The structural development of the Rheinische Schiefergebirge

Abstract

The structural framework of the Rheinische Schiefergebirge is characterized by NW-facing folds with a more or less strongly developed slaty cleavage and by listric overthrusts. The listric overthrusts of the Subvariscan Foredeep often are folded. Nappe displacements are assumed for the southeastern Rheinische Schiefergebirge. Based on their tectonic movement pictures two different types of listric overthrusts can be distinguished in the Rheinische Schiefergebirge: (1) Listric overthrusts forming simultaneously with folding. As the upward decreasing displacement may be compensated by folding, the overthrusts may die out at higher tectonic levels. During folding and thrusting fold-axial planes and thrust planes are rotated to NW. As a result of uplift and rotation to NW, both being related to folding and thrusting, secondary structures occur: low dipping NW-facing overthrusts and, locally, a SE-dipping post-crystalline crenulation cleavage. (2) Listric overthrusts which cut pre-existing NW-facing fold structures. This gives rise to an antithetic rotation of the overthrusted block and thus to a steepening of the originally NW-facing folds and cleavage planes. This rotation is intensified by further tectonic shortening and finally results in NW-dipping axial planes and cleavage planes. As a consequence of the SE-directed rotation of the overthrusted blocks a cleavage fan develops, which becomes overprinted by a predominantly flat-lying post-crystalline second cleavage. The youngest tectonic fabrics are kink bands. They occur in areas of late tectonic uplift and must be interpreted as extension structures. The character of metamorphism in the Rheinische Schiefergebirge is of a low pressure and high temperature with metamorphic temperatures not higher than 350°C except in the metamorphic zone of the Taunus, where temperatures of 400-450°C were reached. The metamorphism in the Rheinische Schiefergebirge is syntectonical with respect to the folding and its associated first cleavage. According to radiometric datings the age of folding in the southernmost part of the Rheinische Schiefergebirge is about 330 Ma and increases continuously towards the north to about 300 Ma. Folding and metamorphic development are continuous processes without distinguishable phase-like events. The Rheinische Schiefergebirge must be regarded as an ensialic orogen. Its geodynamic development is interpreted as being the result of A-subduction, i.e. subduction of lithospheric mantle below continental crust.