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RESEARCH PRODUCT

The N Variscan margin of the Ruhr coal district (Western Germany): structural style of a buried thrust front?

subject

description

The Ruhr coal district represents part of an external fold and thrust belt of the Variscan orogen in central Europe. Extensive mining exposed molasse-type, coal-bearing clastic sequences of Upper Carboniferous age, deformed by folding and thrusting. Thrusts range in scale from decameters to 40 kilometers along strike but displacements are below 2.5 km. They are distributed in various stratigraphic levels and no regional detachment horizon is exposed. Displacement is mainly towards NW (towards the foreland) but also towards SE. The thrusts are bound to stockwerk-tectonics and die out towards depth frequently antithetic to the exposed strata. Folds vary in wavelength from decameters to more than 10 kilometers. Transitions from cylindrical or concentric to chevron-type folds are common. They have also been modelled in experiments. There, varying fold types compensate volume problems, created (e.g.) during concentric folding, without a major detachment. Another mechanism of compensating excess volume during folding is evident, where thrusts transport volume from fold hinge areas to fold limbs. Folds generally deform both footwall and hangingwall of thrusts. Numerous observations prove a contemporary development of thrusts in mechanic relation to the folding-process. Overall orogenic shortening decreases slowly from SE to NW, towards the foreland without a major break. The presented structural observations and experimental data point to the fact that orogenic shortening in the marginal fold and thrust belt of the Ruhr district may well be compensated by folding and minor thrusting without major detachment. Therefore, the buried tip line of a regional Variscan detachment horizon may be situated towards the internal parts of the orogen, southeast of the Ruhr district.