Polyphase Variscan tectonics and metamorphism along a segment of the Saxothuringian-Moldanubian boundary: The Baden-Baden Zone, northern Schwarzwald (F.R.G.)

Abstract

The Baden-Baden Zone (BBZ) separates polymetamorphic gneisses of the Central Schwarzwald Gneiss Complex (CSGC) in the south from accreted crustal slices of different structural and metamorphic evolution in the north. From SSE to NNW, the following slices (units) can be distinguished: (A) gneisses and amphibolites metamorphosed at low pressures and temperatures of the upper amphibolite facies; (B) kyanite-garnet mica schists formed at pressures around 0.9 GPa and at temperatures between 630 and 670°C; (C) Paleozoic metasediments and metabasites metamorphosed under low pressures and temperatures (greenschist facies), and locally imbricated with mylonitic two-mica granites (S-type). Variscan convergence caused mylonitic, oblique, rightlateral deformation of Paleozoic metasediments and metabasites along NE-SW trending, SE dipping shear planes, accompanied by the intrusion of subsequently sheared porphyric two-mica granites (S-type). In unit C strain and metamorphic grade increase from NW to SE. The rocks of units A and B do not show the NE-SW trending mylonitic foliations found in rocks of unit C. Cataclastic strike-slip imbrication of mica-schists (B) and gneisses (A) was followed by the intrusion of a hornblende-biotite granite (I-type; unit D) into the low-grade metamorphics of unit C during the late Lower Carboniferous. Later on, this granite and the porphyric two-mica granites of unit D were subjected to ductile left-lateral strike-slip faulting along E-W to ENE-WSW-trending, steeply S-dipping shear zones. Geometry and kinematics of these shear zones suggest exhumation of the northern block (units C and D) relative to the southern block (units A and B) and thus point to the onset of extension. Synkinematic mineral assemblages in the northern block point to an exhumation from greenschist facies to surface conditions. South of the BBZ, two-mica granites intruded during Namurian times and subsequently were rapidly exhumed. Pebbles of these granites occur in an ENE-WSW-trending molasse basin. Continental red bed sedimentation and rhyolitic volcanism in this basin lasted from Stephanian to late Lower Permian times.