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1 GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany oncken{at}gfz-potsdam.de
2 University of Cologne, Institut für Geologie, Zülpicher Str. 49a, D-50674 Köln, Germany
In the Rhenish Massif and Ardennes, the Rheno-Hercynian fold and thrust belt of the Mid-European Variscides exposes a telescoped complete Late Palaeozoic passive margin, which was detached from the lower crust during Carboniferous collision with a continental arc (Mid-German Crystalline High on the leading edge of Armorica). Geometric analysis and isostatically corrected balanced cross-sections show that the basal detachment propagated from the oceanic realm into the passive margin by repeated ductile footwall failure during lithospheric flexure of the weak lower plate (effective elastic lithospheric thickness <4 km) under the load of the advancing upper plate. Early collision was associated with offscraping of the uppermost slope sediments in frontal accretion mode. Subsequent detachment of the remaining passive margin cover was initiated after subduction of the ocean-continent transition to about 6 kbar depth. Rocks and fabrics from the detachment show that, at this pressure, they crossed the brittle-plastic transition at the fossil 300–400 °C isotherm. Newly failed segments of the basal detachment propagated along this isotherm and branched off towards the foreland by progressively downstepping from the passive margin sediments into the basement of the downflexed lower-plate crust. This evolution coincided with cyclic changes in accretion mode of the lower-plate upper crust to the advancing orogenic wedge (repeated changes from basal to frontal accretion) as well as with a related stepwise propagation of a narrow foreland basin. Propagation of the detachment segments and the related imbricate fans in the lower plate, moreover, was controlled by the geometry of the basin structure by localizing branch lines and ramps along earlier growth faults.
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