Introducing Tectonics, Rock
Structures and Mountain Belts

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Introducing Tectonics,
Rock Structures
and Mountain Belts

Graham Park

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Dunedin Academic Press Ltd
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Contents

Acknowledgements

List of illustrations and tables

Sourced illustrations

Preface

1Introduction

2Large-scale Earth structure

3Plate tectonics

4Deformation, stress and strain

5Fractures, faults and earthquakes

6Folds and folding

7Fabric

8Igneous intrusions

9Structural effects of gravity

10Tectonic interpretation of orogenic belts

11Examples of modern orogenic belts

12Ancient orogenic belts

Appendix

Glossary

Further reading

Acknowledgements

I am indebted to Professor John Winchester and an anonymous reviewer for many helpful comments and suggestions that have resulted in significant improvements to this book.

I also wish to thank my wife Sylvia for her unfailing support and for subjecting the manuscript to the valuable scrutiny of a non-geologist.

List of illustrations and tables

Figure 2.1 Earth from space

Figure 2.2 Main topographic features of the continents and oceans

Figure 2.3 Cutaway model showing the main regions of the Earth’s interior

Figure 2.4 Structure of the Earth’s crust

Figure 2.5 The hypsographic curve

Figure 2.6 Pattern of recent earthquake and volcanic activity

Figure 2.7 Raised beach

Figure 2.8 Post-glacial uplift of Scandinavia

Figure 3.1 Gondwana

Figure 3.2 Climatic zones of the supercontinent of Pangaea

Figure 3.3 Palaeomagnetic evidence for continental drift

Figure 3.4 The ‘conveyor-belt’ model

Figure 3.5 Oceanic magnetic stripe pattern

Figure 3.6 Dating the ocean floor

Figure 3.7 The plates and their boundaries

Figure 3.8 Plate motion on a sphere

Figure 3.9 Constructive boundaries

Figure 3.10 Destructive boundaries

Figure 3.11 Subduction zone beneath the Japan island arc

Figure 3.12 Distribution of hot spots

Figure 3.13 A, the Iceland hot spot; B, sketch section of a plume

Figure 3.14 Mechanisms for plate motion

Figure 4.1 Force and stress

Figure 4.2 Pressure

Figure 4.3 Normal stress and shear stress

Figure 4.4 Types of strain

Figure 4.5 A–D, co-axial strain in three dimensions. E, progressive shear strain

Figure 4.6 Progressive co-axial and rotational strain in two dimensions

Figure 4.7 Two easy ways to measure strain

Figure 4.8 Contraction and extension in layered rocks

Figure 4.9 Strain and fabric

Figure 4.10 Ideal elastic, plastic and viscous strain, visco-elastic behaviour and creep

Figure 4.11 Schematic deformation diagram through the crust

Figure 4.12 Deformation mechanisms in crystalline rocks

Figure 5.1 Fractures and faults

Figure 5.2 Types of fault: A, normal faults; B, reverse faults; and C, wrench faults

Figure 5.3 Measurement of fault displacement

Figure 5.4 Extension

Figure 5.5 A, seismic section from the inner Moray Firth; B, C, structure of petroleum traps

Figure 5.6 Structure of the West Iberian rifted continental margin

Figure 5.7 Thrust faults in profile

Figure 5.8 A, the San Andreas fault; B, C, local extension and compression on a strike-slip fault

Figure 5.9 A, fault breccia; B, pseudotachylite vein intruding cataclastic gneiss

Figure 5.10 Stress conditions for shear fracture

Figure 5.11 Orientation of shear faults in relation to principal stress axes

Figure 5.12 Joints

Figure 5.13 Initiation of a fault

Figure 5.14 Earthquake waves

Figure 6.1 Large-scale folds from the air, Bighorn County, Wyoming

Figure 6.2 Some typical fold styles

Figure 6.3 Fold description

Figure 6.4 Types of fold in profile

Figure 6.5 The folding process: A, parallel; B, similar; and C, chevron folding

Figure 6.6 Strain in folding

Figure 6.7 The role of faults in folding

Figure 6.8 Flow folds in glacier ice, Lewis glacier, Alaska

Figure 6.9 Multilayer buckle folding

Figure 6.10 Fault-induced folding

Figure 6.11 Superimposed folding

Figure 6.12 Shear zones

Figure 7.1 Cleavage

Figure 7.2 A–C, cleavage fans; D, E, symmetric and asymmetric crenulation cleavage

Figure 7.3 Schistosity and gneissosity

Figure 7.4 Asymmetric fabrics: rotation and sense of shear

Figure 7.5 Asymmetric fabrics and shear sense

Figure 7.6 Lineations

Figure 7.7 A, intersection lineation; B, cuspate structure

Figure 8.1 Types of igneous intrusion

Figure 8.2 Methods of emplacement: dilational emplacement of dykes and sills

Figure 8.3 Dykes: some examples.

Figure 8.4 Granitic plutons of the Etive–Glencoe area

Figure 8.5 The Strontian pluton

Figure 8.6 Cone sheets of the Ardnamurchan Central Igneous Complex

Figure 8.7 Mode of formation of cone sheets, ring dykes and radial dykes

Figure 9.1 Gravitational instability of steep slopes

Figure 9.2 Gravity-gliding of Alpine nappes

Figure 9.3 Gravitational spreading

Figure 9.4 Evolution of a salt diapir

Figure 9.5 Salt structures in Iran

Figure 9.6 Flow directions and fold patterns in a salt diapir

Figure 9.7 Diapirism of granite-gneiss basement

Figure 10.1 An idealised collisional orogenic belt

Figure 10.2 Structure of the accretionary prism

Figure 10.3 Inversion

Figure 10.4 Channel flow

Figure 10.5 Terrane amalgamation

Figure 10.6 Oblique collision

Figure 11.1 The break-up of Pangaea

Figure 11.2 The Himalayan orogenic belt

Figure 11.3 Structure of the central Himalayan mountain belt

Figure 11.4 Velocity vectors of India relative to stable Eurasian plate

Figure 11.5 Alpine orogenic belts of the western and central Mediterranean

Figure 11.6 Cartoon cross-sections showing the evolution of the central Alps

Figure 11.7 Structure of the Swiss-Italian sector of the western Alps

Figure 11.8 The Canadian sector of the North American Cordilleran belt

Figure 12.1 Mid-Palaeozoic reconstruction of the continents

Figure 12.2 Main tectonic zones of the British-Irish Caledonides

Figure 12.3 Types of Caledonian structure

Figure 12.4 Cartoon sections showing the evolution of the British Caledonides

Figure 12.5 Tectonic zones of the Caledonides of the N. Atlantic region pre-Atlantic opening

Figure 12.6 Reconstruction of the continents of Laurentia, Amazonia and Baltica at ~990 Ma

Figure 12.7 The Canadian sector of the Grenville orogenic belt

Figure 12.8 Tectonic evolution of the Grenville orogenic belt

Figure 12.9 The Trans-Hudson orogen

Figure 12.10 Tectonic evolution of the Trans-Hudson orogen

Figure 12.11 The western Superior Province

Figure 12.12 Archaean terranes of the Nuuk area, southern West Greenland

Tables

Table 5.1 The Mercalli and Richter scales of earthquake magnitude

Table A1 The geological timescale

Table A2 Main types of igneous rock

Table A3 Some common types of sedimentary rock

Table A4 Some common types of metamorphic rock

Sourced illustrations

The following illustrations are reproduced by permission.

British Geological Survey. ©NERC. All rights reserved. IPR/73-34C: Figure 2.7

Jim Wark airphotos: Figures 5.6, 6.1, 6.8

NASA photo courtesy Space Images: Figure 2.1

NASA photo courtesy Parstimes: Figures 9.5A, B.

The following illustrations have been adapted from published sources.

Figure 2.2: Wyllie, P.J. 1976. The way the Earth works, Wiley, New York.

Figure 2.3: Hamblin, W.K. 1989. The Earth’s dynamic systems, 5th edition. Macmillan, New York.

Figure 2.5: Wyllie, P.J. 1976. The way the Earth works, Wiley, New York.

Figure 2.8: Zeuner, F.E. 1958. Dating the past (4th edn) Methuen, London.

Figure 3.2: Hamblin, W.K. 1989. The Earth’s dynamic systems, 5th edition. Macmillan, New York.

Figure 3.3: McElhinny, N.W. 1973. Palaeomagnetism and plate tectonics, Cambridge University Press.

Figure 3.6: Larson, R.L. & Pitman, W.C. 1972. Bulletin of the Geological Society of America, 83, 3645–3661.

Figure 3.7: Vine, F.J. & Hess, H.H. 1970, The Sea, Wiley, New York.

Figure 3.9A: Bott, M.P. The interior of the Earth, Arnold, London.

Figure 3.9B: Girdler, R.W. & Darracott, B.W. 1972. African poles of rotation. Comments on the Earth Sciences: Geophysics 2, (5), 7–15.

Figure 3.11: Uyeda, S. 1978. The new view of the Earth. Freeman, San Francisco.

Figure 3.12: Morgan, W.J. Deep mantle convection plumes and plate motions. Bulletin of the American Association of Petroleum Geologists, 56, 203–213.

Figure 3.13: Saemundsson, K. 1974. Evolution of the axial rifting zone in northern Iceland. Bulletin of the Geological Society of America, 85, 495–504.

Figure 5.4D: Reston, T.J. 2007. The formation of non-volcanic rifted margins by the progressive extension of the lithosphere: the example of the West Iberian margin. From: Karner, G.D., Manatscheal, G. & Pinheiro, L.M. (eds) Imaging, mapping and modeling continental lithosphere extension and breakup. Geological Society, London, Special Publications, 282, 77–110.

Figure 5.5A: Virtual Seismic Atlas, Robert Butler.

Figure 5.6: Reston, T.J. 2007 (see above).

Figure 5.7C: Elliott, D & Johnston, M.R.W. 1980. Structural evolution in the northern part of the Moine thrust zone. Transactions of the Royal Society of Edinburgh: Earth Sciences, 71, 69–96.

Figure 5.9B: Park, R.G. 1961. The pseudotachylite of the Gairloch district, Ross-shire, Scotland. American Journal of Science, 259, 542–550.

Figure 6.13: Ramsay, J.G. 1980. Shear zone geometry: a review. Journal of Structural Geology, 2, 83–99.

Figure 8.2C: McGregor, M.D. & McGregor, A.G. 1948. British Regional Geology: the Midland Valley of Scotland. HMSO, London.

Figure 8.2E: Nicholson, R. & Pollard, D.D. 1985. Dilation and linkage of en-echelon cracks. Journal of Structural Geology 7, 583–590.

Figure 8.4: Johnstone, G.S. 1966. British Regional Geology: the Grampian Highlands (3rd Edn). HMSO, London.

Figure 8.5: Hutton, D.H.W. 1988. Granite emplacement mechanisms and tectonic controls: inferences from deformation studies. Transactions of the Royal Society of Edinburgh: Earth Sciences, 79, 245–255.

Figure 8.6: Emeleus, C.H. & Bell, B.R. 2003. Scotland: the Tertiary volcanic districts. British Regional Geology, 4th Edition, London, HMSO for the British Geological Survey.

Figure 9.2: Graham, R.H. 1981. Gravity sliding in the Maritime Alps. From: McClay, K.R. & Price, N.J. (eds) Thrust and nappe tectonics. Geological Society, London, Special Publication 9, 335–352.

Figure 9.4: Trusheim, F. 1960. Mechanism of salt migration in Germany. Bulletin of the American Association of Petroleum Geologists, 44, 1519–1540.

Figure 10.1: Hatcher, R. & Williams, R.T. 1986. Mechanical model for single thrust sheets. Geological Society of America Bulletin, 97. 975–985.

Figure 10.2: Westbrook, G.K. 1982. The Barbados Ridge complex: tectonics of a mature forearc system. In: Leggett, J.K. (ed.) Trench-forearc geology: sedimentation and tectonics on modern and ancient active plate margins. Geological Society, London, Special Publications 10, 275–290.

Figure 10.4. Godin, L., Grujic, D., Law, R.D. & Searle, M.P. 2006.Channel flow, ductile extrusion and exhumation in continental collision zones: an introduction. From: Law, R.D., Searle, M.P. & Godin, L. (eds) Channel flow, ductile extrusion and exhumation in continental collision zones. Geological Society, London, Special Publication 268, 1–23.

Figure 11.1: Smith, A.G. & Briden, J.C. 1977. Mesozoic and Cenozoic palaeocontinental maps. Cambridge University Press.

Figure 11.2A. Molnar, P. & Taponnier, P. 1975. Cenozoic tectonics of Asia: effects of a continental collision. Science 189, 419–426.

Figure 11.3A, B. Harrison, T.M. 2006. Did the Himalaya crystallines extrude partially molten from beneath the Tibetan plateau? From: Law, R.D., Searle, M.P. & Godin, L. (eds) Channel flow, ductile extrusion and exhumation in continental collision zones. Geological Society, London, Special Publication 268, 237–254.

Figure 11.3D, 11.4: Searle, M.P, Elliott, J.R, Phillips, R.J. et al. 2011. Crustal-lithospheric structure and continental extrusion of Tibet. Journal of the Geological Society, London, 168, 633–672.

Figure 11.5A. Coward, M.P. & Dietrich, D. 1989. Alpine tectonics: an overview. From: Coward, M.P., Dietrich, D. & Park, R.G. (eds) Alpine tectonics. Geological Society, London, Special Publication 45, 1–29.

Figure 11.7. Ramsay, J.G. 1963. Stratigraphy, structure and metamorphism in the Western Alps. Proceedings of the Geologists Association 74, 357–392.

Figure 11.8A. Coney, P.J., Jones, D.L. & Monger, J.W.H. 1980. Cordilleran suspect terranes. Nature, London, 288, 329–333.

Figure 12.1: Dalziel, I.W.D. 1997. Neoproterozoic-Palaeozoic geography and tectonics: review, hypothesis, environmental speculation. Geological Society of America Bulletin, 109, 16–42.

Figure 12.3A: Elliott, D. & Johnson, M.R.W. 1980. Structural evolution in the northern part of the Moine thrust zone. Transactions of the Royal Society of Edinburgh: Earth Sciences, 71, 69–96.

Figure 12. 3B: Treagus, J.E. 2000. Solid geology of the Schiehallion district. Memoir of the British Geological Survey, HMSO.

Figure 12.3D: Leggett, J.K., McKerrow, W.S. & Eales, M.H. 1979. The Southern Uplands of Scotland: a Lower Palaeozoic accretionary prism. Journal of the Geological Society, London, 136, 755–770.

Figure 12.3F: Coward, M.P. & Siddans, A.W.B. 1979. The tectonic evolution of the Welsh Caledonides. From: Harris, A.L. Holland, C.H. & Leake, B.E. (eds) The Caledonides of the British Isles – reviewed. Geological Society of London, Special Publication 8, 187–198.

Figure 12.5: Dewey, J.F. & Shackleton, R.M. 1984. A model for the evolution of the Grampian tract in the early Caledonides and Appalachians. Nature, London, 312, 115–121.

Figure 12.6. Pisarevsky, S.A., Wingate, M.T.D., Powell, C. McA., Johnson, S., Evans, D.A.D., 2003. Models of Rodinia assembly and fragmentation. From: Yoshida, M., Windley, B., Dasgupta, S. (eds) Proterozoic East Gondwana: supercontinent assembly and breakup. Geological Society, London, Special Publication 206, 35–55.

Figure 12.7, 12.8: Rivers, T. The Grenville Province as a large, hot, long-duration collisional orogen: insights from the spatial and thermal evolution of its orogenic fronts. From: Murphy, J.B., Keppie, J.D. & Hynes, A.J. 2009. Ancient orogens and modern analogues. Geological Society of London, Special Publication 327, 405–444.

Figure 12.9, 12.10: Corrigan, D., Pehrsson, S., Wodicka, N. & de Kemp, E. 2009. The Palaeoproterozoic Trans-Hudson Orogen: a prototype of modern accretionary processes. From: Murphy, J.B., Keppie, J.D. & Hynes, A.J. 2009. Ancient orogens and modern analogues. Geological Society of London, Special Publication 327, 457–479.

Figure 12.11: Card, K.D. & Cieselski, A. 1986. DNAG#1: Subdivision of the Superior Province of the Canadian Shield. Geoscience Canada 13, 5–13.

Figure 12.12: Friend, C.R.L. & Nutman, A.P. 2005. New pieces to the Archaean terrane jigsaw puzzle in the Nuuk region, southern West Greenland: steps in transforming a simple insight into a complex regional tectonothermal model. Journal of the Geological Society, London, 162, 147–162.

Preface

This book is not intended to be a textbook, but is designed to explain the key concepts of tectonics and rock structures to both students and others interested in geology – especially those who may not have a good scientific or mathematical background. The study and understanding of geological structures has traditionally been guided by the rigorous application of mathematics and physics, and conventional textbooks on structural geology have reflected this approach. However, in my experience, many students are discouraged by this aspect of the subject and consequently, in this book, I have avoided mathematical equations altogether, and reduced the geometry to the minimum I judged necessary to understand the concepts. Those who wish to gain a deeper understanding of the subject, or who are engaged on a university-level course in structural geology, are directed to the reading list, which contains several excellent textbooks and on-line sources recommended for further study.

The application of plate tectonic theory has revolutionised structural geology by giving the study of structures a context in which they can be explained. Since the large-scale movements of the plates ultimately control smaller-scale structures, the study of tectonics is therefore the key to understanding the latter. I therefore introduce the reader first to large-scale Earth structure and the theory of plate tectonics. The following four chapters deal in rather more detail with what might be called ‘traditional’ structural geology – the study of the response of rock material to crustal forces, and the explanation for the bewildering variety of rock structures formed thereby. This aspect of geology was transformed in the 1950s and 1960s by rigorous geometrical analysis and the application of the techniques of rock mechanics. I believe that it is important for the student of geology to be aware of this background, without necessarily being able (at least initially!) to understand the mathematical or geometrical detail.

An important development in the latter decades of the last century was the emphasis by structural geologists on the movement history of rock masses relative to each other as revealed by the study of fault systems and shear zones. This enabled structural geology to be more directly related to plate tectonics and helped to integrate geological structures with tectonics; I have tried to emphasise this aspect in the book.

One of the most exciting aspects of geology is the study of the great mountain ranges (‘orogenic belts’), both of the present day and of the past. The final three chapters of the book attempt to explain how knowledge of plate-tectonic theory, geological structures and the processes of deformation may be employed to understand these orogenic belts.

Scientists are addicted to the classification and naming of the things that fall into their domain, and structural geologists are no exception. However, I have tried to avoid the excessive use of terminology, and all technical terms are defined and explained in the Glossary, which readers are encouraged to check when uncertain of the exact meaning of a word used in a geological context (which often differs from its everyday meaning!).

The Appendix contains a set of four tables that set out in a simplified way the main terminology of the geological timescale and of igneous, sedimentary and metamorphic rocks as an aid to those readers who may not be so familiar with these aspects of the subject.

Finally, I hope that the reader might share the excitement of discovering how the all-embracing theory of plate tectonics can help to explain the multitude of complexities revealed in the study of the rocks.

R.G.P.

December 2011

Note: all terms highlighted in bold are defined in the Glossary at the end of the book.

1Introduction

Meaning and scope of the terms ‘tectonic’ and ‘structure’

The adjective ‘tectonic’ merely means ‘structural’, i.e. applying to a structure. However, in geological usage it has come to be applied particularly to large-scale structures – thus ‘tectonic plate’. In order to explain rock structures, it is necessary to understand the forces operating within and on the crust that are responsible for creating them, and to do this, we need to investigate the Earth-scale processes known collectively as plate tectonics.

The term ‘structure’ in everyday usage refers normally to a building or other artificial construction, but in geological terms it has come to mean a body of rock whose shape can be defined geometrically and which has originated by a geological process. The most obvious and best known types of geological structure are folds and faults, which have been produced by the action of geological forces within the Earth’s crust and which can give us an insight into the magnitude of these forces and how they operate. Another group of structures is typical of deep-seated metamorphic rocks where crustal forces have effected thoroughgoing changes in the rock, producing new textures through recrystallisation; the structure produced by such changes is known as the fabric. A fourth category of geological structures consists of igneous intrusions. Such bodies are influenced by the forces acting within the crust during their emplacement and the igneous bodies themselves may cause structural changes in the host rocks.

Kinematic and dynamic models of deformation

Kinematics is the study of movement and dynamics the study of forces, and both types of model have been used in the investigation of geological structures. Dynamic models have traditionally been used by structural geologists employing the techniques of mechanical engineering and materials science. This requires a familiarity with mathematics and physics, which is a potential barrier to understanding for many students. However, whereas a background in these subjects is essential to understanding the deformation of rock in any depth, it is not really necessary to be able to grasp the essentials of structural geology at a basic level.

The kinematic model of deformation has become in many ways more popular as a way of understanding geological structures. In this type of approach, structures are analysed in terms of the relative movement between blocks of crust, and the causal forces are disregarded. Thus, systems of related folds and faults are explained by a single set of relative movements. Such a system can also be integrated into the plate tectonic model by scaling up to a higher order of magnitude but maintaining the same overall movement sense. Examples of this are discussed in Chapters 10–12, where we look in detail at the structure of orogenic belts. At this larger scale, structures are grouped into tectonic regimes, based on their over-riding characteristics, and linked with their plate tectonic setting, thus: extensional regimes are associated with divergent plate boundaries (continental rift zones); compressional regimes with convergent plate boundaries (subduction and collision zones); and strike-slip regimes with transform faults. These plate boundary types are explained in Chapter 3.

Layout of the book

The sequence of chapters reflects the author’s belief that an understanding of the plate tectonic model and the evidence for it is helpful in the understanding of geological structures on the smaller scale. Thus Chapters 2–3 deal with large-scale Earth structure and the theory of plate tectonics. These chapters are followed by chapters 4–9 on the process of deformation and on the various types of geological structure. Finally, Chapters 10–12 deal with the study and understanding of orogenesis and orogenic belts. A brief summary of each chapter follows.

Chapter 2: Large-scale earth structure. This chapter describes the most significant features of the Earth’s crust: the continents and oceans, mountain belts, ridges and trenches; with the Earth’s internal structure, the asthenosphere and lithosphere; and finally with the distribution of present-day tectonic activity in the form of earthquakes and volcanoes.

Chapter 3: Plate tectonics. Modern plate tectonics starts with the theory of continental drift, which, when added to the more recent concept of ocean-floor spreading, led in the 1960s to the development of plate tectonics. The concept of the tectonic plate is explained, together with the three types of plate boundary – ocean ridges, ocean trenches and transform faults – and the processes that accompany them. This is followed by a brief discussion of hot spots and plumes, and finally by a consideration of the driving mechanism of plate motion.

Chapter 4: Deformation, stress and strain. This chapter deals with the processes that govern how structures are formed, and with the methods by which the amount of deformation (the strain) can be measured. The study of deformation requires an understanding of the relationship between force and stress, and between stress and strain. The various types of strain and the methods of measuring strain are discussed, followed by a consideration of the behaviour of rock material under varying conditions of temperature and pressure in response to stress.

Chapter 5: Fractures, faults and earthquakes. The description and geometry of faults and fault systems, and their relationship to their tectonic setting, are described first. This is followed by a discussion of the conditions under which fracturing, and thus faulting, takes place, and the relationship between faulting and earthquakes. Knowledge of how and why earthquakes occur is particularly relevant in today’s world – afflicted as it is by so many recent natural disasters with a geological origin.

Chapter 6: Folds and folding. This chapter deals first with the description and significance of the various types of fold structure in terms of their shape and mode of formation, and examines the nature and geometry of the strain accompanying each. This is followed by a discussion of fold systems, the three-dimensional geometry of folds, and fold superimposition. Shear zones are discussed here as a special case of folding, although they are in reality the deep-seated equivalent of faults. The chapter concludes with a discussion of the conditions under which folding occurs.

Chapter 7: Fabric. The effects of higher temperature and pressure are revealed in metamorphic rocks by special types of structure as well as by the changed mineral composition. These new structures include planar structures (foliation), new rock types, such as schist and gneiss, and new linear structures (lineation); these are included under the general term ‘fabric’. The effects of deformation under these conditions are to form structures that penetrate throughout the whole rock, unlike folds or faults, and involve micro-structural changes at the scale of the single crystal. Study of the fabric can reveal useful information about the deformation process accompanying the metamorphism.

Chapter 8: Igneous intrusions. Igneous intrusions are rock bodies whose shape is controlled by the state of stress (or force field) of the crust into which they have been emplaced; the different types of igneous body are thus of structural significance. Moreover, the process of emplacement itself exerts some structural control on the host rocks and produces structures in them. There are important differences between the structures of small-scale bodies such as dykes and sills and the large bodies (plutons) such as batholiths, laccoliths and lopoliths. Certain plutonic bodies are emplaced passively, by the host rocks moving aside to accommodate them, while others are forcefully emplaced by a mechanism termed diapirism, driven by gravitational pressure. Certain igneous complexes representing the roots of large central volcanoes consist of several different types of igneous body (ring dykes, cone sheets and radial dykes) that are related to each other by the stress field generated by the central magma body. Examples of these are described from western Scotland.

Chapter 9: Structural effects of gravity. The force of gravity has a key influence on deformation, and this chapter explores how gravity controls structures, giving examples of its effects in terms of gravity sliding on both small and large (orogenic) scales, and on the emplacement of salt domes.

Chapter 10: Tectonic interpretation of orogenic belts. This chapter explains how the principles of tectonics and structural geology are used in understanding present-day and ancient mountain belts.

Chapter 11: Examples of modern orogenic belts. Three typical examples are chosen from the present-day orogenic belt system: the central Himalayas, the Western Alps and the Canadian sector of the North American Cordillera. In each case the structure of the belt is explained in terms of its plate-tectonic context, and the sequence of plate movements that are believed to be responsible.

Chapter 12: Ancient orogenic belts. The same approach as in the previous chapter is taken to examples of more ancient orogenic belts – the Lower Palaeozoic Caledonides of the British Isles, the Mid-Proterozoic Grenville belt, and the Early Proterozoic Trans-Hudson belt of Canada. Finally, examples of Archaean orogenesis from Canada and Greenland are examined to explore the question of whether plate tectonics and orogeny operated in a basically similar way in the very distant geological past.

Appendix. There are certain topics of general geological interest, but not directly relevant to structural geology, which most geology students will understand, but which might be unfamiliar to other readers. These include the geological timescale, and the classification of igneous, sedimentary and metamorphic rocks, which are often useful in understanding the tectonic context of certain structures; these are summarised in the form of tables.

What is not included

This book is not intended as a comprehensive survey of the whole subject, and what is selected partly reflects the author’s subjective view of what is most important, and also a desire to keep the book reasonably short. Certain topics are omitted as not directly relevant; these include the interpretation of geological maps – the complex geometry of the interaction between simple structures and the three-dimensional ground surface – and sedimentary structures, i.e. bedding and allied structures formed by the processes of erosion and sedimentation.

2Large-scale earth structure

If we were to view the Earth from Space (Figure 2.1), the most obvious large-scale features would be the continents, the oceans, and the rather linear continental mountain ranges of the Alpine– Himalayan belt and the circum-Pacific belt, which includes the Cordilleran– Andean chain of the western Americas (Figure 2.2). Were we to imagine the water of the oceans removed, another set of mountain ranges would appear in the form of the ocean ridge network, which takes up around one-third of the oceanic surface area. Deep-ocean trenches would also be apparent, although these are an order of magnitude smaller in areal extent. They form a discontinuous system of linear features parallel to and offshore from island arcs, as in the NW Pacific, or close to the continental margin of western America. These large-scale structures are not randomly arranged but have a tectonic significance, being caused by geological processes that have operated over long periods of geological time.

Mountain belts, ridges and trenches

The most prominent and tectonically active mountain ranges form two linear belts shown in red in Figure 2.2. The Alpine–Himalayan belt extends from Gibraltar, through the Alps, Caucasus, and Zagros ranges, and culminates in the huge mass of the Himalayas and related ranges that sweep around northern India southwards to the Indian Ocean. The eastern side of the circum-Pacific belt chain follows the western margin of the Americas from Alaska down through the North American Cordillera to the Andes of South America. On the western Pacific margin, it forms a discontinuous chain including south-east Siberia, Japan and New Guinea. These belts vary considerably both in width and height: the width of the western American belt is over 600 km in the Canadian Rockies but only around 400 km in the southern Andes. The Alpine– Himalayan belt is even more variable: it divides into several branches around the Mediterranean, including the Atlas and Apennine ranges; in central Asia, the belt broadens to around 1500 km where it surrounds the high plateau of Tibet. Ongoing tectonic activity in these belts is expressed by sporadic volcanism and frequent earthquakes. Other prominent mountain belts, such as the Urals, Caledonian and Appalachian ranges, are no longer tectonically active, being the result of processes in more remote geological periods.

The ocean ridges, shown in blue on Figure 2.2, are even more prominent features than the mountain belts in terms of their volume, although of course almost entirely hidden below sea level; they are typically 500–1000 km wide and elevated by as much as 2–3 km from the deep ocean floor. The ridge network consists of three main strands: the Mid-Atlantic, Indian Ocean and circum-Antarctic ridges, the latter extending through the southeast Pacific Ocean to end against the North American coast off Mexico.

The deep-ocean trenches (thin green lines on Figure 2.2) are much smaller features than the ridges, being only about 100–150 km across, but they attain depths of up to 11 km below sea level. They fall into two groups: those of the first group lie offshore from, and parallel to, the western continental margin of the Americas, and those of the second group form a discontinuous series of curved features parallel to the volcanic island arcs of the western and northern Pacific Ocean and south-west of Indonesia in the Indian Ocean. Smaller arcs are located in the Caribbean and between South America and Antarctica.

Earth’s internal structure

A considerable amount is known about the internal structure of the Earth, much of it from the study of earthquake waves. On the scale of the whole Earth (Figure 2.3) the crust represents only a thin outer shell with an average thickness of around 20 km. The bulk of the Earth is made up of the mantle, the base of which lies at a depth of 2900 km below the surface, and is composed largely of solid material with a composition similar to the igneous rock peridotite. It is here that the processes that mostly control what happens at the Earth’s surface originate, as we shall see in the following chapter. The innermost, approximately spherical, region is called the core, and extends to the centre of the Earth, at 6500 km depth. The core is believed to consist mostly of metallic iron, with some lighter elements in addition, such as nickel. In the outer core the metal is in a molten state but the inner core is solid. These constituents are thought to have become molten at an early stage in Earth’s history and drained down towards the centre, forming the core.

The nature of the crust

The crust varies in thickness from about 7 km beneath the oceans, to an average of about 33 km in continental areas, and reaches nearly 80 km in depth beneath certain young mountain belts (see Figure 2.4). The composition of the crust is known in some detail, since material from the various depths of the crust is directly accessible at the surface somewhere because of tectonic movements. There is a great difference between oceanic and continental crust; oceanic crust is composed almost entirely of the volcanic rock basalt, whereas continental rocks are extremely varied in composition, including the whole range of igneous rocks together with all the different types of rock derived from them. However, the average composition has been estimated to be similar to a mixture of granite and basalt.

The present-day coastlines are arbitrary, in the sense that sea level fluctuates through time in response to various geological processes. The formation or removal of ice sheets and the uplift or depression of land masses cause either the retreat or advance of the shorelines across the continental margins. The true margin of the continents may be taken as a line about halfway down the continental slope, where the nature of the underlying crust changes from continental type to oceanic type (Figure 2.5). Measured in this way, the ‘continents’ would occupy around 40% of the Earth’s surface area rather than less than the 30% that currently consists of land. Looking in more detail at the Earth’s surface, it becomes apparent that a large proportion of it consists of either plains or plateaux on land, or the deep-ocean floor; these show little variation in relief until interrupted by the extreme elevations and depressions of the mountain ranges and deep sea trenches, which occupy only about 3% of the surface area. Between these two dominant levels is a region of intermediate depths, amounting to perhaps 15% of the total area, representing the continental slopes and the ocean ridges. The reason for this distribution, as we shall see, is that the zones of high relief result from localised tectonic instabilities, whereas the areas of low relief are more stable.

The differences between continental and oceanic areas are due to differences in the composition, and consequently the density, of the underlying crust, as shown in Figure 2.4. Continental crust, with a composition corresponding to a mixture of granite and basalt, has a mean density of around 2.8, whereas oceanic crust, composed largely of basalt, has a mean density of around 2.9. Moreover, continental crust has a mean thickness of about 33 km and oceanic crust is much thinner, averaging only about 7 km. This explains their difference in mean height (or depth) with respect to sea level. Because the Earth is in a state of approximate gravitational balance (termed isostasy), the weight of any particular sector is similar to that of any other. Consequently, the less dense continents must attain a higher level than the denser oceans if they are to have the same gravitational effect (i.e. have the same weight). In other words, we can imagine the continents as being more ‘buoyant’ than the oceans, as if they were icebergs floating on the sea. Although the mantle underlying the crust is made up of solid rock (approximating to peridotite in composition), it is able to flow in the solid state at a very slow rate of centimetres per year, enabling it to gradually adjust to the pressure of gravitational differences in the crust above. Thus, when a particular part of the crust is subjected to an additional load, by the creation of a mountain range for example, its base adjusts by sinking downwards and displacing mantle material sideways to form a ‘mountain root’ as shown in Figure 2.4B; like an iceberg, much more of the volume of the thickened crust of a mountain range lies beneath sea level than above! The reverse happens when crust is thinned to form an ocean basin, for example; the top of the mantle rises and the surface is depressed, but the whole is still in gravitational balance.

Asthenosphere and lithosphere

The region of the mantle in which flow takes place lies at a depth of about 100 km beneath the surface, and is termed the low velocity layer because of the fact that seismic waves travel more slowly through it than would be expected if it were composed of normal mantle material, indicating that its density is lower than that of the surrounding mantle. It is therefore regarded as a zone of relative weakness and is termed the asthenosphere (see Figure 2.4B). The region above this level, composed partly of mantle and partly of crust, is stronger and is termed the lithosphere. As we shall see in the next chapter, it is the lithosphere from which the tectonic plates are formed, and their ability to move across the Earth’s surface is due to the relative weakness of the underlying asthenosphere.

Present-day tectonic effects: earthquakes, volcanoes and aseismic movements

The Earth is subject to continuous tectonic activity, as testified by the occurrence of earthquakes (seismicity) and vulcanicity of varying severity, much of it with disastrous consequences. This activity is not random but is concentrated in well-defined zones (Figure 2.6).

Earthquakes

Although earthquakes can occur almost anywhere, the great majority of them (and all the most severe ones) are concentrated in narrow zones along the centres of the ocean ridges and in a rather wider zone along the Alpine–Himalayan and circum-Pacific mountain belts delineated in Figure 2.2. The earthquakes along the ocean ridge network are generally quite shallow, less than ~50 km in focal depth, but those near the ocean trenches range from shallow to deep – up to 700 km in depth, indicating that the tectonic process responsible for the latter is different. The earthquakes along the young mountain belts are intermediate between the two extremes, generally lying above about 300 km. The reason for these differences will become apparent when we discuss plate tectonics in the next chapter.

Volcanoes

The distribution of vulcanicity, like that of earthquakes, is distinctly non-random; there are three main categories of occurrence. The majority of active volcanoes are concentrated along the circum-Pacific and Alpine–Himalayan belts, as shown in Figure 2.6; secondly, there are numerous occurrences along the ocean ridges and on subsidiary ridges within the ocean basins, as well as in Hawaii. A third category is associated with continental rift zones such as the African rift valley. There are characteristic differences in the type of lava emitted from these volcanoes – oceanic vulcanicity is typically basaltic whereas that associated with the circum-Pacific and Indonesian belts is dominated by andesitic vulcanicity. The reasons for both the distribution and the compositional difference are explained by the plate tectonic theory to be discussed in the following chapter.

Aseismic tectonic activity

Crustal movements currently taking place can be measured by repeated recording of location and height using sensitive instruments. The larger and more obvious movements are associated with the major tectonic belts already recognised from their seismic and volcanic signatures, but less intense movements can be recorded well away from these belts anywhere on the Earth’s surface. These movements are generally not accompanied by earthquake activity, that is, they are aseismic and show both horizontal and vertical movements of the order of millimetres to centimetres per year.

Relatively recent vertical movements of the land surface relative to sea level can also be studied by observing the positions of raised beaches (e.g. Figure 2.7) and buried forests. This evidence shows us that since the last (Pleistocene) Ice Age, the land surface of Scotland and Scandinavia has been uplifted due to the slow recovery after the release of the weight of the ice sheet. In the case of Scandinavia, the old shore-line has been uplifted into a dome-like shape by up to 250 metres (Figure 2.8); this gives average uplift rates of up to 10 millimetres per year. Larger post-glacial uplift rates for Iceland have been calculated in the range 2–9 centimetres per year.

cratons‘mobile’ beltsArchaean