ALBERT

Description: The kinematic evolution of the Rhenohercynian fold-and-thrust belt of, the Central-European Variscides can be reconstructed from incremental strain and palaeostress data, synkinematic metamorphism, geochronology, synorogenic sediments, and the geometry of shortening. This data set allows to infer the mechanical development with respect to the Mohr–Coloumb theory of critically tapered orogenic wedges. Growth of the Rhenohercynian orogenic wedge was kinematically partitioned into a broad orogen-normal moving and a narrow obliquely moving domain. The ratio of the final widths of these domains varies from 1.6 to 2.3. This low ratio indicates strong basal coupling. Growth averages are 14 km/Ma of shortening and a strain rate of 6×10-16 s-1. The mode of mass transfer from lower to upper plate, which controlled the accretion geometry and the particle paths, evolved in the long term from dominantly basal to frontal accretion. This evolution was accompanied by a decrease in taper from precollisional 18°–28° to final 5°–8°. 4–6 accretionary subsystems assemble the wedge. Each is characterised by a sequence of basal to frontal accretion, both rooting in a common detachment. The development of the subsystems is supported by the observation of stepwise migration of a foreland-bulge ponding flysch subbasins. These features imply shorter termed (ca. 2 Ma) cyclic mass transfer above a high friction detachment with episodes of basal accretion followed by episodes of a quickly forward-propagating thrust front. Transfer of the basal detachment from ductile mid-crustal to brittle conditions via a crustal ramp and particle flow across the ramp provoked large-scale adjustment of wedge geometry. Internal wedge thickening is spatially and chronologically related to the formation of the ramp and was supported by surficial erosion of ca. 3 kbar for keeping a critical taper. Three growth stages of the wedge can be reconstructed. The high taper of the precollisional stage plots in the tectonically erosive field in a taper-stability diagram. Then taper decreases to 10° after emplacement of a flysch nappe stack. During the final stage of contraction an eastern and a western branch develop with geometries fluctuating around a critically stable state. The two branches link with differences in large-scale wedge architecture, bulk contraction, and detachment geometry, the latter being controlled by the former basin geometry.