Gravity and Other Geophysical Studies Relating to the Crustal Structure of South-East Scotland
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GRAVITY AND OTHER GEOPHYSICAL STUDIES RELATING TO THE CRUSTAL STRUCTURE OF SOUTH-EAST SCOTLAND by Evangelos Lagios B.Sc., Athens University Thesis presented for the degree of Doctor of Philosophy of the Universit of Edinburgh in the Faculty of Science OCTOBER 1979 (. 1w co • .1 -i -. DECLARATION I hereby declare that the work presented in this thesis is my own unless otherwise stated in the text, and that the thesis has been composed by myself. Evangelos Lagios - ii - CONFESSIONS... 7t'ameva mata Chapita twameva 7ameva bundhu cha sukha twameva Twarneva vidya draminam twameva 7Wameva saivan mama deva deva Twameva sarvan mama deva deva Thrameva sarvan mama deva deva - iii - To Elizabeth - iv - ACOWLEDGEMENTS I would like to express my sincere thanks to all those who helped me in any way to complete this thesis. I am thankful to Professor K M Creer, Head of the Geophysics Department for allowing me full use of the Departmental facilities to carry out my project, and for his strong support for financial assistance. I am grateful and particularly indebted to Dr Roger G Eipkin, my supervisor and initiator in Geophysics, not only for suggesting the project, but also for the kindness he has shown me during my stay in Edinburgh and his valuable help which I received in innumerable ways. Dr Hipkin accompanied me many times in fieldwork and particularly during the most difficult part of it, by walking up and down the hills at Lammerinuir and Tweeddale in the Southern Uplands. I am thankful to him not only for his constant guidance and advice, but also for his extremely helpful and constructive criticisms during the progress of this thesis. For me it was a great pleasure and joy working with him. I express my sincere thanks to Graham Dawes for his help and advice which I received in computing, especially during the implementation of the contouring routines and his occasional helpful company in fieldwork, which was fully appreciated and is gratefully acknowledged. - I am deeply indebted to my wife, Elizabeth, for her strong support, help in fieldwork and encouragement throughout my work. I am pleased to extend my warm.thanks to Drs Chris Swain and David Powell for their computer programs given to me for my work. I am thankful to Drs Bruce Hobbs, Rosemary Hutton, Roy Gill and John Donato for useful discussions; especially to Dr Gill for giving me samples for density measurements. I would like to thank Mr McQuillin, Head of Marine Geophysics Unit, IGS, for his 1permission to include in my thesis open-file IGS maps. Also, I express my thanks to Professor J Drakopoülos, Athens University for his strong support for financial assistance and the encouragement he gave me to come to Edinburgh for post- graduate studies. The latter is gratefully acknowledged. My friends and colleagues, K Dimitropoulos and K Makropoulos; the first for his assistance and useful discussions and the second for the strong support he has shown me for financial assistance. I would like to thank my parents for their financial support and constant encouragement for which I will never be able to repay them. - vi - The Schilljzj Foundation, the National Hellenic Research Foundation and the Royal Society of London for financial assistance. Mr John Mcinnes for computing help with the SACM package. Miss Routh Thomas who kindly did the susceptibility measurements. All other members of the Geophysics Department and all my student colleagues, especially, Khalid Fahmi, for the enjoyable time I have had with them during the last three years. Finally, Miss Linda Nisbet who kindly gave up many of her hours to type this thesis, an achievement for which I am indebted to her. - vii - ABS TRACE About 2500 gravity stations were established during a gravity survey of south-east Scotland. The reduction of the gravity observations, including computations of terrain corrections out to 22 km was made automatically by a sequence of computer programs and the Bouguer gravity anomaly map was plotted using different modern contouring routines. The densities of about 200 rock samples were measured and a new method of estimating the surface density of rock formations, by fitting trend surfaces to the Bouguer anomaly, is described. The gravity base network was adj.usted manually but a least-squares analysis is also described which includes the instrumental drift. A computer program, based on this algorithm was written to perform the calculations and an RMS error of 0.096 g.u. was obtained for the adjustment of the network. The Bouguer gravity anomaly map was interpreted with control from aeromagnetic and seismic information. The long-wavelength negative gravity anomaly of the Southern Uplands was interpreted as due to a granite batholith underlying the whole north-eastern part of the region. Particularly in the Tweeddale area, the detailed gravity model was also supported by aeromagnetic modelling. Three bosses near Peebles appear to come within 1½ km of the surface while another near Lammer Law may be shallower still. The interpretation with a granite batholith underlying the area and extending to a depth of 10 - 12 km - viii - is generally compatible with other geophysical studies in the same area (seismic and magnetotelluric) and explains the formation of some interesting geological faulting features (the Southern Uplands Fault System) as well as the different rates of sedimentation between East Lothian and Midlothian. The Devonian basins of Lauder, EyenDuth and Oldharnstocks have been studied gravintrically. The first appears to be a simple V-shaped valley filled with Old Red Sandstone sediments, with a thickness of about 550 m near Lauder. The last two are faulted-bounded and have greater sedinEnt thickness: about 900 m for Oldhamstocks and perhaps 1900 m for Eyemouth. The Cove and Innerwick Faults bounding the main Oldhamstocks Basin have estimated maximum throws of 800 m and 500 m respectively, while the Eyemouth and Coldingham Bay Faults both have throws of about 600 m. The Great Conglomerate of Monynut Edge, west of Oldhamstocks, has a thickness which is unlikely to exceed 400 m. Finally, an investigation of the Lower Carboniferous volcanics in East Lothian and the inner Firth of Forth was based on combined gravity and magnetic models. The maximum thickness of the volcanjcs was found to be about 470 m, a few kilometres offshore from Aberlady. They were found not to be the main cause of the north-westerly gravity gradient in the area, which is an edge effect of the underlying granite batholith. - viiii - CONTENS Page Title.............................................................. Declaration........................................................ Confessions....................................................... Dedication......................................................... iv Acknowledgements ................................................... v Abstract.......................................................... viii. Contents ............................. .............................. x Listof figures .................................................... xiii Listof tables ..................................................... xvi CHAPTER I COLOGIcAL AND GCPY2IAL ?VIff 1.1 Tectonic Review ............................................. j 1.2 Geological history .......................................... 1.3 Geophysical Review .......................................... 41 CHAPTER II FI3LD'VC3K AND uArA CEDUCTICT. 2.1 Location of Gravity Measurements ............................ 20 2.2 Elevation and Position Measurements .......................... 22 2.3 Gravity Measurements ......................................... 23 2.3.1 Instri.ent-Drift characteristics ............................. 23 2.3.2 Data Reduction ............................................... 26 2.3.3 Terrain Corrections .......................................... 28 2.3.3.1 Digitisation Scheme ........................................ 28 2.3.3.2 Description of the Terrain Correction, TERCOR, Computer Program .................................................... 29 2.3.4 Accuracy of Terrain Corrections .............................. 32 2.4 Density Measurements .......................................... 35 2.4.1 Laboratory Density Measurements ............................... 35 2.4.2 Least Squares Density Determination .......................... 42 2.5 Gravity Anomalies ............................................ 44 2.5.1 Error estimation of the Gravity Anomalies .................... 47 CHAPTER III G.RAVITYBAZZ STATI01 NTVTORK ADJU3T1T 3.1 Introduction ................................................. 50 3.2 Manual Adjustment of the Network ............................. 5]. 3.3 Least-Squares Adjustment ..................................... 55 3.3.1 Intrcduction .................................................. 55 3.3.2 Definitions .................................................. 56 3.3.3 Notation ..................................................... 56 3.3.4 Observational Equation ........................................ 57 3.3.5 Variance ..................................................... 57 - x-- Page 3.3.6 Normal Equations ......................................... 57 3.3.7 Error Analysis ........................................... 59 3.3.8 Programming - Results ...................................... 63 3.3.9 Calibration Factor Tests ................................. 71 CHAPTER IV TH3 BOUGUER GRAVITY. ANOMALY MAP AND CO'iTOU?.ING WiTIN.S 4.1 Introduction ............................................. 76 4.2 Bouguer