Electromagnetic imaging of Variscan crustal structures in SW Iberia:: the role of interconnected graphite

The western part of the Iberian Peninsula (Iberian Massif) is the best exposed fragment of the Variscan orogen in Europe. Its southern half was generated by an oblique collision between three continental terranes belonging to the margins of Laurassia (Avalonia) - the South Portuguese Zone (SPZ) - and Gondwana - the Ossa Morena Zone (OMZ) and the Central Iberian Zone (CIZ). The boundaries between them are considered to be suture zones. A 200 kin long magnetotelluric profile across the three Variscan terranes was done in a NNE direction, approximately perpendicular to the main tectonic features. The results of two-dimensional inversion of the MT dataset reveal high-conductivity zones coinciding with the transitions SPZ/OMZ and OMZ/CIZ. These conductive bodies related to the sutures at depth were interpreted as graphite enrichments along shear planes formed due to the overall transpressive regime. A high-conductivity layer extending along the whole OMZ was found at a depth of 15-25 km, the top of which spatially correlates with a broad reflector detected by a recently acquired deep seismic reflection profile. The high conductivity was interpreted as caused by the Precambrian Serie Negra graphite-rich rocks. Carbon and oxygen X-ray mapping with electron microprobe on polished sections of Serie Negra samples from OMZ revealed the presence of interconnected graphite, which supports the hypothesis that graphite is determinant for the high conductivity. Two graphite types, which help to record the geological evolution, were identified: graphite accumulations in the schistosity surfaces produced by folding and metamorphism, and metallic films of graphite developed along late faults. The conductive layer shows blobs of higher conductivity suggesting macro-anisotropy. Additional mylonitisation and shearing produced by thrusting at depth can be the origin of these zones of enhanced conductivity, given that the detachment level is located within the Serie Negra. Several high-resistivity features were found in the upper crust, related to Devonian and Carboniferous successions and probably to some unexposed plutons in the SPZ and the Palaeozoic series of OMZ plus some granitic intrusions. In the CIZ, a high-resistivity zone extending to the whole crust is correlated with extensive late Variscan granite intrusions. (C) 2003 Elsevier B.V. All rights reserved.