3-D electromagnetic imaging of a Palaeozoic plate-tectonic boundary segment in SW Iberian Variscides (S Alentejo, Portugal)

In SW Iberian Variscides, the boundary between the South Portuguese Zone (SPZ) and the Ossa Morena Zone (OMZ) corresponds to a major tectonic suture that includes the Beja Acebuches Ophiolite Complex (BAOC) and the Pulo do Lobo Antiform Terrane (PLAT). Three sub-parallel and approximately equidistant MT profiles were performed, covering a critical area of this Palaeozoic plate-tectonic boundary in Portugal; the profiles, running roughly along an NE-SW direction, are sub-perpendicular to the main Variscan tectonic features. Results of the three-dimensional (3-D) modelling of MT data allow to generate, for the first time, a 3-D electromagnetic imaging of the OMZ-SPZ boundary, which reveals different conductive and resistive domains that display morphological variations in depth and are intersected by two major sub-vertical corridors; these corridors coincide roughly with the NE-SW, Messejana strike-slip fault zone and with the WNW-ESE, Ferreira-Ficalho thrust fault zone. The distribution of the shallow resistive domains is consistent with the lithological and structural features observed and mapped, integrating the expected electrical features produced by igneous intrusions and metamorphic sequences of variable nature and age. The development in depth of these resistive domains suggests that: (1) a significant vertical displacement along an early tectonic structure, subsequently re-taken by the Messejana fault-zone in Late-Variscan times, has to be considered to explain differences in deepness of the base of the Precambrian-Cambrian metamorphic pile; (2) hidden, syn- to late-collision igneous bodies intrude the meta-sedimentary sequences of PLAT; (3) the roots of BAOC are inferred from 12 km depth onwards, forming a moderate resistive band located between two middle-crust conductive layers extended to the north (in OMZ) and to the south (in SPZ). These conductive layers overlap the Iberian Reflective Body (evidenced by the available seismic reflection data) and are interpreted as part of an important middle-crust décollement developed immediately above or coinciding with the top of a graphite-bearing granulitic basement.