Global Tectonics E-Book

Philip Kearey

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 1150

Veröffentlichungsjahr: 2013

Beschreibung

The third edition of this widely acclaimed textbook provides a comprehensive introduction to all aspects of global tectonics, and includes major revisions to reflect the most significant recent advances in the field.

• A fully revised third edition of this highly acclaimed text written by eminent authors including one of the pioneers of plate tectonic theory
  
• Major revisions to this new edition reflect the most significant recent advances in the field, including new and expanded chapters on Precambrian tectonics and the supercontinent cycle and the implications of plate tectonics for environmental change
  
• Combines a historical approach with process science to provide a careful balance between geological and geophysical material in both continental and oceanic regimes
  
• Dedicated website available at www.blackwellpublishing.com/kearey/

Leseprobe

Contents

Preface

Acknowledgments

The geologic timescale and stratigraphic column

1 Historical perspective

1.1 Continental drift

1.2 Sea floor spreading and the birth of plate tectonics

1.3 Geosynclinal theory

1.4 Impact of plate tectonics

2 The interior of the Earth

2.1 Earthquake seismology

2.2 Velocity structure of the Earth

2.3 Composition of the Earth

2.4 The crust

2.5 Ophiolites

2.6 Metamorphism of oceanic crust

2.7 Differences between continental and oceanic crust

2.8 The mantle

2.9 The core

2.10 Rheology of the crust and mantle

2.11 Isostasy

2.12 Lithosphere and asthenosphere

2.13 Terrestrial heat flow

3 Continental drift

3.1 Introduction

3.2 Continental reconstructions

3.3 Geologic evidence for continental drift

3.4 Paleoclimatology

3.5 Paleontologic evidence for continental drift

3.6 Paleomagnetism

4 Sea floor spreading and transform faults

4.1 Sea floor spreading

4.2 Transform faults

5 The framework of plate tectonics

5.1 Plates and plate margins

5.2 Distribution of earthquakes

5.3 Relative plate motions

5.4 Absolute plate motions

5.5 Hotspots

5.6 True polar wander

5.7 Cretaceous superplume

5.8 Direct measurement of relative plate motions

5.9 Finite plate motions

5.10 Stability of triple junctions

5.11 Present day triple junctions

6 Ocean ridges

6.1 Ocean ridge topography

6.2 Broad structure of the upper mantle below ridges

6.3 Origin of anomalous upper mantle beneath ridges

6.4 Depth–age relationship of oceanic lithosphere

6.5 Heat flow and hydrothermal circulation

6.6 Seismic evidence for an axial magma chamber

6.7 Along-axis segmentation of oceanic ridges

6.8 Petrology of ocean ridges

6.9 Shallow structure of the axial region

6.10 Origin of the oceanic crust

6.11 Propagating rifts and microplates

6.12 Oceanic fracture zones

7 Continental rifts and rifted margins

7.1 Introduction

7.2 General characteristics of narrow rifts

7.3 General characteristics of wide rifts

7.4 Volcanic activity

7.5 Rift initiation

7.6 Strain localization and delocalization processes

7.7 Rifted continental margins

7.8 Case studies: the transition from rift to rifted margin

7.9 The Wilson cycle

8 Continental transforms and strike-slip faults

8.1 Introduction

8.2 Fault styles and physiography

8.3 The deep structure of continental transforms

8.4 Transform continental margins

8.5 Continuous versus discontinuous deformation

8.6 Strain localization and delocalization mechanisms

8.7 Measuring the strength of transforms

9 Subduction zones

9.1 Ocean trenches

9.2 General morphology of island arc systems

9.3 Gravity anomalies of subduction zones

9.4 Structure of subduction zones from earthquakes

9.5 Thermal structure of the downgoing slab

9.6 Variations in subduction zone characteristics

9.7 Accretionary prisms

9.8 Volcanic and plutonic activity

9.9 Metamorphism at convergent margins

9.10 Backarc basins

10 Orogenic belts

10.1 Introduction

10.2 Ocean–continent convergence

10.3 Compressional sedimentary basins

10.4 Continent–continent collision

10.5 Arc–continent collision

10.6 Terrane accretion and continental growth

11 Precambrian tectonics and the supercontinent cycle

11.1 Introduction

11.2 Precambrian heat flow

11.3 Archean tectonics

11.4 Proterozoic tectonics

11.5 The supercontinent cycle

12 The mechanism of plate tectonics

12.1 Introduction

12.2 Contracting Earth hypothesis

12.3 Expanding Earth hypothesis

12.4 Implications of heat flow

12.5 Convection in the mantle

12.6 The forces acting on plates

12.7 Driving mechanism of plate tectonics

12.8 Evidence for convection in the mantle

12.9 The nature of convection in the mantle

12.10 Plumes

12.11 The mechanism of the supercontinent cycle

13 Implications of plate tectonics

13.1 Environmental change

13.2 Economic geology

13.3 Natural hazards

Review questions

References

Index

Color plates

This edition first published 2009, © 2009 by Philip Kearey, Keith A. Klepeis, Frederick J. Vine

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Library of Congress Cataloguing-in-Publication Data

Kearey, P.

Global tectonics. – 3rd ed. / Philip Kearey, Keith A. Klepeis, Frederick J. Vine

p. cm.

Includes bibliographical references and index.

ISBN 978-1-4051-0777-8 (pbk. : alk. paper)     1. Plate tectonics–Textbooks.     I. Klepeis, Keith A.     II. Vine, F. J.     III. Title.

QE511.4.K43 2008

551.1′36–dc22

2007020963

A catalogue record for this book is available from the British Library.

Preface

As is well known, the study of tectonics, the branch of geology dealing with large-scale Earth structures and their deformation, experienced a major breakthrough in the 1960s with the formulation of plate tectonics. The simultaneous confirmation of sea floor spreading and continental drift, together with the recognition of transform faults and subduction zones, derived from the interpretation of new and improved data from the fields of marine geology and geophysics, and earthquake seismology. By 1970 the essentials of plate tectonics – the extent of plates, the nature of the plate boundaries, and the geometry and kinematics of their relative and finite motions – were well documented.

As further details emerged, it soon became apparent that plates and plate boundaries are well-defined in oceanic areas, where the plates are young, relatively thin,but rigid, and structurally rather uniform, but that this is not true for continental areas. Where plates have continental crust embedded in them they are generally thicker,older and structurally more complex than oceanic plates. Moreover the continental crust itself is relatively weak and deforms more readily by fracture and even by flow. Thus the nature of continental tectonics is more complex than a simple application of plate tectonic theory would predict and it has taken much longer to document and interpret. An important element in this has been the advent of Global Positioning data that have revealed details of the deformation field in complex areas.

The other major aspect of plate tectonics in which progress initially was slow is the driving mechanism for plate motions. Significant progress here had to await the development of new seismologic techniques and advances in laboratory and computer modeling of convection in the Earth’s mantle.

Since 1990, when the first edition of Global Tectonics appeared, there have been many developments in our understanding of Earth structure and its formation, particularly in relation to continental tectonics and mantle convection. As a consequence, approximately two-thirds of the figures and two-thirds of the text in this third edition are new. The structure of the book is largely unchanged. The order in which data and ideas are presented is in part historical, which may be of some interest in itself, but it has the advantage of moving from simple to more complex concepts, from the recent to the distant past, and from the oceanic to the continental realms. Thus one moves from consideration of the fundamentals of plate tectonics, which are best illustrated with reference to the ocean basins, to continental tectonics, culminating in Precambrian tectonics, and a discussion of the possible nature of the implied convection in the mantle.

The book is aimed at senior undergraduate students in the geological sciences and postgraduate students and other geoscientists who wish to gain an insight into the subject. We assume a basic knowledge of geology, and that for a full description of geophysical and geochemical methodology it will be necessary to refer to other texts. We have attempted to provide insights into the trends of modern research and the problems still outstanding, and have supplied a comprehensive list of references so that the reader can follow up any item of particular interest. We have included a list of questions for the use of tutors in assessing the achievement of their students in courses based on the book. These are mainly designed to probe the students’ integrative powers, but we hope that in their answers students will make use of the references given in the text and material on relevant websites listed on the book’s website at: http://www.blackwellpublishing.com/kearey

The initial impact of the plate tectonic concept, in the fields of marine geology and geophysics and seismology, was quickly followed by the realization of its relevance to igneous and metamorphic petrology, paleontology, sedimentary and economic geology, and all branches of goescience. More recently its potential relevance to the Earth system as a whole has been recognized. In the past, processes associated with plate tectonics may have produced changes in seawater and atmospheric chemistry, in sea level and ocean currents, and in the Earth’s climate. These ideas are briefly reviewed in an extended final chapter on the implications of plate tectonics. This nextension of the relevance of plate tectonics to the atmosphere and oceans, to the evolution of life, and possibly even the origin of life on Earth is particularly gratifying in that it emphasizes the way in which the biosphere, atmosphere, hydrosphere, and solid Earth are interrelated in a single, dynamic Earth system.

K.A. KLEPEIS F.J. VINE

A companion resources website for this book is available at : http://www.blackwellpublishing.com/kearey

Acknowledgments

The first two editions of Global Tectonics were largely written by Phil Kearey. Tragically Phil died, suddenly, in 2003 at the age of 55, just after starting work on a third edition. We are indebted to his wife, Jane, for encouraging us to complete a third edition. Phil had a particular gift for writing succinct and accessible accounts of often difficult concepts, which generations of students have been thankful for. We are very conscious of the fact that our best efforts to emulate his style have often fallen short.

We thank Cynthia Ebinger, John Hopper, John Oldow, and Peter Cawood for providing thoughtful reviews of the original manuscript. Ian Bastow, José Cembrano, Ron Clowes, Barry Doolan, Mian Liu, Phil Hammer, and Brendan Meade provided helpful comments on specific aspects of some chapters. KAK wishes to thank Gabriela Mora-Klepeis for her excellent research assistance and Pam and Dave Miller for their support.

K.A.K.F.J.V.

The Geologic Timescale and Stratigraphic column

1

Historical perspective

1.1 CONTINENTAL DRIFT

Although the theory of the new global tectonics, or plate tectonics, has largely been developed since 1967, the history of ideas concerning a mobilist view of the Earth extends back considerably longer (Rupke, 1970; Hallam, 1973a; Vine, 1977; Frankel, 1988). Ever since the coastlines of the continents around the Atlantic Ocean were first charted, people have been intrigued by the similarity of the coastlines of the Americas and of Europe and Africa. Possibly the first to note the similarity and suggest an ancient separation was Abraham Ortelius in 1596 (Romm, 1994). In 1620, Francis Bacon, in his Novum Organum, commented on the similar form of the west coasts of Africa and South America: that is, the Atlantic coast of Africa and the Pacific coast of South America. He also noted the similar configurations of the New and Old World, “both of which are broad and extended towards the north, narrow and pointed towards the south.” Perhaps because of these observations, for there appear to be no others, Bacon is often erroneously credited with having been first to notice the similarity or “fit” of the Atlantic coastlines of South America and Africa and even with having suggested that they were once together and had drifted apart. In 1668, François Placet, a French prior, related the separation of the Americas to the Flood of Noah. Noting from the Bible that prior to the flood the Earth was one and undivided, he postulated that the Americas were formed by the conjunction of floating islands or separated from Europe and Africa by the destruction of an intervening landmass, “Atlantis.” One must remember, of course, that during the 17th and 18th centuries geology, like most sciences, was carried out by clerics and theologians who felt that their observations, such as the occurrence of marine fossils and water-lain sediments on high land, were explicable in terms of the Flood and other biblical catastrophes.

Another person to note the fit of the Atlantic coastlines of South America and Africa and to suggest that they might once have been side by side was Theodor Christoph Lilienthal, Professor of Theology at Königsberg in Germany. In a work dated 1756 he too related their separation to biblical catastrophism, drawing on the text, “in the days of Peleg, the earth was divided.” In papers dated 1801 and 1845, the German explorer Alexander von Humbolt noted the geometric and geologic similarities of the opposing shores of the Atlantic, but he too speculated that the Atlantic was formed by a catastrophic event, this time “a flow of eddying waters … directed first towards the north-east, then towards the north-west, and back again to the north-east … What we call the Atlantic Ocean is nothing else than a valley scooped out by the sea.” In 1858 an American, Antonio Snider, made the same observations but postulated “drift” and related it to “multiple catastrophism” – the Flood being the last major catastrophe. Thus Snider suggested drift sensu stricto, and he even went so far as to suggest a pre-drift reconstruction (Fig. 1.1).

Figure 1.1Snider’s reconstruction of the continents (Snider, 1858).

The 19th century saw the gradual replacement of the concept of catastrophism by that of “uniformitarianism” or “actualism” as propounded by the British geologists James Hutton and Charles Lyell. Hutton wrote “No powers are to be employed that are not natural to the globe, no action to be admitted of except those of which we know the principle, and no extraordinary events to be alleged in order to explain a common appearance.” This is usually stated in Archibald Geikie’s paraphrase of Hutton’s words, “the present is the key to the past,” that is, the slow processes going on at and beneath the Earth’s surface today have been going on throughout geologic time and have shaped the surface record. Despite this change in the basis of geologic thought, the proponents of continental drift still resorted to catastrophic events to explain the separation of the continents. Thus, George Darwin in 1879 and Oswald Fisher in 1882 associated drift with the origin of the Moon out of the Pacific. This idea persisted well into the 20th century, and probably accounts in part for the reluctance of most Earth scientists to consider the concept of continental drift seriously during the first half of the 20th century (Rupke, 1970).

Figure 1.2Taylor’s mechanism for the formation of Cenozoic mountain belts by continental drift (after Taylor, 1910).

A uniformitarian concept of drift was first suggested by F.B. Taylor, an American physicist, in 1910, and Alfred Wegener, a German meteorologist, in 1912. For the first time it was considered that drift is taking place today and has taken place at least throughout the past 100–200 Ma of Earth history. In this way drift was invoked to account for the geometric and geologic similarities of the trailing edges of the continents around the Atlantic and Indian oceans and the formation of the young fold mountain systems at their leading edges. Taylor, in particular, invoked drift to explain the distribution of the young fold mountain belts and “the origin of the Earth’s plan” (Taylor, 1910) (Fig. 1.2 and Plate 1.1 between pp. 244 and 245).

The pioneer of the theory of continental drift is generally recognized as Alfred Wegener, who as well as being a meteorologist was an astronomer, geophysicist, and amateur balloonist (Hallam, 1975), and he devoted much of his life to its development. Wegener detailed much of the older, pre-drift, geologic data and maintained that the continuity of the older structures, formations and fossil faunas and floras across present continental shorelines was more readily understood on a pre-drift reconstruction. Even today, these points are the major features of the geologic record from the continents which favor the hypothesis of continental drift. New information, which Wegener brought to his thesis, was the presence of a widespread glaciation in Permo-Carboniferous times which had affected most of the southern continents while northern Europe and Greenland had experienced tropical conditions. Wegener postulated that at this time the continents were joined into a single landmass, with the present southern continents centered on the pole and the northern continents straddling the equator (Fig. 1.3). Wegener termed this continental assembly Pangea (literally “all the Earth”) although we currently prefer to think in terms of A. du Toit’s idea of it being made up of two supercontinents (du Toit, 1937) (Fig. 11.27). The more northerly of these is termed Laurasia (from a combination of Laurentia, a region of Canada, and Asia), and consisted of North America, Greenland, Europe, and Asia. The southerly supercontinent is termed Gondwana (literally “land of the Gonds” after an ancient tribe of northern India), and consisted of South America, Antarctica, Africa, Madagascar, India, and Australasia. Separating the two supercontinents to the east was a former “Mediterranean” sea termed the paleo-Tethys Ocean (after the Greek goddess of the sea), while surrounding Pangea was the proto-Pacific Ocean or Panthalassa (literally “all-ocean”).

Figure 1.3Wegener’s reconstruction of the continents (Pangea), with paleoclimatic indicators, and paleopoles and equator for (a) Carboniferous and (b) Permian time. I, ice; C, coal; S, salt; G, gypsum; D, desert sandstone; hatched areas, arid zones (modified from Wegener, 1929, reproduced from Hallam, 1973a, p. 19, by permission of Oxford University Press).

Wegener propounded his new thesis in a book Die Entstehung der Kontinente and Ozeane (The Origin of Continents and Oceans), of which four editions appeared in the period 1915–29. Much of the ensuing academic discussion was based on the English translation of the 1922 edition which appeared in 1924, consideration of the earlier work having been delayed by World War I. Many Earth scientists of this time found his new ideas difficult to encompass, as acceptance of his work necessitated a rejection of the existing scientific orthodoxy, which was based on a static Earth model. Wegener based his theory on data drawn from several different disciplines, in many of which he was not an expert. The majority of Earth scientists found fault in detail and so tended to reject his work in toto. Perhaps Wegener did himself a disservice in the eclecticism of his approach. Several of his arguments were incorrect: for example, his estimate of the rate of drift between Europe and Greenland using geodetic techniques was in error by an order of magnitude. Most important, from the point of view of his critics, was the lack of a reasonable mechanism for continental movements. Wegener had suggested that continental drift occurred in response to the centripetal force experienced by the high-standing continents because of the Earth’s rotation. Simple calculations showed the forces exerted by this mechanism to be much too small. Although in the later editions of his book this approach was dropped, the objections of the majority of the scientific community had become established. Du Toit, however, recognized the good geologic arguments for the joining of the southern continents and A. Holmes, in the period 1927–29, developed a new theory of the mechanism of continental movement (Holmes, 1928). He proposed that continents were moved by convection currents powered by the heat of radioactive decay (Fig. 1.4). Although differing considerably from the present concepts of convection and ocean floor creation, Holmes laid the foundation from which modern ideas developed.

Between the World Wars two schools of thought developed – the drifters and the nondrifters, the latter far outnumbering the former. Each ridiculed the other’s ideas. The nondrifters emphasized the lack of a plausible mechanism, as we have already noted, both convection and Earth expansion being considered unlikely. The nondrifters had difficulty in explaining the present separation of faunal provinces, for example, which could be much more readily explained if the continents were formerly together, and their attempts to explain these apparent faunal links or migrations also came in for some ridicule. They had to invoke various improbable means such as island stepping-stones, isthmian links, or rafting. It is interesting to note that at this time many southern hemisphere geologists, such as du Toit, Lester King, and S.W. Carey, were advocates of drift, perhaps because the geologic record from the southern continents and India favors their assembly into a single supercontinent (Gondwana) prior to 200 Ma ago.

Very little was written about continental drift between the initial criticisms of Wegener’s book and about 1960. In the 1950s, employing methodology suggested by P.M.S. Blackett, the paleomagnetic method was developed (Section 3.6), and S.K. Runcorn and his co-workers demonstrated that relative movements had occurred between North America and Europe. The work was extended by K.M. Creer into South America and by E. Irving into Australia. Paleomagnetic results became more widely accepted when the technique of magnetic cleaning was developed in which primary magnetization could be isolated. Coupled with dating by faunal or newly developed radiometric methods, the paleomagnetic data for Mesozoic to Recent times showed that there had been significant differences, beyond the scope of error, in the motions between various continents.

An important consideration in the development of ideas relating to continental drift was that prior to World War II geologists had, necessarily, only studied the land areas. Their findings had revealed that the continental crust preserves a whole spectrum of Earth history, ranging back to nearly 4000 Ma before the present, and probably to within a few hundred million years of the age of the Earth and the solar system itself. Their studies also revealed the importance of vertical movements of the continental crust in that the record was one of repeated uplift and erosion, subsidence, and sedimentation. But as J. Tuzo Wilson, a Canadian geophysicist, said, this is like looking at the deck of a ship to see if it is moving.

Figure 1.4The concept of convection as suggested by Holmes (1928), when it was believed that the oceanic crust was a thick continuation of the continental “basaltic layer”. (a) Currents ascending at A spread laterally, place a continent under tension and split it, providing the obstruction of the old ocean floor can be overcome. This is accomplished by the formation of eclogite at B and C, where sub-continental currents meet sub-oceanic currents and turn downwards. The high density of the eclogite causes it to sink and make room for the continents to advance. (b) The foundering of eclogite at B and C contributes to the main convective circulation. The eclogite melts at depth to form basaltic magma, which rises in ascending currents at A, heals the gaps in the disrupted continent and forms new ocean floor. Local swells, such as Iceland, would be formed from old sial left behind. Smaller current systems, initiated by the buoyancy of the basaltic magma, ascend beneath the continents and feed flood basalts or, beneath “old” (Pacific) ocean floor, feed the outpourings responsible for volcanic islands and seamounts (redrawn from Holmes, 1928).

1.2 SEA FLOOR SPREADING AND THE BIRTH OF PLATE TECTONICS

If there is a possibility that the continental areas have been rifted and drifted apart and together, then presumably there should be some record of this within the ocean basins. However, it is only since World War II and notably since 1960 that sufficient data have been obtained from the 60% of the Earth’s surface covered by deep water for an understanding of the origin and history of the ocean basins to have emerged. It transpires that, in contrast to the continents, the oceanic areas are very young geologically (probably no greater than 200 Ma in age) and that horizontal, or lateral, movements have been all-important during their history of formation.

In 1961, following intensive surveying of the sea floor during post-war years, R.S. Dietz proposed the mechanism of “sea floor spreading” to explain continental drift. Although Dietz coined the term “sea floor spreading,” the concept was conceived a year or two earlier by H.H. Hess. He suggested that continents move in response to the growth of ocean basins between them, and that oceanic crust is created from the Earth’s mantle at the crest of the mid-ocean ridge system, a volcanic submarine swell or rise which occupies a median position in many of the world’s oceans (Fig. 1.5). Oceanic crust is much thinner than continental crust, having a mean thickness of about 7km, compared with the average continental thickness of about 40 km; is chemically different; and is structurally far less complex. The lateral motion of the oceanic crust was believed to be driven by convection currents in the upper mantle in the fashion of a conveyer belt. In order to keep the surface area of the Earth constant, it was further proposed that the oceanic crust is thrust back down into the mantle and resorbed at oceanic trenches. These are vast bathymetric depressions, situated at certain ocean margins and associated with intense volcanic and earthquake activity. Within this framework the continents are quite passive elements – rafts of less dense material which are drifted apart and together by ephemeral ocean floors. The continents themselves are a scum of generally much older material that was derived or separated from the Earth’s interior either at a very early stage in its history or, at least in part, steadily throughout geologic time. Instead of blocks of crust, we now think in terms of “plates” of comparatively rigid upper mantle and crust, perhaps 50–100 km thick and which we term lithosphere (a term originally coined by R.A. Daly many years ago and meaning “rock layer”). Lithospheric plates can have both continental and oceanic crust embedded in them.

Figure 1.5The concept of sea floor spreading (after Hess, 1962).

The theory of sea floor spreading was confirmed in the period 1963–66 following the suggestion of F.J. Vine and D.H. Matthews that the magnetic lineations of the sea floor might be explained in terms of sea floor spreading and reversals of the Earth’s magnetic field (Section 4.1). On this model the conveyor belt of oceanic crust is viewed as a tape recorder which registers the history of reversals of the Earth’s magnetic field.

A further precursor to the development of the theory of plate tectonics came with the recognition, by J.T. Wilson in 1965, of a new class of faults termed transform faults, which connect linear belts of tectonic activity (Section 4.2). The Earth was then viewed as a mosaic of six major and several smaller plates in relative motion. The theory was put on a stringent geometric basis by the work of D.P. McKenzie, R.L. Parker, and W.J. Morgan in the period 1967–68 (Chapter 5), and confirmed by earthquake seismology through the work of B. Isacks, J. Oliver, and L.R. Sykes.

The theory has been considerably amplified by intensive studies of the geologic and geophysical processes affecting plate margins. Probably the aspect about which there is currently the most contention is the nature of the mechanism that causes plate motions (Chapter 12).

Although the basic theory of plate tectonics is well established, understanding is by no means complete. Investigating the implications of plate tectonics will fully occupy Earth scientists for many decades to come.

1.3 GEOSYNCLINAL THEORY

Prior to the acceptance of plate tectonics, the static model of the Earth encompassed the formation of tectonically active belts, which formed essentially by vertical movements, on the site of geosynclines. A review of the development of the geosyncline hypothesis and its explanation in terms of plate tectonics is provided by Mitchell & Reading (1986).

Geosynclinal theory envisaged elongate, geographically fixed belts of deep subsidence and thick sediments as the precursors of mountain ranges in which the strata were exposed by folding and uplift of the geosynclinal sediments (Dickinson, 1971). A plethora of specific nomenclature evolved to describe the lithological associations of the sedimentary fill and the relative locations of the geosynclines.

The greatest failing of geosynclinal theory was that tectonic features were classified without there being an understanding of their origin. Geosynclinal nomenclature consequently represented an impediment to the recognition of a common causal mechanism. The relation of sedimentation to the mobilistic mechanism of plate tectonics (Mitchell & Reading, 1969) allowed the recognition of two specific environments in which geosynclines formed, namely rifted, or trailing, continental margins and active, or leading, continental margins landward of the deep oceanic trenches. The latter are now known as subduction zones (Chapter 9). Although some workers retain geosynclinal terminology to describe sedimentary associations (e.g. the terms eugeosyncline and miogeosyncline for sediments with and without volcanic members, respectively), this usage is not recommended, and the term geosyncline must be recognized as no longer relevant to plate tectonic processes.

1.4 IMPACT OF PLATE TECTONICS

Plate tectonics is of very great significance as it represents the first theory that provides a unified explanation of the Earth’s major surface features. As such it has enabled an unprecedented linking of many different aspects of geology, which had previously been considered independent and unrelated. A deeper understanding of geology has ensued from the interpretation of many branches of geology within the basic framework provided by plate tectonics. Thus, for example, explanations can be provided for the past distributions of flora and fauna, the spatial relationships of volcanic rock suites at plate margins, the distribution in space and time of the conditions of different metamorphic facies, the scheme of deformation in mountain belts, or orogens, and the association of different types of economic deposit.

Recognition of the dynamic nature of the apparently solid Earth has led to the realization that plate tectonic processes may have had a major impact on other aspects of the Earth system in the past. Changes in volcanic activity in general, and at mid-ocean ridges in particular, would have changed the chemistry of the atmosphere and of seawater. Changes in the net accretion rate at mid-ocean ridges could explain major changes in sea level in the past, and the changing configuration of the continents, and the uplift of mountain belts would have affected both oceanic and atmospheric circulation. The nature and implications of these changes, in particular for the Earth’s climate, are explored in Chapter 13.

Clearly some of these implications were documented by Wegener, notably in relation to the distribution of fauna and flora in the past, and regional paleoclimates. Now, however, it is realized that plate tectonic processes impact on the physics and chemistry of the atmosphere and oceans, and on life on Earth, in many more ways, thus linking processes in the atmosphere, oceans, and solid Earth in one dynamic global system.

The fact that plate tectonics is so successful in unifying so many aspects of Earth science should not be taken to indicate that it is completely understood. Indeed, it is the critical testing of the implications of plate tectonic theory that has led to modifications and extrapolations, for example in the consideration of the relevance of plate tectonic processes in continental areas (Section 2.10.5) and the more distant geologic past (Chapter 11). It is to be hoped that plate tectonic theory will be employed cautiously and critically.

FURTHER READING

Hallam, A. (1973) A Revolution in the Earth Sciences: from continental drift to plate tectonics. Oxford University Press, Oxford, UK.

LeGrand, H.E. (1988) Drifting Continents and Shifting Theories. Cambridge University Press, Cambridge, UK.

Marvin, U.B. (1973) Continental Drift: the evolution of a concept. Smithsonian Institution, Washington, DC.

Oreskes, N. (1999) The Rejection of Continental Drift: theory and method in American Earth Science. Oxford University Press, New York.

Oreskes, N. (ed.) (2001) Plate Tectonics: an insider’s history of the modern theory of the Earth. Westview Press, Boulder.

Stewart, J.A. (1990) Drifting Continents and Colliding Paradigms: perspectives on the geoscience revolution. Indiana University Press, Bloomington, IN.

2

The interior of the Earth

2.1 EARTHQUAKE SEISMOLOGY

2.1.1 Introduction

Much of our knowledge of the internal constitution of the Earth has come from the study of the seismic waves generated by earthquakes. These waves follow various paths through the interior of the Earth, and by measuring their travel times to different locations around the globe it is possible to determine its large-scale layering. It is also possible to make inferences about the physical properties of these layers from a consideration of the velocities with which they transmit the seismic waves.

2.1.2 Earthquake descriptors

Earthquakes are normally assumed to originate from a single point known as the focus or hypocenter (Fig. 2.1), which is invariably within about 700 km of the surface. In reality, however, most earthquakes are generated by movement along a fault plane, so the focal region may extend for several kilometers. The point on the Earth’s surface vertically above the focus is the epicenter. The angle subtended at the center of the Earth by the epicenter and the point at which the seismic waves are detected is known as the epicentral angle Δ. The magnitude of an earthquake is a measure of its energy release on a logarithmic scale; a change in magnitude of one on the Richter scale implies a 30-fold increase in energy release (Stein & Wysession, 2003).

Figure 2.1Illustration of epicentral angle Δ.

2.1.3 Seismic waves

The strain energy released by an earthquake is transmitted through the Earth by several types of seismic wave (Fig. 2.2), which propagate by elastic deformation of the rock through which they travel. Waves penetrating the interior of the Earth are known as body waves, and consist of two types corresponding to the two possible ways of deforming a solid medium. P waves, also known as longitudinal or compressional waves, correspond to elastic deformation by compression/dilation. They cause the particles of the transmitting rock to oscillate in the direction of travel of the wave so that the disturbance proceeds as a series of compressions and rarefactions. The velocity of a P wave Vp is given by:

where k is the bulk modulus, μ the shear modulus (rigidity), and ρ the density of the transmitting medium. S waves, also known as shear or transverse waves, correspond to elastic deformation of the transmitting medium by shearing and cause the particles of the rock to oscillate at right angles to the direction of propagation. The velocity of an S wave Vs is given by:

Because the rigidity of a fluid is zero, S waves cannot be transmitted by such a medium.

Figure 2.2Focus and epicenter of an earthquake and the seismic waves originating from it (after Davies, 1968, with permission from Iliffe Industrial Publications Ltd).

A consequence of the velocity equations for P and S waves is that the P velocity is about 1.7 times greater than the S velocity in the same medium. Consequently, for an identical travel path, P waves arrive before S waves. This was recognized early in the history of seismology, and is reflected in the names of the body waves (P is derived from primus and S from secundus). The passage of body waves through the Earth conforms to the laws of geometric optics in that they can be both refracted and reflected at velocity discontinuities.

Seismic waves whose travel paths are restricted to the vicinity of a free surface, such as the Earth’s surface, are known as surface waves. Rayleigh waves cause the particles of the transmitting medium to describe an ellipse in a vertical plane containing the direction of propagation. They can be transmitted in the surface of a uniform half space or a medium in which velocity changes with depth. Love waves are transmitted whenever the S wave velocity of the surface layer is lower than that of the underlying layer. Love waves are essentially horizontally polarized shear waves, and propagate by multiple reflection within this low velocity layer, which acts as a wave guide.

Surface waves travel at lower velocities than body waves in the same medium. Unlike body waves, surface waves are dispersive, that is, their different wavelength components travel at different velocities. Dispersion arises because of the velocity stratification of the Earth’s interior, longer wavelengths penetrating to greater depths and hence sampling higher velocities. As a result, surface wave dispersion studies provide an important method of determining the velocity structure and seismic attenuation characteristics of the upper 600 km of the Earth.

2.1.4 Earthquake location

Earthquakes are detected by seismographs, instruments that respond to very small ground displacements, velocities, or accelerations associated with the passage of seismic waves. Since 1961 there has been an extensive and standardized global network of seismograph stations to monitor earthquake activity. The original World-Wide Standardized Seismograph Network (WWSSN), based on analogue instruments, has gradually been superseded since 1986 by the Global (Digital) Seismograph Network (GSN). By 2004 there were 136 well-distributed GSN stations worldwide, including one on the sea floor between Hawaii and California. It is hoped that this will be the first of several in oceanic areas devoid of oceanic islands for land-based stations. Digital equipment greatly facilitates processing of the data and also has the advantage that it records over a much greater dynamic range and frequency bandwidth than the earlier paper and optical recording. This is achieved by a combination of high frequency, low gain and very broadband seismometers (Butler et al., 2004). Most countries have at least one GSN station and many countries also have national seismometer arrays. Together these stations not only provide the raw data for all global and regional seismological studies but also serve an important function in relation to monitoring the nuclear test ban treaty, and volcano and tsunami warning systems.

Earthquakes occurring at large, or teleseismic, distances from a seismograph are located by the identification of various phases, or seismic arrivals, on the seismograph records. Since, for example, the direct P and S waves travel at different velocities, the time separation between the arrival of the P phase and the S phase becomes progressively longer as the length of the travel path increases. By making use of a standard model for the velocity stratification of the Earth, and employing many seismic phases corresponding to different travel paths along which the seismic waves are refracted or reflected at velocity discontinuities, it is possible to translate the differences in their travel times into the distance of the earthquake from the observatory. Triangulation using distances computed in this way from many observatories then allows the location of the epicenter to be determined.

The focal depths of teleseismic events are determined by measuring the arrival time difference between the direct phase P and the phase pP (Båth, 1979). The pP phase is a short path multiple event which follows a similar path to P after first undergoing a reflection at the surface of the Earth above the focus, and so the P–pP time difference is a measure of focal depth. This method is least accurate for foci at depths of less than 100 km as the P–pP time separation becomes very small. The focal depths of local earthquakes can be determined if a network of seismographs exists in the vicinity of the epicenter. In this case the focal depth is determined by triangulation in the vertical plane, using the P–S time difference to calculate the distance to the focus.

Figure 2.3Elastic rebound mechanism of earthquake generation.

2.1.5 Mechanism of earthquakes

Most earthquakes are believed to occur according to the elastic rebound theory, which was developed after the San Francisco earthquake of 1906. In this theory an earthquake represents a sudden release of strain energy that has built up over a period of time.

In Fig. 2.3a a block of rock traversed by a pre-existing fracture (or fault) is being strained in such a way as eventually to cause relative motion along the plane of the fault. The line AB is a marker indicating the state of strain of the system, and the broken line the location of the fault. Relatively small amounts of strain can be accommodated by the rock (Fig. 2.3b). Eventually, however, the strain reaches the level at which it exceeds the frictional and cementing forces opposing movement along the fault plane (Fig. 2.3c). At this point fault movement occurs instantaneously (Fig. 2.3d). The 1906 San Francisco earthquake resulted from a displacement of 6.8 m along the San Andreas Fault. In this model, faulting reduces the strain in the system virtually to zero, but if the shearing forces persist, strain would again build up to the point at which fault movement occurs. The elastic rebound theory consequently implies that earthquake activity represents a stepwise response to persistent strain.

2.1.6 Focal mechanism solutions of earthquakes

The seismic waves generated by earthquakes, when recorded at seismograph stations around the world, can be used to determine the nature of the faulting associated with the earthquake, to infer the orientation of the fault plane and to gain information on the state of stress of the lithosphere. The result of such an analysis is referred to as a focal mechanism solution or fault plane solution. The technique represents a very powerful method of analyzing movements of the lithosphere, in particular those associated with plate tectonics. Information is available on a global scale as most earthquakes with a magnitude in excess of 5.5 can provide solutions, and it is not necessary to have recorders in the immediate vicinity of the earthquake, so that data are provided from regions that may be inaccessible for direct study.

According to the elastic rebound theory, the strain energy released by an earthquake is transmitted by the seismic waves that radiate from the focus. Consider the fault plane shown in Fig. 2.4 and the plane orthogonal to it, the auxiliary plane. The first seismic waves to arrive at recorders around the earthquake are P waves, which cause compression/dilation of the rocks through which they travel. The shaded quadrants, defined by the fault and auxiliary planes, are compressed by movement along the fault and so the first motion of the P wave arriving in these quadrants corresponds to a compression. Conversely, the unshaded quadrants are stretched or dilated by the fault movement. The first motion of the P waves in these quadrants is thus dilational. The region around the earthquake is therefore divided into four quadrants on the basis of the P wave first motions, defined by the fault plane and the auxiliary plane. No P waves propagate along these planes as movement of the fault imparts only shearing motions in their directions; they are consequently known as nodal planes.

Figure 2.4Quadrantal distribution of compressional and dilational P wave first motions about an earthquake.

Simplistically, then, a focal mechanism solution could be obtained by recording an earthquake at a number of seismographs distributed around its epicenter, determining the nature of the first motions of the P waves, and then selecting the two orthogonal planes which best divide compressional from dilational first arrivals, that is, the nodal planes. In practice, however, the technique is complicated by the spheroidal shape of the Earth and the progressive increase of seismic velocity with depth that causes the seismic waves to follow curved travel paths between the focus and recorders. Consider Fig. 2.5. The dotted line represents the continuation of the fault plane, and its intersection with the Earth’s surface would represent the line separating compressional and dilational first motions if the waves generated by the earthquake followed straight-line paths. The actual travel paths, however, are curved and the surface intersection of the dashed line, corresponding to the path that would have been followed by a wave leaving the focus in the direction of the fault plane, represents the actual nodal plane.

It is clear then, that simple mapping of compressional and dilational first motions on the Earth’s surface cannot readily provide the focal mechanism solution. However, the complications can be overcome by considering the directions in which the seismic waves left the focal region, as it is apparent that compressions and dilations are restricted to certain angular ranges.

Figure 2.5Distribution of compressional and dilational first arrivals from an earthquake on the surface of a spherical Earth in which seismic velocity increases with depth.

A focal mechanism solution is obtained firstly by determining the location of the focus by the method outlined in Section 2.1.4. Then, for each station recording the earthquake, a model for the velocity structure of the Earth is used to compute the travel path of the seismic wave from the focus to the station, and hence to calculate the direction in which the wave left the focal region. These directions are then plotted, using an appropriate symbol for compressional or dilational first motion, on an equal area projection of the lower half of the focal sphere, that is, an imaginary sphere of small but arbitrary radius centered on the focus (Fig. 2.5). An equal area net, which facilitates such a plot, is illustrated in Fig. 2.6. The scale around the circumference of such a net refers to the azimuth, or horizontal component of direction, while dips are plotted on the radial scale from 0° at the perimeter to 90° at the center. Planes through the focus are represented on such plots by great circles with a curvature appropriate to their dip; hence a diameter represents a vertical plane.

Let us assume that, for a particular earthquake, the fault motion is strike-slip along a near vertical fault plane. This plane and the auxiliary plane plot as orthogonal great circles on the projection of the focal sphere, as shown on Fig. 2.7. The lineation defined by the intersection of these planes is almost vertical, so it is apparent that the direction of movement along the fault is orthogonal to this intersection, that is, near horizontal. The two shaded and two unshaded regions of the projection defined by the nodal planes now correspond to the directions in which compressional and dilational first motions, respectively, left the focal region. A focal mechanism solution is thus obtained by plotting all the observational data on the projection of the focal sphere and then fitting a pair of orthogonal planes which best divide the area of the projection into zones of compressional and dilational first motions. The more stations recording the earthquake, the more closely defined will be the nodal planes.

Figure 2.6Lambert equal area net.

Figure 2.7Ambiguity in the focal mechanism solution of a strike-slip fault. Regions of compressional first motions are shaded.

2.1.7 Ambiguity in focal mechanism solutions

It is apparent from Fig. 2.7 that the same distribution of compressional and dilational quadrants would be obtained if either nodal plane represented the actual fault plane. Thus, the same pattern of first motions would be obtained for sinistral motion along a north–south plane as for dextral motion along an east–west plane.

Figure 2.8Ambiguity in the focal mechanism solution of a thrust fault. Shaded areas represent regions of compressional first motions (C), unshaded areas represent regions of dilational first motions (D), f refers to a fault plane, ap to an auxiliary plane. Changing the nature of the nodal planes as in (a) and (c) does not alter the pattern of first motions shown in (b), the projection of the lower hemisphere of the focal sphere.

In Fig. 2.8a an earthquake has occurred as a result of faulting along a westerly dipping thrust plane f1. f1 and its associated auxiliary plane ap1 divide the region around the focus into quadrants which experience either compression or dilation as a result of the fault movement. The directions in which compressional first motions C1 and C2 and dilational first motions D1 and D2 leave the focus are shown, and C2 and D2 are plotted on the projection of the focal sphere in Fig. 2.8b, on which the two nodal planes are also shown. Because Fig. 2.8a is a vertical section, the first motions indicated plot along an east–west azimuth. Arrivals at stations at other azimuths would occupy other locations within the projection space. Consider now Fig. 2.8c, in which plane ap1 becomes the fault plane f2 and f1 the auxiliary plane ap2. By considering the movement along the thrust plane it is obvious that the same regions around the fault are compressed or dilated, so that an identical focal sphere projection is obtained. Similar results are obtained when the faulting is normal (Fig. 2.9). In theory the fault plane can be distinguished by making use of Anderson’s simple theory of faulting (Section 2.10.2) which predicts that normal faults have dips of more than 45° and thrusts less than 45°. Thus, f1 is the fault plane in Fig. 2.8 and f2 the fault plane in Fig. 2.9.

Figure 2.9Ambiguity in the focal mechanism solution of a normal fault. Legend as for Fig. 2.8.

It is apparent that the different types of faulting can be identified in a focal mechanism solution by the distinctive pattern of compressional and dilational regions on the resulting focal sphere. Indeed, it is also possible to differentiate earthquakes that have originated by a combination of fault types, such as dip-slip accompanied by some strike-slip movement. The precision with which the directions of the nodal planes can be determined is dependent upon the number and distribution of stations recording arrivals from the event. It is not possible, however, to distinguish the fault and auxiliary planes.

Figure 2.10(a) P wave radiation pattern for a type I and type II earthquake source mechanism; (b) S wave radiation pattern from a type I source (single couple); (c) S wave radiation pattern from a type II source (double couple).

At one time it was believed that distinction between the nodal planes could be made on the basis of the pattern of S wave arrivals. P waves radiate into all four quadrants of the source region as shown in Fig. 2.10a. However, for this simple model, which is known as a type I, or single-couple source, S waves, whose corresponding ground motion is shearing, should be restricted to the region of the auxiliary plane (Fig. 2.10b). Recording of the S wave radiation pattern should then make it possible to determine the actual fault plane. It was found, however, that instead of this simple pattern, most earthquakes produce S wave radiation along the direction of both nodal planes (Fig. 2.10c). This observation initially cast into doubt the validity of the elastic rebound theory. It is now realized, however, that faulting occurs at an angle, typically rather less than 45% to the maximum compressive stress, σ1, and the bisectors of the dilational and compressional quadrants, termed P and T, respectively, approximate to the directions of maximum and minimum principal compressive stress, thus giving an indication of the stress field giving rise to the earthquake (Fig. 2.10c) (Section 2.10.2).

This type II, or double-couple source mechanism gives rise to a four-lobed S wave radiation pattern (Fig. 2.10c) which cannot be used to resolve the ambiguity of a focal mechanism solution. Generally, the only constraint on the identity of the fault plane comes from a consideration of the local geology in the region of the earthquake.

2.1.8 Seismic tomography

Tomography is a technique whereby three-dimensional images are derived from the processing of the integrated properties of the medium that rays encounter along their paths through it. Tomography is perhaps best known in its medical applications, in which images of specific plane sections of the body are obtained using X-rays. Seismic tomography refers to the derivation of the three-dimensional velocity structure of the Earth from seismic waves. It is considerably more complex than medical tomography in that the natural sources of seismic waves (earthquakes) are of uncertain location, the propagation paths of the waves are unknown, and the receivers (seismographs) are of restricted distribution. These difficulties can be overcome, however, and since the late 1970s seismic tomography has provided important new information on Earth structure. The method was first described by Aki et al. (1977) and has been reviewed by Dziewonski & Anderson (1984), Thurber & Aki (1987), and Romanowicz (2003).

Seismic tomography makes use of the accurately recorded travel times of seismic waves from geographically distributed earthquakes at a distributed suite of seismograph stations. The many different travel paths from earthquakes to receivers cross each other many times. If there are any regions of anomalous seismic velocity in the space traversed by the rays, the travel times of the waves crossing this region are affected. The simultaneous interpretation of travel time anomalies for the many criss-crossing paths then allows the anomalous regions to be delineated, providing a threedimensional model of the velocity space.

Both body waves and surface waves (Section 2.1.3) can be used in tomography analysis. With body waves, the actual travel times of P or S phases are utilized. The procedure with surface waves is more complex, however, as they are dispersive; that is, their velocity depends upon their wavelength. The depth of penetration of surface waves is also wavelength-dependent, with the longer wavelengths reaching greater depths. Since seismic velocity generally increases with depth, the longer wavelengths travel more rapidly. Thus, when surface waves are utilized, it is necessary to measure the phase or group velocities of their different component wavelengths. Because of their low frequency, surface waves provide less resolution than body waves. However, they sample the Earth in a different fashion and, since either Rayleigh or Love waves (Section 2.1.3) may be used, additional constraints on shear velocity and its anisotropy are provided.

The normal procedure in seismic tomography is to assume an initial “one-dimensional” model of the velocity space in which the velocity is radially symmetrical. The travel time of a body wave from earthquake to seismograph is then equal to the sum of the travel times through the individual elements of the model. Any lateral velocity variations within the model are then reflected in variations in arrival times with respect to the mean arrival time of undisturbed events. Similarly, the dispersion of surface waves across a heterogeneous model differs from the mean dispersion through a radially symmetrical model. The method makes use of a simplifying assumption based on Fermat’s Principle, which assumes that the ray paths for a radially symmetrical and laterally variable velocity model are identical if the heterogeneities are small and that the differences in travel times are caused solely by heterogeneity in the velocity structure of the travel path. This obviates the necessity of computing the new travel path implied by refractions at the velocity perturbations.

There are two main approaches to seismic tomography depending upon how the velocity heterogeneity of the model is represented. Local methods make use of body waves and subdivide the model space into a series of discrete elements so that it has the form of a threedimensional ensemble of blocks. A set of linear equations is then derived which link the anomalies in arrival times to velocity variations over the different travel paths. A solution of the equations can then be obtained, commonly using matrix inversion techniques, to obtain the velocity anomaly in each block. Global methods express the velocity variations of the model in terms of some linear combination of continuous basic functions, such as spherical harmonic functions.

Local methods can make use of either teleseismic or local events. In the teleseismic method (Fig. 2.11) a large set of distant seismic events is recorded at a network of seismographs over the volume of interest. Because of their long travel path, the incident wave fronts can be considered planar. It is assumed that deviations from expected arrival times are caused by velocity variations beneath the network. In practice, deviations from the mean travel times are computed to compensate for any extraneous effects experienced by the waves outside the volume of interest. Inversion of the series of equations of relative travel time through the volume then provides the relative velocity perturbations in each block of the model. The method can be extended by the use of a worldwide distribution of recorded teleseismic events to model the whole mantle. In the local method the seismic sources are located within the volume of interest (Fig. 2.12). In this case the location and time of the earthquakes must be accurately known, and ray-tracing methods used to construct the travel paths of the rays. The inversion procedure is then similar to that for teleseisms. One of the uses of the resulting three-dimensional velocity distributions is to improve focal depth determinations.

Figure 2.11Geometry of the teleseismic inversion method. Velocity anomalies within the compartments are derived from relative arrival time anomalies of teleseismic events (redrawn from Aki et al., 1977, by permission of the American Geophysical Union. Copyright © 1977 American Geophysical Union).

Figure 2.12Geometry of the local inversion method.

Figure 2.13Great circle paths from two earthquakes (stars) to recording stations (dots) (after Thurber & Aki, 1987).

Global methods commonly make use of both surface and body waves with long travel paths. If the Earth were spherically symmetrical, these surface waves would follow great circle routes. However, again making use of Fermat’s Principle, it is assumed that ray paths in a heterogeneous Earth are similarly great circles, with anomalous travel times resulting from the heterogeneity. In the single-station configuration, the surface wave dispersion is measured for the rays traveling directly from earthquake to receiver. Information from only moderate-size events can be utilized, but the source parameters have to be well known. The great circle method uses multiple circuit waves, that is, waves that have traveled directly from source to receiver and have then circumnavigated the Earth to be recorded again (Fig. 2.13). Here the differential dispersion between the first and second passes is measured, eliminating any undesirable source effects. This method is appropriate to global modeling, but can only use those large magnitude events that give observable multiple circuits.

2.2 VELOCITY STRUCTURE OF THE EARTH

Knowledge of the internal layering of the Earth has been largely derived using the techniques of earthquake seismology. The shallower layers have been studied using local arrays of recorders, while the deeper layers have been investigated using global networks to detect seismic signals that have traversed the interior of the Earth.

The continental crust was discovered by Andrija Mohorovičić from studies of the seismic waves generated by the Croatia earthquake of 1909 (Fig. 2.14). Within a range of about 200 km from the epicenter, the first seismic arrivals were P waves that traveled directly from the focus to the recorders with a velocity of 5.6 km s–1. This seismic phase was termed Pg. At greater ranges, however, P waves with the much higher velocity of 7.9 km s–1 became the first arrivals, termed the Pn phase. These data were interpreted by the standard techniques of refraction seismology, with Pn representing seismic waves that had been critically refracted at a velocity discontinuity at a depth of some 54 km. This discontinuity was subsequently named the Mohorovičić discontinuity, or Moho, and it marks the boundary between the crust and mantle. Subsequent work has demonstrated that the Moho is universally present beneath continents and marks an abrupt increase in seismic velocity to about 8 km s–1. Its geometry and reflective character are highly diverse and may include one or more sub-horizontal or dipping reflectors (Cook, 2002). Continental crust is, on average, some 40 km thick, but thins to less than 20 km beneath some tectonically active rifts (e.g. Sections 7.3, 7.8.1) and thickens to up to 80 km beneath young orogenic belts (e.g. Sections 10.2.4, 10.4.5) (Christensen & Mooney, 1995; Mooney et al., 1998).

A discontinuity within the continental crust was discovered by Conrad in 1925, using similar methods. As well as the phases Pg and Pn he noted the presence of an additional phase P* (Fig. 2.15) which he interpreted as the critically refracted arrival from an interface where the velocity increased from about 5.6 to 6.3 km s–1