Crustal‐scale thrusting and extension in the Hercynian Schwarzwald and Vosges, central Europe

Abstract

The Hercynian outcrop areas in the Schwarzwald of SW Germany and Vosges of easternmost France consist from north to south of three major lithotectonic complexes: very low‐ to medium grade metamorphics of the Saxothuringian Belt, the polymetamorphic Central Gneiss Complex, and the tectonic melange of the Southern Complex. Juxtaposition of these three complexes occurred by oblique convergent motions in early Carboniferous time. The Saxothuringian Belt is part of a major SE dipping accretionary wedge that consists of lower Paleozoic meta‐sedimentary and metaigneous rocks and superimposed upper Devonian to lower Carboniferous synorogenic volcanics and elastics. The Central Gneiss Complex consists of polymetamorphic paragneisses, orthogneisses, metabasites, and meta‐ultramafics dipping to the NW. The Southern Complex consists of an imbricated melange of north to NW dipping synorogenic elastics, volcanics, and blocks of ultramafics and gneissic basement. The Central Gneiss Complex (CGC) experienced pervasive Hercynian low‐pressure/high‐temperature metamorphisrn before it was emplaced against and thrust over the sedimentary and volcanic fill of the adjacent synorogenic basins. This convergence (340–330 Ma) produced mylonitic‐cataclastic shear zones in the CGC. The most significant shear zones are the SE verging Todtnau fault zone between the CGC and the Southern Complex and the NW verging Lubine fault zone between the Central Gneiss Complex and the rocks of the Saxothuringian Belt. These two fault zones enclose a bivergent “pop‐up” of the polymetamorphic basement. The Todtnau fault zone probably extends for more than 100 km along strike and can be traced to a depth of 10 km by deep seismic reflection studies. Detachment probably occurred along a mid crustal zone of anatexis expressed by a domain of low seismic reflectivity between about 10 and 15 km. Crustal thickening during early Carboniferous convergence led to an extensional phase of crustal reordering (between about 330 and 280 Ma) that was accompanied by the massive rise of “posttectonic” granitic plutons from middle to high crustal levels. Simultaneous creation of a structural “lamination” in the refractory lower crust neglected the main convergent tectonic boundaries; it produced a basin‐and‐range type lower crust and mantle‐crust transition in early Permian time and involved a major advective input of heat.