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THE STRUCTURE of SOME ATLANTIC ISLANDS AS DEDUCED from GRAVITY and OTHER GEOPHYSICAL DATA a Thesis Submitted for the Degree of D

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THE STRUCTURE of SOME ATLANTIC ISLANDS AS DEDUCED from GRAVITY and OTHER GEOPHYSICAL DATA a Thesis Submitted for the Degree of D THE STRUCTURE OF SOME ATLANTIC ISLANDS AS DEDUCED FROM GRAVITY AND OTHER GEOPHYSICAL DATA A thesis submitted for the degree of Doctor of Philosophy by Duncan John Macfarlane, B.Sc., D.I.C. December, 1968 Geophysics Department, Imperial College, London, S.W.7. 2 ABSTRACT The results of reconnaissance gravity surveys on twenty-five of the Atlantic oceanic islands are presented. Digital computer programs are developed for processing of the gravity data, notably for terrain correction and density determination. Fourteen of these land gravity surveys are used in conjunction with nearby marine gravity and seismic refraction data, to study the subsurface structure of the islands. All the islands are volcanic structures formed at centres along deep fissures in the earth's crust. They are characterized by high Bouguer anomalies ranging from 125 - 289 mgal of which about one third comes from a primary volcanic pipe or high-level magma chamber. The islands can be divided into two groups on the basis of gravity field, crustal structure and depth of magma generation. Those on the Mid-Atlantic Ridge have lower maximum and regional Bouguer anomaly values; they rise from shallower water depths, above a thinner crust which is underlain by lower density mantle material; the magma for the islands on the Ridge is believed to be generated at shallow levels and volcanic centres are regarded as 3 intrusions of differentiated, fused mantle material, while the volcanic centres of the other oceanic islands are probably high-level reservoirs for magma which is generated deep in the mantle. 4 CONTENTS Page CHAPTER 1 INTRODUCTION 7 1.1 Structure of the Atlantic Ocean Floor 8 1.2 The Oceanic Islands 13 1.3 Theories of Oceanic Genesis 21 1.4 History of the Atlantic Ocean 29 CHAPTER 2 FIELD OPERATIONS A1'D DATA PROCESSING 33 2.1 Field Operations 33 2.2 Data Reduction 35 2.3 Gravimetric Terrain Correction by Digital Computer 43 2.4 Regionals and Residuals 57 2.5 Estimation of Anomalous Mass 61 2.6 Marine Gravity Data Processing 63 CHAPTER 3 DETERMINATION OF THE DENSITY TO BE USED IN THE BOUGUER CORRECTION 72 3.1 Introduction 72 3.2 Direct Measurement of Rock Samples 72 3.3 Estimation from Observed Compressional Velocities 74 3.4 Estimation from Surface Gravity Data 80 3.5 Density of the Island below Sea-Level 87 CHAPTER 4 A GRLVITY SURVEY OF ASCENSION ISLAND 90 CHAPTER 5 GRAVITY SURVEYS OF SOME OF THE AZORES ISLANDS 109 5.1 Introduction 109 5.2 A Gravity Survey of Santa Maria 123 5.3 A Gravity Survey of S. Miguel 139 5.4 A Gravity Survey of Terceira 148 5.5 A Gravity Survey of S. Jorge 153 5.6 A Gravity Survey of Pico 159 5.7 A Gravity Survey of Faial 164 5.8 A Gravity Survey of Flores 173 5.9 General Discussion 178 5.10 Conclusions 192 5 CHAPTER 6 GRAVITY SURVEYS OF SOME OF THE CANARY ISLANDS 193 6.1 Introduction 193 6.2 A Gravity Survey of Lanzarote 204 6.3 A Gravity Survey of Gran Canaria 221 6.4 A Gravity Survey of Tenerife 229 6.5 A Gravity Survey of Hierro 238 6.6 General Discussion 247 6.7 Conclusions 255 CHAPTER 7 GRAVITY SURVEYS OF TWO ISLANDS IN THE DEEP OCEAN 256 7.1 A Gravity Survey of Fernando de Noronha 256 7.2 A Gravity Survey of Madeira Island 262 CHAPTER 8 GRAVITY OBSERVATIONS BY THE PORTUGUESE SEKVICO METEOROLOGICO NACIONAL 273 8.1 The Cape Verde Islands 273 8.2 The Islands of sab Tom4 and Principe 299 8.3 Discussion 302 CHAPTER 9 CONCLUSION 304 ACKNOWLEDGEMENTS 320 BIBLIOGRAPHY 322 APPENDIX SUMMARY OF GRAVITY OBSERVATIONS ON THE ATLANTIC OCEANIC ISLANDS 356 - 6 - 0* **: JAN MAYEN ICELAND 60* 45* AZORES •BERMUDA MADEIRA • 30° CANARY • -. ISLANDS CAPE, • VERDE 15* ISLANDS GUINEA 0* - ISLANDS • . FERNANDO NORONHA ASCENSION 15* •ST. HELENA 30* TRISTAN DA CUNHA •GOUGH ISLAND - 45° 1 I I I 75° 60° 45* 30* 15* 15° FIG. H. ATLANTIC ISLANDS 7 CHAPTER 1 INTRODUCTION Oceanic islands were seized on by the early geologists as being the only accessible exposures of the vast area of the earth's surface that is covered by sea. While the rapid advances in marine geology and geophysics in recent years have brought the whole of the oceanic crust within direct or indirect roach, and have shown that the islands must be regarded rather as anomalies of the oceanic crust, scientific interest in these islands has increased rather than diminished. Much of this interest has been in the hope that study of the islands may indirectly yield information concerning the structure and history of the ocean basins but the islands are also regarded as of interest in their own right. In this thesis, an attempt is made primarily on the basis of original gravity data on many of the Atlantic islands named in Fig. 1.1 to define a common structure for oceanic islands and to deduce their possible evolution in the light of the known facts and present theories of the earth's crust. In this introductory chapter, the geological environment of the islands, i.e. the Atlantic Ocean, is described and the 8 main facts and theorias regarding oceanic islands are reviewed. The chapter closes with a brief summary of current theories of oceanic genesis and their bearing on the possible history of the islands. 1.1 Structure of the Atlantic Ocean Floor. The structure of the ocean floor (see Fig. 1.2) can be considered in three main divisions : 1) the deep ocean basins, 2) the continental margins, and 3) the Mid-Atlantic Ridge. 1.1.1 The Deep Ocean Basins. These are large flat-bottomed areas of the deep-ocean floor lying between the continental margins and the Mid-Atlantic Ridge. Their smoothness appears to be due to a thick layer of sediments, since gravity and magnetic observations indicate that the sub-bottom relief is large. The small abyssal hills which border the basins on their seaward side are considered (Heezen et al, 1959) to represent the original topography buried beneath these abyssal plains. Following current usage, the term "crust" is applied to the upper part of the lithosphere above the Mohorovicic dis- continuity. At the Mohorovicic discontinuity or "Moho" the seismic velocities increase suddenly to 7.6 - 8.3 km/sec. Worzel (1965b) presents from a compilation of all the seismic refraction data available up to 1964, an average crustal section for the ocean basins (see Fig. 2.7). This 9 FIG 12. Physiographic provinces in the Central Atlantic, after Heezen and Tharp, 1959 and 1961. 40°W 20° 0° 20°E A A . • • A N 4 4 4 A • . 40° 4.4 AZORES , ....... Continental slope e" Abyssol plain A Ave ..4•. •••• MAD El RA •=•• r ts, Abyssol hills Rift mountains and A . rift volley AA CANARY A A A • ' ISLANDS Fracture zone 4 "4 4 A A A A 4 A A Seamounts 4 ,t 4 4 A 'CONTINENTAL 20° A A SHELF Aseismic ridge A I I A It 4ill CAPE A A VERDE ISLANDS •A I A A A A A A A A A A s A GUINEA ' 0° IS LANDS , A. A A A • . FERNANDO DE •*ea NORONHA AAAA 4 A t AA A A A I' ASCENSION I SLANDef :• 01 ••A A A IT A A •A A A r, A, A A A A 4 4 A SY HELENA A A A A A *-, A A A '‘,.‘`‘ A \ A A A 20° - - A A A A ••I A A A A S A A A A "‘ A A AJ •• ^ ( A A 4 / fr A A el A A / 1 A A A A A / A A A'1 10 consists of 4.9 km of water, 0.7 km of sediments of density 2.3 gm/cc and velocity 2.5 km/sec, 1.7 km of layer 2, or "basement" of density 2.55 gm/cc and velocity 5.1 km/sec, and 4.2 km of layer 3 or "oceanic layer" of density 2.9 gm/cc and velocity 6.7 km/sec overlying mantle material of density 3.4 gm/cc and velocity 8.1 km/sec. Layer 2 is considered to be basalt (Hill, 1960) and layer 3 to be partially serpen- tinized peridodite (Hess, 1965; Nicholls, 1965). The total thickness of the ocean crust and the velocities for the different layers are markedly different from continental crust (Fig. 2.'40. 1.1.2 Continental Margins. Adjoining the continents is the smooth gently-sloping continental shelf. Depths on the shelf are less than 450 metres and the width may vary from a few kilometres to over 350 km. The edge of the continental shelf is marked by a sharp increase in gradient as the relatively steep continental slope grades sharply down to about 2000 m. Below 2000 m a more gentle slope which Heezen et al (1959) have named the continental rise, links the continental slope to the abyssal plains at depths of greater than 7300 m (4000 fathoms). Except for submarine canyons and sepmounts, the bathymetric relief is slight. A summary of the structure of coastal margins by Worzel (1965a) indicates that the transition from continental crust of 20 - 30 km thickness to oceanic crust, occurs in a distance 11 of 100 - 200 km with the greatest part of the change occuring 30 - 50 km on either side of the 2000 m line. It appears that the continental-type basement layer wedges out towards the ocean with a major discontinuity in crustal structure at the base of the continental slope.
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