains (NE-France). Medium to coarse-grained sandstones are the dominating lithology alternating with fine-grained units. Sedimentary facies and their stacking patterns indicate a prograding evolution of sandy submarine fans from an outer-fan facies to channel-lobe-transional deposits and coarse conglomerates. Sedimentological and tectonical data confirm the strong geological relationship between the sedimentary units of the Southern Vosges and the Badenweiler-Lenzkirch Zone in the southern Schwarzwald.The grain composition of the turbidite sandstones is of metagranodioritic and volcanic origin. It reflects the erosion of an uplifted plutonic basement, episodically mixed with detritus from basaltic to dacitic active volcanoes. Trace element patterns and element ratios in sandstones from mafic-volcanic as well as from non-volcanic stratigraphic levels point to a subduction related setting. Trace element patterns in sandstones from dacite-volcanic active stages have normalized values, which can be compared with those from overthickened active margins or collisional belts. However, large-scale frontal collisional processes between the Protoalpine and the Moldanubian Zone cannot be evidenced by tectonic or provenance data. Instead, the favoured plate tectonic scenarios for the depositional setting are an arc-hinterland collision followed by dextral transpressive deformation through largescale wrench faults or a transpressively deformed active margin basin. In the first case a prograding retro-arc foreland is regarded as the tectonic setting of the submarine fans, in the second case an extensional or strike-slip setting in the fore-arc or back-arc area can also be imagined. Some turbidite complexes share petrographic and sedimentological similarities which in combination with the occurrences of overturned units point to an accretionary tectonic environment. Therefore the first model is favoured here. Our results are of interest for the paleogeographic reconstruction of the southern border of the Variscan Moldanubian Zone.