On the detectability of a highly conductive layer in the upper mantle beneath the Black Forest crystalline using magnetotelluric methods

Abstract

SUMMARY Simultaneous geomagnetic and telluric variations (600 s-4 hr) were observed at two stations on the Rhine-Graben sediments and on the neighbouring Black Forest crystalline in order to extend the pulsation observations (10-600 s) of former magnetotelluric (MT) surveys to longer periods. These observations completely describe the anomalous magnetic and telluric fields in terms of a set of transfer functions using the Black Forest station as 'normal'. In the period range from 30 s to 4 hr the horizontal and vertical magnetic transfer functions of the Rhine-Graben anomaly are characterized by continuously decreasing amplitudes. The local Schmucker induction arrows point uniformly 45 south-east for all long-period geomagnetic transfer functions, and the NW-SE-striking direction in the upper mantle derived from the induction arrows is also consistent with the preferred telluric direction in the Black Forest. In both areas large offsets of apparent resistivity and phase curves for the E- and B-polarization directions are observed, except that the phase curves converge above 400s in the long-period range. The interpretation of the period dependences of the observed magnetic and telluric transfer functions from 600 s to 4 hr by a 2-D conductivity model requires a highly conductive layer in the upper mantle at a depth of 120 km beneath the Black Forest. This confirms Bahr's (1992) model for a highly conductive layer in the upper mantle that has also been found south of the Alps (Braitenberg et al. 1994) and which is interrupted by low conductive alpine roots. However, no information about such a layer beneath the Rhine Graben can be derived. 2-D model studies have shown that an improved depth resolution of highly conductive layers in the upper mantle is possible using telluric phases, but the transfer functions for the vertical magnetic fields are not sensitive enough for the determination of the depth of an upper mantle conductive layer.