Integrated two-dimensional lithospheric conductivity modelling in the pyrenees using field-scale and laboratory measurements

Recent magnetotelluric (MT) studies have shown that the lower crust in the Pyrenees contains a high conductivity zone consistent with a subducting con tinental stab, whose conductivity is 0.33 S/m. Partial melting has been int erpreted to be the most plausible explanation for this high conductivity. H ere we report a two-dimensional conductivity model of the lithosphere by in tegrating field-scale and laboratory determinations of the conductivity of continental crustal and mantle rocks. The laboratory data provide empirical formulas which allow us to determine the fluid saturated rock and melt con ductivity when temperature, pressure and lithology are known, Consequently, we have also calculated the density, lithostatic pressure, and several alt ernative temperature profiles for use in the model from gravity, seismic an d thermal field data. These can be used with a prescribed melt fraction to predict the electrical conductivity at depth, which can be compared with th e MT conductivity data. Alternatively, the laboratory data can be combined with the MT conductivity data to predict the melt fraction at depth. The pr imary outputs of the modelling are conductivity and melt fraction predictio n profiles for six mixing models; (i) Waff's model/Hashin-Shtrikman (HS) up per bound, (ii) HS lower bound, (iii) parallel layers, (iv) perpendicular l ayers, (v) random melt areas, and (vi) a modified Archie's law that takes a ccount of the presence of two conducting phases. The modelling results indi cate that a good match to the MT data can be obtained along the whole profi le by the influence of pressure, temperature and the fluid phase with the o nly exception being the subducted slab, where a minimum of 4.7% melt fracti on is necessary to explain the data. (C) 2000 Elsevier Science B.V. All rig hts reserved.