Paleomagnetism: Continents and Oceans (International Geophysics)

PALEOMAGNETISM

PALEOMAGNETISM

This is Volume 73 in the INTERNATIONAL GEOPHYSICS SERIES A series of monographs and textbooks Edited by RENATA DMOWSKA, JAMES R. HOLTON, and H. THOMAS ROSSBY A complete list of books in this series appears at the end of this volume.

PALEOMAGNETISM Continents and Oceans

MICHAEL W.McELHINNY Gondwana Consultants Hat Head, New South Wales, 2440 Australia

PHILLIP L McFADDEN Australian Geological Survey Organisation Canberra, 2601 Australia

ACADEMIC PRESS A Harcourt Science and Technology Company

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Front cover photograph: A depiction of a dipole field. Courtesy of Michael W. McElhinny and Phillip L. McFadden. Back cover photograph: Global paleogeographic map for Late Permian. (See Figure 7.11 for more details.)

This book is printed on acid-free paper.

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Harcourt/Academic Press A Harcourt Science and Technology Company 200 Wheeler Road, Burhngton, Massachusetts 01803 http://www.harcourt-ap.com Library of Congress Catalog Card Number: 99-65104 International Standard Book Number: 0-12-483355-1 International Standard Serial Number: 0074-6142 PRINTED IN THE UNITED STATES OF AMERICA 99 00 01 02 03 04 MM 9 8 7 6

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Contents

Preface

xi

Chapter 1 Geomagnetism and Paleomagnetism 1.1 Geomagnetism 1.1.1 Historical 1.1.2 Main Features of the Geomagnetic Field 1.1.3 Origin of the Main Field 1.1.4 Variations of the Dipole Field with Time

1 1 3 7 12

1.2 Paleomagnetism 1.2.1 Early Work in Paleomagnetism 1.2.2 Magnetism in Rocks 1.2.3 Geocentric Axial Dipole Hypothesis 1.2.4 Archeomagnetism 1.2.5 Paleointensity over Geological Times 1.2.6 Paleosecular Variation

14 14 16 18 22 25 26

Chapter 2 Rock Magnetism 2.1 Basic Principles of Magnetism 2.1.1 Magnetic Fields, Remanent and Induced Magnetism 2.1.2 Diamagnetism and Paramagnetism 2.1.3 Ferro-, Antiferro-, and Ferrimagnetism 2.1.4 Hysteresis

31 31 34 35 37

2.2 Magnetic Minerals in Rocks 2.2.1 Mineralogy 2.2.2 Titanomagnetites 2.2.3 Titanohematites

38 38 40 45

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Contents

2.2.4 Iron Sulfides and Oxyhydroxides 2.3 Physical Theory of Rock Magnetism 2.3.1 Magnetic Domains 2.3.2 Theory for Single-Domain Grains 2.3.3 Magnetic Viscosity 2.3.4 Critical Size for Single-Domain Grains 2.3.5 Thermoremanent Magnetization 2.3.6 Crystallization (or Chemical) Remanent Magnetization 2.3.7 Detrital and Post-Depositional Remanent Magnetization 2.3.8 Viscous and Thermo viscous Remanent Magnetization 2.3.9 Stress Effects and Anisotropy

46 48 48 51 54 56 60 64 68 71 74

Chapter 3 Methods and Techniques 3.1 Sampling and Measurement 3.1.1 Sample Collection in the Field 3.1.2 Sample Measurement

79 79 82

3.2 Statistical Methods 3.2.1 Some Statistical Concepts 3.2.2 The Fisher Distribution 3.2.3 Statistical Tests 3.2.4 Calculating Paleomagnetic Poles and Their Errors 3.2.5 Other Statistical Distributions

84 84 87 91 98 99

3.3 Field Tests for Stability 3.3.1 Constraining the Age of Magnetization 3.3.2 The Fold Test 3.3.3 Conglomerate Test 3.3.4 Baked Contact Test 3.3.5 Unconformity Test 3.3.6 Consistency and Reversals Tests

100 100 101 108 109 111 112

3.4 Laboratory Methods and Applications 3.4.1 Progressive Stepwise Demagnetization 3.4.2 Presentation of Demagnetization Data 3.4.3 Principal Component Analysis 3.4.4 Analysis of Remagnetization Circles

114 114 119 124 125

3.5 Identification of Magnetic Minerals and Grain Sizes 3.5.1 Curie Temperatures 3.5.2 Isothermal Remanent Magnetization 3.5.3 The Lowrie-Fuller Test 3.5.4 Hysteresis and Magnetic Grain Sizes

127 127 128 131 133

Contents

3.5.5 Low-Temperature Measurements

vii

13 5

Chapter 4 Magnetic Field Reversals 4.1 Evidence for Field Reversal 4.1.1 Background and Definition 4.1.2 Self-Reversal in Rocks 4.1.3 Evidence for Field Reversal

137 137 139 141

4.2 The Geomagnetic Polarity Time Scale 4.2.1 Polarity Dating of Lava Flows 0-6 Ma 4.2.2 Geochronometry of Ocean Sediment Cores 4.2.3 Extending the GPTS to 160 Ma

143 143 146 149

4.3 Magnetostratigraphy 4.3.1 Terminology in Magnetostratigraphy 4.3.2 Methods in Magnetostratigraphy 4.3.3 Quality Criteria for Magnetostratigraphy 4.3.4 Late Cretaceous-Eocene: The Gubbio Section 4.3.5 Late Triassic GPTS 4.3.6 Superchrons

154 154 15 5 157 158 159 162

4.4 Polarity Transitions 4.4.1 Recording Polarity Transitions 4.4.2 Directional Changes 4.4.3 Intensity Changes 4.4.4 Polarity Transition Duration 4.4.5 Geomagnetic Excursions

164 164 166 171 172 174

4.5 Analysis of Reversal Sequences 4.5.1 Probability Distributions 4.5.2 Filtering of the Record 4.5.3 Nonstationarity in Reversal Rate 4.5.4 Polarity Symmetry and Superchrons

175 175 177 179 180

Chapter 5 Oceanic Paleomagnetism 5.1 Marine Magnetic Anomalies 5.1.1 Sea-Floor Spreading and Plate Tectonics 5.1.2 Vine-Matthews Crustal Model 5.1.3 Measurement of Marine Magnetic Anomalies 5.1.4 Nature of the Magnetic Anomaly Source

183 183 188 189 191

5.2 Modeling Marine Magnetic Anomalies 5.2.1 Factors Affecting the Shape of AnomaHes

195 195

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Contents

5.2.2 Calculating Magnetic Anomalies

199

5.3 Analyzing Older Magnetic Anomalies 5.3.1 The Global Magnetic Anomaly Pattern 5.3.2 Magnetic Anomaly Nomenclature 5.3.3 The Cretaceous and Jurassic Quiet Zones

204 204 208 209

5.4 Paleomagnetic Poles for Oceanic Plates 5.4.1 Skewness of Magnetic Anomalies 5.4.2 Magnetization of Seamounts 5.4.3 Calculating Mean Pole Positions from Oceanic Data

212 212 214 216

5.5 Evolution of Oceanic Plates 5.5.1 The Hotspot Reference Frame 5.5.2 Evolution of the Pacific Plate

221 221 224

Chapter 6 Continental Paleomagnetism 6.1 Analyzing Continental Data

227

6.2 Data Selection and Reliability Criteria 6.2.1 Selecting Data for Paleomagnetic Analysis 6.2.2 Reliability Criteria 6.2.3 The Global Paleomagnetic Database

228 228 228 230

6.3 Testing the Geocentric Axial Dipole Model 6.3.1 The Past 5 Million Years 6.3.2 The Past 3000 Million Years 6.3.3 Global Paleointensity Variations 6.3.4 Paleoclimates and Paleolatitudes

232 232 236 239 241

6.4 Apparent Polar Wander 6.4.1 The Concept of Apparent Polar Wander 6.4.2 Determining Apparent Polar Wander Paths 6.4.3 Magnetic Blocking Temperatures and Isotopic Ages

245 245 246 249

6.5 Phanerozoic APWPs for the Major Blocks 6.5.1 Selection and Grouping of Data 6.5.2 North America and Europe 6.5.3 Asia 6.5.4 The Gondwana Continents

251 251 252 261 269

Chapter 7 Paleomagnetism and Plate Tectonics 7.1 Plate Motions and Paleomagnetic Poles 7.1.1 Combining Euler and Paleomagnetic Poles

281 281

Contents

7.1.2 Making Reconstructions from Paleomagnetism

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287

7.2 Phanerozoic Supercontinents 7.2.1 Laurussia 7.2.2 Paleo-Asia 7.2.3 Gondwana 7.2.4 Pangea 7.2.5 Paleogeography: 300 Ma to the present

289 289 291 295 298 301

7.3 Displaced Terranes 7.3.1 Western North America 7.3.2 The East and West Avalon Terranes 7.3.3 Armorica 7.3.4 The Western Mediterranean 7.3.5 South and East Asia

303 303 306 308 310 312

7.4 Rodinia and the Precambrian 7.4.1 Rodinia 7.4.2 Paleomagnetism and Rodinia 7.4.3 Earth History: Ma to the Present 7.4.4 Precambrian Cratons

315 315 317 321 323

7.5 Non-Plate Tectonic Hypotheses 7.5.1 True Polar Wander 7.5.2 An Expanding Earth?

325 325 330

References

333

Index

311

This Page Intentionally Left Blank

Preface

This book is the sequel to Palaeomagnetism and Plate Tectonics written by Michael W. McElhinny, first published in 1973. The aim of that book was to explain the intricacies of paleomagnetism and of plate tectonics and then to demonstrate that paleomagnetism confirmed the validity of the new paradigm. Today it is no longer necessary to explain plate tectonics, but paleomagnetism has progressed rapidly over the past 25 years. Furthermore, magnetic anomaly data over most of the oceans have been analyzed in the context of sea-floor spreading and reversals of the Earth's magnetic field. Oceanic data can also be used to determine paleomagnetic poles by combining disparate types of data, from deep-sea cores, seamounts, and magnetic anomalies. Our aim here is to explain paleomagnetism and its contribution in both the continental and the oceanic environment, following the general outline of the initial book. We demonstrate the use of paleomagnetism in determining the evolution of the Earth's crust. Our intention has been to write a text that can be understood by Earth-science undergraduates at about second-year level. To make the text as accessible as possible, we have kept the mathematics to a minimum. The book can be considered a companion volume to The Magnetic Field of the Earth by Ronald T. Merrill, Michael W. McElhinny, and Phillip L. McFadden, which was published in the same series in 1996. There is inevitably some overlap between the books, occurring mostly in Chapter 4. However, the emphasis is different, with this text concentrating more on the geological aspects. Chapter 1 introduces geomagnetism and explains the basis of paleomagnetism in that context. It follows the original book quite closely. Chapter 2 is about rock magnetism and the magnetic minerals that are important in paleomagnetism. The theory of rock magnetism is an essential part of understanding how and why paleomagnetism works. Chapter 3 deals with field and laboratory methods and techniques. The chapter concludes with a summary of some methods for identifying magnetic minerals. Chapter 4 describes the evidence for magnetic field reversals and their paleomagnetic applications. The development of the

XI

xii

Preface

geomagnetic polarity time scale and its application to magnetostratigraphy are highlighted, together with the analysis of reversal sequences. Oceanic paleomagnetism, including the modeling and interpretation of marine magnetic anomalies, is discussed in Chapter 5. Methods for determining pole positions using oceanic paleomagnetic data are also covered. Global maps in color show the age of the ocean floor and of the evolution of the Pacific Ocean. Chapter 6 summarizes the results from continental paleomagnetism and includes methods of data selection and combination to produce apparent polar wander paths. Reference apparent polar wander paths are then compiled and presented for each of the Earth's major crustal blocks. Chapter 7 puts it all together and relates the results to global tectonics. Here we emphasize only the major features of global tectonic history that can be deduced from paleomagnetism. Van der Voo (1993) gives an excellent detailed account of the application of paleomagnetism to tectonics, and it is not our intention, in a single chapter, to provide readers with that level of detail and analysis. Color paleogeographic maps illustrate continental evolution since the Late Permian. A new and exciting development in global tectonics is the hypothesis of a Neoproterozoic supercontinent named Rodinia. Paleomagnetism is playing and will continue to play an important role in determining its configuration and evolution. With this in mind we discuss Earth history from 1000 Ma to the present through a combination of geology with paleomagnetism. In writing the book we have had discussions with many colleagues. We thank Jean Besse, Dave Engebretson, Dennis Kent, Zheng-Xiang Li, Roger Larson, Dietmar Muller, Andrew Newell, Neil Opdyke, Chris Powell, Phil Schmidt, Chris Scotese, Jean-Pierre Valet, and Rob Van der Voo for their assistance in providing us with materials. Our special thanks go to Charlie Barton, Steve Cande, Jo Lock, Helen McFadden, Ron Merrill, and Sergei Pisarevsky, who read parts of the book and made valuable comments. Mike McElhinny thanks Vincent Courtillot and the Institute de Physique du Globe de Paris for providing financial assistance for a visit to that institute in 1997, during which time he commenced writing the book. Phil McFadden thanks Helen Hunt and Christine Hitchman for their assistance in preparing the manuscript, and Neil Williams and Trevor Powell for their continued support. Hat Head and Canberra April 1999

Michael W. McElhinny Phillip L. McFadden

Chapter One

Geomagnetism and Paleomagnetism

1.1 Geomagnetism 1.1.1 Historical The properties of lodestone (now known to be magnetite) were known to the Chinese in ancient times. The earhest known form of magnetic compass was invented by the Chinese probably as early as the 2" century B.C., and consisted of a lodestone spoon rotating on a smooth board (Needham, 1962; see also Merrill et ai, 1996). It was not until the 12^^ century A.D. that the compass arrived in Europe, where the first reference to it is made in 1190 by an English monk, Alexander Neckham. During the 13*^ century, it was noted that the compass needle pointed toward the pole star. Unlike other stars, the pole star appeared to be fixed in the sky, so it was concluded that the lodestone with which the needle was rubbed must obtain its "virtue" from this star. In the same century it was suggested that, in some way, the magnetic needle was affected by masses of lodestone on the Earth itself. This produced the idea of polar lodestone mountains, which had the merit at least of bringing magnetic directivity down to the Earth from the heavens for the first time (Smith, 1968). Roger Bacon in 1216 first questioned the universality of the north-south directivity of the compass needle. A few years later Petrus Peregrinus questioned the idea of polar lodestone deposits, pointing out that lodestone deposits exist in many parts of the world, so why should the polar ones have preference? Petrus Peregrinus reported, in his Epistola de Magnete in 1269, a remarkable series of experiments with spherical pieces of lodestone (Smith, 1970a). He defined the

2

Paleomagnetism: Continents and Oceans

concept of polarity for the first time in Europe, discovered magnetic meridians, and showed several ways of determining the positions of the poles of a lodestone sphere, each method illustrating an important magnetic property. He thus discovered the dipolar nature of the magnet, that the magnetic force is both strongest and vertical at the poles, and became the first person to formulate the law that like poles repel and unlike poles attract. The Epistola bears a remarkable resemblance to a modem scientific paper. Peregrinus used his experimental data as a source for new conclusions, unlike his contemporaries who sought to reconcile observations with pre-existing speculation. Although written in 1269 and widely circulated during the succeedmg centuries, the Epistola was not published in printed form under Peregrinus' name until 1558. Magnetic declination was known to the Chinese from about 720 A.D. (Needham, 1962; Smith and Needham, 1967), but knowledge of this did not travel to Europe with the compass. It was not rediscovered until the latter part of the 15^ century. By the end of that century, following the voyages of Columbus, the great age of exploration by sea had begun and the compass was well established as an aid to navigation. Magnetic inclination (or dip) was discovered by Georg Hartmann m 1544, but this discovery was not publicized. In 1576 it was independently discovered by Robert Norman. Mercator, in a letter in 1546, first realized from observations of magnetic declination that the point which the needle seeks could not lie in the heavens, leading him to fix the magnetic pole firmly on the Earth. Norman and Borough subsequently consolidated the view that magnetic directivity was associated with the Earth and began to realize that the cause was not the polar region but lay closer to the center of the Earth. In 1600, William Gilbert published the results of his experimental studies in magnetism in what is usually regarded as the first scientific treatise ever written, entitled De Magnete. However, credit for writing the first scientific treatise should probably be given to Petrus Peregrinus for his Epistola de Magnete; Gilbert, whose work strongly influenced the course of magnetic study, must certainly have leaned heavily on this previous work (Smith, 1970a). He investigated the variation in inclination over the surface of a piece of lodestone cut into the shape of a sphere and summed up his conclusions in his statement "magnus magnes ipse est globus terrestris'' (the Earth globe itself is a great magnet). Gilbert's work, confirming that the geomagnetic field is primarily dipolar, thus represented the culmination of many centuries of thought and experimentation on the subject. His conclusions put a stop to the wild speculations that were then common concerning magnetism and the magnetic needle. Apart from the roundness of the Earth, magnetism was the first property to be attributed to the body of the Earth as a whole. Newton's theory of gravitation came 87 years later with the publication of his Principia,

Geomagnetism and Paleomagnetism

3

1.1.2 Main Features of the Geomagnetic Field If a magnetic compass needle is weighted so as to swing horizontally, it takes up a definite direction at each place and its deviation from geographical or true north is called the declination (or magnetic variation), D. In geomagnetic studies D is reckoned positive or negative according as the deviation is east or west of true north. In paleomagnetic studies D is always measured clockwise (eastwards) from the present geographic north and consequently takes on any angle between 0° and 360°. The direction to which the needle points is called magnetic north and the vertical plane through this direction is called the magnetic meridian. A needle perfectly balanced about a horizontal axis (before being magnetized), so placed that it can swing freely m the plane of the magnetic meridian, is called a dip needle. After magnetization it takes up a position inclined to the horizontal by an angle called the inclination (or dip), I. The inclination is reckoned positive when the north-seeking end of the needle points downwards (as in the northern hemisphere) or negative when it points upwards (as in the southern hemisphere). The main elements of the geomagnetic field are illustrated in Fig. 1.1. The total intensity F, declination Z), and inclination /, completely define the field at any point. The horizontal and vertical components of F are denoted by H and Z. Z is reckoned positive downwards as for /. The horizontal component can be X North (geographic)

Fig. 1.1. The main elements of the geomagnetic field. The deviation, D, of a compass needle from true north is referred to as the declination (reckoned positive eastwards). The compass needle lies in the magnetic meridian containing the total field F, which is at an angle /, termed the inclination (or dip), to the horizontal. The inclination is reckoned positive downwards (as in the northern hemisphere) and negative upwards (as in the southern hemisphere). The horizontal (H) and vertical (Z) components of Fare related as given by (1.1.1) to (1.1.3). From Merrill etal. (1996).

4

Paleomagnetism: Continents and Oceans

resolved into two components, X (northwards) and Y (eastwards). The various components are related by the equations: H = FcosI, X=HcosD,

Z = FsmI,

Y=HsmD,

F' =H^-hZ'

tmI = Z/H;

tmD =

= X' + Y^+Z^

Y/X;

(1.1.1) (1.1.2) (1.1.3)

Variations in the geomagnetic field over the Earth's surface are illustrated by isomagnetic maps. An example is shown in Fig. 1.2, which gives the variation of inclination over the surface of the Earth for the year 1995. A complete set of isomagnetic maps for this epoch is given in Merrill et al. (1996). The path along which the inclination is zero is called the magnetic equator, and the magnetic poles (or dip poles) are the principal points where the inclination is vertical, i.e. ±90°. The north magnetic pole is situated where / = +90°, and the south magnetic pole where / = -90°. The strength, or intensity, of the Earth's magnetic field is commonly expressed in Tesla (T) in the SI system of units (see §2.1.1 for discussion of magnetic fields). The maximum value of the Earth's magnetic field at the surface is currently about 70 |LtT in the region of the south magnetic pole. Small variations are measured in nanotesla (1 nT = 10"^ T). Gilbert's observation that the Earth is a great magnet, producing a magnetic field similar to a uniformly magnetized sphere, was first put to mathematical analysis by Gauss (1839) (see §1.1.3). The best-fit geocentric dipole to the Earth's magnetic field is inclined at 10!/2°to the Earth's axis of rotation. If the axis of this geocentric dipole is extended, it intersects the Earth's surface at two points that in 1995 were situated at 79.3°N, 71.4°W (in northwest Greenland)

Fig. 1.2. Isoclinic (lines of constant inclination) chart for 1995 showing the variation of inclination in degrees over the Earth's surface.

Geomagnetism and Paleomagnetism Geomagnetic \

N v ] y Geographic pole

North magnetic pole(/=+90^^)

South magnetic pole (1 = -90'^) g ^^^.^Geomagnetic Geographic pole south pole Fig. 1.3. Illustrating the distinction between the magnetic, geomagnetic, and geographic poles and equators. From McElhinny (1973a).

and 79.3°S, 108.6°E (in Antarctica). These points are called the geomagnetic poles (boreal and austral, or north and south respectively) and must be carefully distinguished from the magnetic poles (see preceding paragraph). The great circle on the Earth's surface coaxial with the dipole axis and midway between the geomagnetic poles is called the geomagnetic equator and is different from the magnetic equator (which is not in any case a circle). Figure 1.3 distinguishes between the magnetic elements (which are those actually observed at each point) and the geomagnetic elements (which are those related to the best fitting geocentric dipole). In 1634, Gellibrand discovered that the magnetic inclination at any place changed with time. He noted that whereas Borough in 1580 had measured a value of 11.3°E for the declination at London, his own measurements in 1634 gave only 4.1°E. The difference was far greater than possible experimental error. The gradual change in magnetic field with time is called the secular variation and is observed in all the magnetic elements. The secular variation of the direction of the geomagnetic field at London and Hobart since about 1580 is shown in Fig. 1.4. At London the changes in declination have been quite large, from 11!/2°E in 1576 to 24°W in 1823, before turning eastward again. For a sunilar time interval the declination changes in Hobart have been less extreme.

Paleomagnetism: Continents and Oceans

Declination 350°

340° T

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0° n—J

1

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10° 1

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