ALBERT

Description: The eastern part of MVE-90 discussed in this issue extends across the largest continuous Hercynian basement exposure in Germany, between the Franconian Line and the Lausitz Massif. The profile is parallel to the SW-NE axis of the Saxothuringian Zone at the northern margin of the Bohemian Massif and allows a detailed discussion of the hitherto controversially interpreted structures, for example: is the Münchberg Gneiss Massif a nappe complex or parautochthonous, what is the cause of the gravity high underneath, are the patterns of the Erzgebirge clearly explained by compressional or extensional tectonics, what is the extent and thickness of the granites and their relationship to the gravity low at the northern margin of the Bohemian Massif, what is the origin and nature of the deep regional NW-trending fault systems (e.g. Franconian Line, Gera-Jáchymov), what is the geological significance and tectonic character of the long-lived N-S-and NW-lineament-like zones of Pritzwalk­Naab, Flöha, Elbe and how does the structure of the Lausitz Massif fit in? The steep-angle reflection profile with 80-fold vibroseis coverage is of very good quality and is complemented by 8 short cross lines. Refraction and wide-angle reflection seismic studies complete the survey. Many shallow dipping reflectors in the upper crust are readily related to structures exposed at the surface. In the middle crust lens-like structures are interpreted as duplex (compressional) or phacoid-like (extensional) structures. The lower crust is seismically well laminated and characterized by flat dipping conjugate structures. Among additional, non-seismic methods, reprocessing of gravity data has yielded new aspects for interpretation and modelling. Besides attempts of a regional geodynamic modelling of the Saxothuringian Zone between the northern margin of the Bohemian Massif and the Mid-German Crystalline Zone, relationships between extreme Hercynian crustal thickening (〉80 km) and subsequent crustal thinning (30 km) are discussed on the basis of petrology and seismic data. The MVE-90 profile helps, more than previous DEKORP profiles, to distinguish more distinctly between extensional and compressional structures and to search for solutions for crustal stretching and subcrustal erosion processes. A controversial discussion concerning the volume of Hercynian granite cannot be decided by seismic, but it may be solved by combining seismic and gravity data.