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Abstract

Below Permo-Mesozoic cover reflection seismics revealed crocodile pattern and dipping reflector-band geometries in the Upper Crust of the Rhenohercynian and Northern Phyllite Zones that may be analogues to seismic structures ob­served beneath the Rheinisches Schiefergebirge. Particularly for the mid-crustal antiformal Grebenau Structure a Variscan strike direction is indicated by a quasi 3-D observation. At the frontal edge of the collided Saxothuringian micro­plate the Mid-German Crystalline Zone shows up as a 30 km wide culmination its SE-flank being marked by strong SE-dipping reflection bands. These pattems are thus likely to represent thrust arrays, antiformal stacks, basement cul­minations due to the late-Variscan collision. The seismic pattem of the Saxothuringian upper crust in general is far more complex than the rather uniformly NW-verging reflector pattem of the Rhenoherzynian, Northem Phyllite and Mid-German Crystalline Zones. In the Saxothuringian reflective structures dipping either way as well as strong diffractions are observed to 7 sTWT. Part of this reflection pattern may be interpreted as indicating a continuation of the regional structures known from the NW­corner of the Bohemian Massif. In particular a number of strongly reflective, partly steeply NW-dipping structures in the middle crust just SE of the Mid-German Crystalline Zone are tentatively assigned to the root of the early-Palaeozoic Vesser-Doberlug Magmatic Belt. Further SE reflective elements of the upper crust may be linked to Palaeozoic limestones or metavolcanics adjacent to and beneath the Lower Carboniferous flysch of the Teuschnitz Syncline. The Permo-Carboniferous Kraichgau-Saale Trough atop the Saxothuringian is shown by the seismic section to be an asymmetric basin attaining maximum thicknesses of about 1,6 km near the Mid-German Crystalline Zone. A related extensional fault system at the NW basin marging may be represented partly by the reflectors on the SE-flank of the Mid-German Crystalline Zone thought to be rejuvenated Variscan thrusts. Preliminary estimates of extension based on the simplest assumptions indicate stretching of the subbasinal crust by at least 5 % during the rift-phase. Extension due to the Permo-Carboniferous collapse as a whole probably exceeds this value considerably. The lower crust beneath the Hessian Depression and up to the border of the Bohemian Massif exhibits generally a well-developed seismic lamination except for a sprinkle of conspicuously reflection-free areas of unknown significance. Subvolcanic alteration of the crust is discussed as a potential explanation. The reflection-Moho continuously descends from around 9.S sTWT observed throughout the Hessian Depression to 11 sTWT near the Bohemian Massif any trace of a Variscan orogenic crustal root being undetectable by reflection seismics. From this observation and from the absence of the lower crustal lamination beneath the North German Basin it is concluded that the Jamination formed during the Permo-Carboniferous collapse of the Variscan belt.