Lateral variations of the electrical conductivity structure across South Scotland

Abstract

Magnetotelluric measurements in the period range 10 - 1000 seconds have been made at ten new sites in South Scotland and the data analysed to give apparent resistivities and phases and induction vectors for each site.

Nine of the sites lie on a linear traverse stretching from Kinloch Rannoch in the Highlands to Borthwickbrae in the Borders. The apparent resistivities and phases from these sites have been modelled by both one and two-dimensional apparent resistivity structures. The technique of modelling averaged resistivity and phase values as a first approximation, has been found to be a useful way of simplifying the initial modelling procedure.

The final two dimensional model across the traverse shows horizontal discontinuities in electrical conductivity at both the Highland Boundary and Southern Uplands Faults, with a highly conducting zone extending to a depth of 90km below the Southern Uplands. Heat flow measurements and thermal conductivity estimates have been used to predict temperature-depth profiles for the Highlands, the Midland Valley and the Southern Uplands. These in turn have been used to interpret the electrical conductivity model in terms of the possible causes of high conductivity. In the Highlands and Midland Valley the conducting zones have been identified with electronic conduction in hydrated rocks and the process of dehydration at the boundary between amphibolitic and granulitic facies. As well as these mechanisms it is possible that beneath the Southern Uplands, partial melting may occur at depths around 70km.

The two-dimensional model has also been fitted to the vertiiv cal magnetic field data across the traverse. However, additional data from other sites in the South of Scotland have been analysed also, and an interpretation of the vertical magnetic field in terms of an equivalent line current has been made. The depth to the line current, and its position beneath the Southern Uplands, agrees with the depth and position of the highly conducting zone in the two-dimensional induction model. It is likely though that both conduction and induction contribute to the measured vertical field.

The results of the modelling tend to support the suggestion that the anomalously high conductivity beneath the Southern Uplands is a result of the tectonic processes involved in the closing of the Iapetus Ocean. There is also some indication of the possibility of an ancient subduction zone beneath the Highland Boundary Fault.