The State of Stress in the North German Basin

摘要

Stress indications form boreholes and wells from the North German Basin (NGB) were compiled in order to describe the actual state of stress in terms of stress direactions and stress gradients. Published data [1,2], as well as new evaluations gained from the eastern part of the basin, result in a uniform stress pattern: Different states of stress are established in the three layers of the sedimentary basin, in the suprasalinar (Triassic and yonger sediments), the salinar (Zechstein) and the subsalinar (lower Permian and older sediments and crystalline rocks resp.). Apparently, a mostly mechanical decoupling between the three layers is due to the salinar rocks the three layers is due to the salinar rocks with their isotropic state of stress. Both the horizontal stress directions and the stress gradients differ significantly in the three layes. The overlying suprasalinar shows different directions of maximum horizontal stress component scattering around an E-W orientation and with amounts of minimal stress both lower and higher than vertical stress component scattering around an E-W orientation and with amounts of minimal stress both lower and higher than vertical stress component. The salinar layer in the middle has an isotropic stress orientation without any horizontal or vertical preference and with lithostatic amounts or gradients of stress. The underlying subsalinar shows a strong N-S direction of maximum horizontal stress component in the central part of the NGB with small deviations to NNW-SSE in the western part and to NNE-SSW in the eastern part. However, the effective gradient of the minimum horizontal amounts of about 13 MPa/km. Nevertheless, in regions with reduced thickness of salinar layer or with dominant faults, a remarkable modification of the state of stress is to be expected. In terms of an actually effective tectonic regime, the state of stress in the NGB is characterized as a normal faulting state with E-W extension in the subsalinar layer. As a rule, the amount of the vertical stess component is higher than the minimum horizontal stress component and there are no indications that the maximum horizontal stress component exceeds the vertical stress in the subsalinar layer. At the same time, the less dense and buoyant salinar sequences modify the state of stress in the uppermost suprasalinar sediments. As a result we find a descending basin with remarkable Quaternary thicknesses.