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 Middle and Southern Urals, is characterised by broad, large amplitude magnetic anomalies due to magnetic rocks within the crystalline basement. These anomalies most commonly have east to north-east trends and contrast markedly with the north-trending, shorter wavelength anomalies over the Uralide orogen to the east. The trace of a line drawn along the truncation of the characteristic EEC anomalies is offset to the west of the Main Uralian Fault in the Southern Urals, crossing Precambrian units which were subject to relatively modest westward-directed thrusting during the Palaeozoic Uralian orogeny. In contrast, the magnetic models require either a major down-to-the-east displacement of an underlying magnetic basement or juxtaposition of dissimilar basement units. Integration of magnetic modelling of the EEC basement with structural mapping and seismic interpretation confirms that the truncation of the magnetic basement is difficult to explain in terms of structures developed during the Uralian orogeny. There is a correlation between the edge of the magnetic basement and the western limit of deformation associated with an earlier (Late Vendian) orogeny. The feature truncating the crystalline basement may have been initiated during the Vendian event, but our preferred interpretation is that it is an earlier structure, perhaps formed by rifting in Riphean times. The edge of the crystalline basement block subsequently acted as a buttress, significantly influencing later deformation. This margin of the EEC thus appears to have been the focus for rifting and orogenic episodes spanning a time interval of more than 1 Ga. (C) 2000 Elsevier Science B,V. All rights reserved.