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 continental slab, whose conductivity is 0.33 S/m. Partial melting has been interpreted to be the most plausible explanation for this high conductivity. Here we report a two-dimensional conductivity model of the lithosphere by integrating 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 conductivity when temperature, pressure and lithology are known. Consequently, we have also calculated the density, lithostatic pressure, and several alternative temperature profiles for use in the model from gravity, seismic and thermal field data. These can be used with a prescribed melt fraction to predict the electrical conductivity at depth, which can be compared with the MT conductivity data. Alternatively, the laboratory data can be combined with the MT conductivity data to predict the melt fraction at depth. The primary outputs of the modelling are conductivity and melt fraction prediction profiles for six mixing models; (i) Waff’s model/Hashin–Shtrikman (HS) upper bound, (ii) HS lower bound, (iii) parallel layers, (iv) perpendicular layers, (v) random melt areas, and (vi) a modified Archie’s law that takes account of the presence of two conducting phases. The modelling results indicate that a good match to the MT data can be obtained along the whole profile by the influence of pressure, temperature and the fluid phase with the only exception being the subducted slab, where a minimum of 4.7% melt fraction is necessary to explain the data.