Combined seismic and magnetotelluric imaging of upper crystaline crust in the Southern Bohemian Massif

Combined engineering-scale seismic and magnetotelluric methods are not usually thought of as useful tools for investigating the uppermost (51 km deep) crystalline crust, although combined methods are making progress at the larger oil exploration scale in sedimentary basins (Nagy 1996). By `engineeringscale' we mean those field methods which can be applied by a team of two or three persons with one or two vehicles, with equipment normally used for engineering site surveys, and with survey field costs of only a few hundred dollars per day. We report here a case history where a combination of such modest geophysical resources has been used to try to clarify some regional structural and tectonic relationships which orthodox geological mapping had failed to resolve. The Southern Bohemian Massif in Austria is a deeply eroded remnant of the Variscan orogen of central Europe, comprising medium-grade metamorphic rocks of Precambrian to Palaeozoic age, extensively intruded by granitic plutons of Variscan age. The paper deals with the location of the boundary between two major divisions of the massif, the Moldanubian in the west and the Moravian in the east, in the neighbourhood of the Messern Arc. The geological problem is essentially whether the Moravian Bittesch Gneiss is identical to or distinct from the Moldanubian Dobra Gneiss in the west. Wieseneder et al. (1976) point to the remarkable similarities between the two gneiss units. The two outcrops are separated at the surface by the Variegated Sequence (Fig. 1) and by granulites and paragneisses. Fuchs & Matura (1980) suggest that the surface outcrops are linked at depth by a synclinorial (trough-shaped) structure. If the synclinorial structure is correct, it implies that the boundary between the Moldanubian and the Moravian must lie some 30 km to the west, at the western border of the Dobra Gneiss. If, on the other hand, as Thiele (1976) assumes, the Variegated Sequence and granulites have been thrust eastwards over the Bittesch Gneiss, there is no genetic relation between the two gneiss sequences. This question is the subject of our combined seismic and magnetotelluric investigations. Although our spatially very limited data turn out to be consistent with either models, we demonstrate that modest geophysical techniques can be used to image structures within crystalline basement.