Lithospheric and upper-mantle structures, results of electromagnetic soundings in Europe

Abstract

Information on electrical conductivity of the subsurface at a crustal scale is inferred mainly from magnetotelluric (MT) and magnetometer data. The latter include both magnetometer array studies (MV) and geomagnetic depth soundings (GDS). Locally, controlled source data (e.g. data from d.c. and very low frequency (VLF) resistivity surveys, frequency soundings and airborne electromagnetic mappings) provide information on near-surface structures, e.g. on exposures of deeper conductors. The variety of electromagnetic methods provides an efficient zooming ability for structures from a regional scale of hundreds of kilometres to small local details of some metres. Thus both the mapping of large-scale crustal conductivity structures and a detail study of the anomalous structures are possible. The electrical conductivity of the crust and upper mantle is described within the plate tectonic framework: examples of structures which become electrically conductive at different phases of the process are shown. Using the results of electromagnetic soundings in various tectonic environments in Europe it is shown how the crustal conductors can be interpreted as ancient and modern tectonic markers. These include structures from the Fennoscandian Shield (Precambrian extensional basins and collisional zones), the Carpathians, Scotland and Ireland (suture zones), and the Pannonian Basin (extensional basin and thrust zones), as well as conductors detected along the European Geotraverse (e.g. the terrain boundaries and the décollement of the Variscan thrusting). An example which considers the lower-crustal conductivity from the Fennoscandian Shield shows rather large lateral variations in conductance of the lower crust varying from 1 S to several hundreds of siemens. Estimates of the depth of the asthenospheric conducting layer within the European territory also vary considerably. Shallowest depths are detected beneath extensional regions (the Pannonian Basin and the Rhinegraben; 40–80 km), whereas depths of 200 km or over (or the absence of a conducting layer) are detected beneath the Alps and old Precambrian cratons. Among the key areas where electromagnetic investigations should play an essential role are the deep structure of the East European platform (boundaries of Archaean and Proterozoic terrains), the structure of the Teisseyre-Tornquist zone, the nature of the present-day continental lower crust, and the characteristics of the electrical asthenosphere beneath Europe.