Carboniferous convergence and subsequent crustal extension in the southern Schwarzwald (SW Germany)

Abstract

Detailed structural analysis in the southwestern part of the Variscan Sehwarzwald Massif (SW Germany) indicates polyphase, synmetamorphic deformation in ductile shear zones. The tectono-melainnrphir evolution is characterized by orogenic crustal shortening and subsequent late- orogenic crustal extension in Carboniferous times. Convergence is responsible for an KSK trending, north dipping thrust zone with intense deformation in orthogneissic S-C type mylonites Superposed on schistose and folded metasediments presumably lower Carboniferous in age. Southeastward thrust-’“g parallel to pervasive stretching lineation, similar to the pre-dominant oblique convergent structures ill the central part of the massif, is related to crustal stacking. Relations of early granite intrusions with the outlasting retrograde tectonics Point to a Lower Carboniferous (Late-Visean) age of shortening.

Subsequent crustal extension is indicated by a broad N-S trending and west dipping ductile shear zone within high grade meetamorphic (I1T7LP) gneisses. Retrograde stretching lineatone marked by sillimanite to chlorite anr consistent with a top-to-the-west shearing on the western flank of a large progressively warping domai structure. Intensely sheared and boudinaged granitic rocks are syn-tectonic and seal the age of extension at about 325 Ma (Lower/ Upper Carboniferous boundary). During progressively cataclastic stages of tectonic denudation the still active detachment controlled formation of an adjacent late Paleozoic (Stephano-Pcrniian) continental basin supersedding high-grade gneiss. As elsewhere in the Varisean belt, the late extensional process in the tectono-”“‘tainorphie evolution of the southern Sehwarzwald is related rapid uplift, exhumation and thinning by a gravitational collaps of a previously thickened crust.