Geoelectric studies of the crust and upper mantle in Northern Scotland

A brief review of regional geoelectric studies around the North Atlantic is given. The theoretical basis for both magnetotelluric (MT) and geomagnetic deep sounding (GDS) methods are discussed and modelling procedures for one-, twoand three-dimensional Earth structures are reviewed. A description of the diakoptic program for two-dimensional induction studies (Brewitt-Taylor and Johns, 1977) and its use in the geoelectric modelling in this study are given. The digital recording system used is described and the fieldwork logistics adopted are also presented. Measurements of five components of MT field variations in the period band 10 1200s have been made at twenty sites across the Moine Thrust and the Great Glen along a NW-SE traverse in Northern Scotland. Three of the sites lay along the length of the Glen. By means of power spectral and standard two-dimensional MT analysis techniques, Earth response parameters have been obtained for the region. Apart from the complex geology and tectonic history of the region, the adjacent seas have a considerable effect on the induction parameters in Northern Scotland. Using experimental data from sites on the Lewisian Foreland and numerical modelling, a method has been found for minimising coast effects on interpreted geoelectric structure. The first two-dimensional geoelectric model for the crust and the upper mantle across Northern Scotland is presented. The model is complex but agrees well with surface geology. Zones of large resistive bodies which extend to 10-20 km in the geoelectric model correlate well with known granitic bodies in the region. On the shield fragment, a depth of 30 km found for the geoelectric interface also agrees with the depth to the Moho from seismic refraction studies. There are significant lateral variations in the conductivity structure along the Great Glen around which both the crust and upper mantle are conducting. Under the metamorphic zone, the lower crust and the uppermost mantle are conducting. Hydrated rocks and partial melting are the likely cause of the low resistivities of 50-500 ohm m detected below the upper crust. The good conductor in the Great Glen and the widespread resistive zones of large granitic bodies north of the Highland Boundary Fault may be related to the past tectonic activities in Northern Scotland. The region merits further study and various suggestions for such work are given. ACKNOWLEDGEMENTS I thank my supervisor, Dr. R. Hutton who made me interested in solid Earth problems in Nigeria and also suggested the project in Northern Scotland. Her advice, encouragement and hospitality to me and my wife were very invaluable. I am also grateful to Dr. D. Rooney, her former research student who helped me during the execution of the first phase of the fieldwork in 1977. My gratitude also goes to the Federal Government of Nigeria for a postgraduate scholarship and to the Department of Geophysics for study and research facilities. I thank Mr. G. Dawes for computing advice and especially for cassette data processing programs. My thanks also go to Drs. C. Brewitt-Taylor, J. Sik, B. Hobbs, D. Summers and J. Weaver for various discussions and reprints on different aspects of induction studies. I am particularly indebted to Dr. Brewitt-Taylor for his two-dimensional modelling program. Mrss. D. McKirdy and M. Ingham, my colleagues are also thanked for various discussions and good companionship in our office and in the field. The technical help and support from Mrss. I. McDonald and A. Jackson are gratefully acknowledged and I am also thankful to Mrs. J. Whan who typed the tables in this thesis. My thanks go too to various final year Geophysics undergraduates who helped during the fieldwork. Discussions with Dr. M.R. W.Johnson and other geologists in the University Geology Department and in the I.G. S. Highland Unit are also gratefully acknowledged. I am also grateful to the staff of I.G. S. Geomagnetism Unit for data transcription done in their laboratory at the initial phase of this study. I thank various landowners, factors and farmers on whose fields the measurements were made. Their hospitality and friendliness are gratefully remembered. I would also like to express my gratitude to our friends, Dr. and Mrs. D. Beamish who provided a home in Edinburgh for me and my family for a greater part of the study period. Finally, my thanks go to my wife, Grace whose love and patience made the long absence for this study possible.