Salt tectonics of the eastern border of the Leinetal Graben, Lower Saxony, Germany, as deduced from seismic reflection data

To study the salt-related tectonic evolution of the Leinetal Graben, located in the southernmost part of the Central European Basin (CEB) in Germany, we acquired two (1.8- and 3.2-km-long) P-wave reflection seismic profiles across the eastern border faults of the graben. The profiles were acquired with a minivibro along a 1.8-km active spread, densely sampled by geophones spaced at 5 m. The resulting sections showed stratigraphic and fault geometries to a depth of approximately 1200 m. Using two deep boreholes for calibration, we interpreted Mesozoic strata down to the Triassic Zechstein salt and the faults that affected these strata. We recognized two sets of faults: (1) steep, planar faults that are closely clustered and terminated in the Zechstein salt (type 1) and (2) shallow faults that connected between the first set of faults and have very variable dip, depending on the lithology they intersect (type 2). We discovered that the faults do not cross cut the Zechstein salt, but instead they decoupled at this layer. The present-day structure can be interpreted using a two-stage tectonic model. Either there was “downbuilding” during the Triassic, “rafting” of lower Buntsandstein blocks on the Zechstein salt, or both. This resulted in a proto-Leinetal Graben Zechstein diapir surrounded by depocenters. During the Late Cretaceous/Early Tertiary inversion phase, the diapir collapsed, first along type 1 steep faults that detached in the salt layer, and later along type 2 faults; the latter formed as the result of continued extension. Recognition of such early halokinesis was important for the understanding of the behavior of the salt in the CEB and salt tectonics in general.