A comparison of electromagnetic distortion and resolution of upper mantle conductivities beneath continental Europe and the Mediterranean using islands as windows

To investigate the potential of utilising islands as windows through which oceanic mantle conductivities might be imaged, simultaneous magnetotelluric (MT) measurements were carried out continuously over a 3-month duration at two continental stations—one in the Swabian Alps, southern Germany, the other in the Bourgogne, central France—and on the Mediterranean islands of Montecristo (MCHR) and Mallorca (MALL). The MT transfer functions to 1.5 cycles per day (cpd) and single-site Z/H transfer functions for the first four harmonics of the solar quiet (Sq) daily field variations are presented. The transfer functions derived at the island sites are corrected for the distortive and inductive effects arising from the Mediterranean bathymetry relative to the transfer functions calculated at the continental stations using three-dimensional thin sheet modelling. The corrections are formulated in terms of the magnetic perturbation tensor, W. Static shifts in the MT transfer functions are corrected using the Z/H transfer functions, and by comparison with hypothesised resistivity jumps at the transition zones. Models are derived jointly from the residual MT and Z/H transfer functions. Constraints on the resistivities of the upper mantle are investigated. Despite the widely different tectonic settings, no significant differences in the conductivities of the upper mantle below 100 km are detected. The disadvantages of utilising widely spaced islands as windows to the mantle are highlighted by departures from one-dimensionality in the complex tectonic setting of the Mediterranean region.