Evolution of the Northeast German Basin - inferences from a 3D structural model and subsidence analysis

摘要

A 3D structural model of the Northeast German Basin was evaluated with special emphasis on its evolution as an intracontinental depression. The study includes investigations on subsidence history and structural setting of the basin. Thickness evolution and calculated tectonic subsidence volumes of Permian to Quaternary sediments in the Northeast German Basin indicate that the subsidence history was related to five stages of basin evolution which differ in their subsidence mechanisms. For the initial rift phase in the Late Carboniferous to Early Permian, a dominant thermal event and subordinate horizontal stresses were indicated by thickness variation evolution and by structural evidence. The main part of basin subsidence occurred in a NW-SE-oriented basin in the subsequent phase of thermal relaxation with maximum subsidence from Early Permian (Rotliegend) to Middle Triassic (Muschelkalk). From Middle Triassic the thermal subsidence pattern was superposed by further tectonic events. In the Middle Triassic regional extension led to a reconfiguration of the southern part of the basin, where new NNE-SSW-trending troughs (Rheinsberg and Gifhorn Troughs) developed. In the Jurassic the northwestern part of the basin was uplifted while in the south the Keuper subsiding areas continued to sink and NW-SE-trending depressions, related to salt margins, became important. Differentiation continued into Cretaceous times when regional compression caused uplift of the southeastern part of the basin and basin margins. A final subsidence phase occurred in the Cenozoic. This was accompanied by intensive salt movement. Recent basin configuration reflects the superposition of structural elements resulting from different evolution stages. The main structural characteristics of the basin are: (1) a vertical tectonic zonation in a pre-Zechstein succession, which lacks significant internal structures, and a strongly deformed post-Zechstein succession, which was decoupled due to the thick Zechstein sa and (2) a marked asymmetry of the basin with a shallow northern slope and a steep bounding fault at the southern margin (Elbe Fault System). The northwestern part of the basin shows the structural properties of an intracratonic sag basin with persisting subsidence and with minor salt mobilisation. In contrast, initial structures in the southeastern part are strongly overprinted by younger tectonic events including Middle Triassic to Jurassic extension, Late Cretaceous inversion and Late- to post-Cretaceous salt movements. Tectonic elements that deform the whole sedimentary succession are restricted to the basin's southern, eastern and northeastern margins where salt thickness decreases. Combined volumetric and backstripping investigations show that 2/3 of the total subsidence was induced by the sediment load and 1/3 was caused by tectonics. The tectonic subsidence history varies laterally across the basin. While tectonic subsidence curves in the northwestern part of the basin show fast subsidence during the Permian, and Early Triassic decreasing exponentially with time, subsidence curves in the southeastern part indicate repeated tectonic activity. However, the tectonic subsidence volume created during Permian to Late Triassic is significantly higher than the tectonic subsidence during younger phases of basin history.