Large regional structures from puzzle pieces – the hidden Triassic rift on the western flank of the Eichsfeld-Altmark-Swell

The incorporation of regional geologic knowledge is essential to solve multiple geological questions in any defined research area meaning that small-scale observations must fit with general regional interpretations as well as the tectonic and sedimentary setting. Conversely, in areas with sparse subsurface information, a conceptional knowledge of the observed area must be developed by incorporating and combining small-scale indications into a sound regional geologic model. In this contribution, we collect some of these small-scale “puzzle pieces” observable along the western flank of the Eichsfeld-Altmark-Swell in Central Germany. This structure is assumed to form a roughly NNE-SSW-trending Permo-Triassic sedimentary high, which is constrained by reduced thickness, facies changes and unconformities in Late Permian to Late Triassic strata. In parts it is accompanied by areas where Zechstein is directly overlain by shaly to evaporitic Upper Buntsandstein/Middle Muschelkalk while several hundred meters of sandy Lower and Middle Buntsandstein are missing. We combine local observations published in cross-sections, outcrop situations, few boreholes and reflection seismics, which occur along a some 200 kilometres long strip in central Germany and attempt to put them in a regional geologic context. Our results for the spatial relationship of sedimentary basins and bordering normal faults as well as associated salt structures indicate that the western flank of the Altmark Swell was affected by huge sub-horizontal, decoupled movements (up to 5 km extension) along ramp-flat normal faults involving two evaporite detachments. Although sparse, some indications are available for gently dipping basement faults that accommodated these movements below the Zechstein salt.