ELECTRICAL-RESISTIVITY STRUCTURE OF THE EASTERN MOROCCAN ATLAS SYSTEMAND ITS TECTONIC IMPLICATIONS

The electrical resistivity structure of the crust and upper mantle of the Atlas Mountain System was studied using magnetotelluric and geomag netic deep soundings. Field experiments were done in eastern Morocco a long a traverse from the Anti Atlas to the Rides Rifaines in two campa igns in 1983 and 1988. Zones of very low electrical resistivity could be identified in the various structural settings at different depth ra nges, most likely connected directly to the tectonic evolution of the mountain belts. A mid-crustal low resistivity layer with total conduct ance (thickness-resistivity ratio) of about 2000 Siemens stretches fro m the southern border of the High Atlas towards the Middle Atlas. This layer seems to characterize the base of crustal detachment, e.g., the plane for large horizontal over-thrusting, and supports the idea of t hick- and thin-skinned tectonics involved in Atlasic mountain building . In the western Middle Atlas an upper-crustal low resistivity layer ( at depth < 10 km) was found in the area where volcanic activity was pr esent, pointing towards a direct relation between low electrical resis tivity and volcanic or postvolcanic events. North of the Middle Atlas resistivity structures change totally: The Pre and parts of the Sub-Ri f have a highly conductive cover, presumably connected to the molasse basin. Total conductance was calculated to reach 6000 Siemens. No furt her conductive structures, like, e.g., the ones found beneath the High and Middle Atlas, are seen within the resistive crust, but at much gr eater depth within the upper mantle.