A high-resolution magnetotelluric survey of the Iapetus Suture Zone in southwest Scotland

Abstract

SUMMARY Magnetic variation (MV) surveys show a 40 km wide zone of high electrical conductivity in the middle crust extending across the Southern Uplands of Scotland from the North Sea to Galloway. A new magnetotelluric (MT) survey across the conductor in Galloway was designed to improve the resolution of structure in the depth range 1–15 km, and, at the same time, assess the improvement in resolution that could be achieved in a practical situation by optimizing data acquisition, processing, analysis and modelling. A 40 km long profile was measured, doubled for part of its length at a separation of 1 km, with a typical site spacing of 2 km or less. It was supplemented by shorter parallel and orthogonal lines, giving a total of 40 sites. Galvanic distortion models involving regional 2-D structures gave poor fits to the impedance tensors both for the array as a whole and for individual sites, confirming the picture from induction arrow maps that many sites were influenced by 3-D structures on scales of a few kilometres and upwards. Other indicators of the electrical strike (regional and local MV measurements, the spatial structure of the Groom–Bailey regional impedance phase and of the rotated off-diagonal phase), gave values more in agreement with the geological strike of N52°E, which was adopted as the most appropriate coordinate system into which to resolve the data. The MT impedances were accordingly rotated into directions N38°W (‘TM’) and N52°E (‘TE’), and inverted using the 2-D code of Rodi & Mackie. The investigation explored the sensitivity of the outcome to a wide range of starting model resistivities and roughness parameters, and to different data subsets. The ‘TE’ mode data were insensitive to structural detail, and the best fit achieved was unacceptable, suggesting the influence of structure outside the plane of the section. The ‘TM’ mode data could be fitted satisfactorily by varying the model roughness and gave the best control on the geometry of the conductive bodies. Forward modelling showed that the model generated from the ‘TM’ mode also fitted the MV data adequately. The inversion resolved the conductive zone into distinct blocks, with edges matching faults mapped in the surface geology: the Leadhills, Fardingmullach and Orlock Bridge Faults, and the Moffat Valley lineament. In the resistivity image, the faults are vertical structures, disrupting other features to a depth of 15 km, which agrees with the view that, although they originated as thrusts, they were reactivated by strike-slip motion during oblique closure of the Iapetus Ocean. Within some of the blocks are highly conductive regions, no more than 4 km wide. The most likely explanation is graphite or other metallic mineralization, localized in shear zones.