Magnetic evidence for the geometry and evolution of the eastern margin of the East European Craton in the Southern Urals

The eastern margin of the East European Craton (EEC), adjacent to the Middl e and Southern Urals, is characterised by broad, large amplitude magnetic a nomalies due to magnetic rocks within the crystalline basement. These anoma lies most commonly have east to north-east trends and contrast markedly wit h the north-trending, shorter wavelength anomalies over the Uralide orogen to the east. The trace of a line drawn along the truncation of the characte ristic EEC anomalies is offset to the west of the Main Uralian Fault in the Southern Urals, crossing Precambrian units which were subject to relativel y modest westward-directed thrusting during the Palaeozoic Uralian orogeny. In contrast, the magnetic models require either a major down-to-the-east d isplacement of an underlying magnetic basement or juxtaposition of dissimil ar basement units. Integration of magnetic modelling of the EEC basement wi th structural mapping and seismic interpretation confirms that the truncati on of the magnetic basement is difficult to explain in terms of structures developed during the Uralian orogeny. There is a correlation between the ed ge of the magnetic basement and the western limit of deformation associated with an earlier (Late Vendian) orogeny. The feature truncating the crystal line basement may have been initiated during the Vendian event, but our pre ferred interpretation is that it is an earlier structure, perhaps formed by rifting in Riphean times. The edge of the crystalline basement block subse quently acted as a buttress, significantly influencing later deformation. T his margin of the EEC thus appears to have been the focus for rifting and o rogenic episodes spanning a time interval of more than 1 Ga. (C) 2000 Elsev ier Science B,V. All rights reserved.