Intrusion vs. inversion at the rims of the Northwest German Basin: the gravity highs of Bramsche and Pritzwalk revisited. 3D structural modelling by the aid of the "Digital Geotectonic Atlas"

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One of the still unsolved problems of regional importance for the both evolution and present structure of the Northwest German Basin concerns the existence or non-existence of the so called "Massif of Bramsche" and, farther to the East, the "East Elbian Massif in the area of Pritzwalk". Both regions are characterized by extremely high Bouguer gravity and positive magnetic field anomalies and were interpreted in terms of mafic intrusions (plutons) at depths of some 6 - 10 km. The high thermal maturity in the southern area of the Lower Saxony Basin, the CO2 - risk and missing reservour properties are being regarded as an aftermath of the intrusion of the Bramsche massif, which is supposed to have intruded into the crust in the Upper Cretaceous. The existence of a similar massive in the area of the "Pritzwalk high" (Mecklenburg/Brandenburg) has been doubted over the last decade: According to recent geophysical investigations the masses causing the strong positive anomaly are no longer assumed to arise from shallower depths but arise from the lower crust. Regarding the area of the positive gravity anomaly of Bramsche, its former interpretation was called into question on the base of to more recent findings: It was demonstrated that the level of maturities along boreholes is more likely to be explanined by an increased amount of burial with subsequent tectonic inversion and not due an intrusion. Additionally, vp/vs ratios of 1.72 do not point to a gabbroic body but more likely to a dolomite. If this instrusion in the suspected form never existed, the regional structural and thermal development of the southern rim of the Northwest German Basin would need to be judged differently. The existence of possible Mesozoic and Paleozoic gas reservoirs in the vicinity of the high would also need to be re-evaluated. The "Geotectonic Atlas of NW Germany" (BGR, Hannover) contains a unique collection of geoscientific information and a compilation of digital depth information representing both the structure and physical properties of a sedimentary basin. The compilation is based on data, experience and knowledge from long-standing hydrocarbon investigations in the NW German basin. Therfore, it offers the chance to foster interdisciplinary studies and the development of research and modelling with strong ties to model database concepts and modelling techniques. Using the data set provided by the atlas, it is possible to find answers to questions related to the evaluation of models for the evolution of the basin rims, and to 3D modelling of geological processes and structures. The questions that arise can be addresed without further time consuming and expensive data acquisition outside the laboratory.