The Mapping of Geological Structures E-Book

K. R. McClay

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: Geological Society of London Handbook Series
• Sprache: Englisch

Seitenzahl: 152

Veröffentlichungsjahr: 2013

Beschreibung

Detailed mapping and analysis of the structural features of rocks enable the 3D geometry of their structures to be reconstructed. The resulting evidence of the stresses and movement patterns which rocks have undergone indicates the processes by which they were formed, and allows evaluation of past deformations of the earth's crust. Written to show how one actually describes, measures and records rock structures such as folds and faults with the emphasis on accuracy, detail and on-going interpretation throughout, this handbook gives students and enthusiasts the practical information and guidance which allows their fieldwork to become vastly more rewarding.

"...the author is to be congratulated on producing such an excellent text. The whole range of mapping techniques that an undergraduate student will require are described and the book will still be immense help to post-graduates setting out on their research work. The book represents extremely good value and is thoroughly recommended."
--C.R.L. Friend, Mineralogical Magazine

Leseprobe

Contents

Acknowledgements

1  Introduction

1.1  Objectives

1.2  Fieldwork

1.3  Tectonic and structural regimes

1.4  Bedding

1.5  Synsedimentary versus tectonic structures

1.6  Basic references

1.7  Safety

2  Mapping techniques

2.1  Equipment

2.2  Stereographic projections

2.3  How to measure structures

2.4  The field map and aerial photographs

2.5  Field notebook

2.6  Map symbols

2.7  Oriented samples

2.8  Photography

3  Fold structures

3.1  Basic fold nomenclature

3.2  Fold types

3.3  Analysis and classification of folds

3.4  Symmetries of parasitic minor folds

3.5  Vergence

3.6  Strain in folds

3.7  Folds associated with Faults

3.8  Kink bands

3.9  Mapping of folds

4  Foliations

4.1  Common foliations

4.2  Axial-planar foliations

4.3  Foliations and folds

4.4  Mapping foliations

5  Linear structures

5.1  Lineations associated with folding

5.2  Mineral stretching and elongation lineations

5.3  Lineations formed by boudins, mullions or rodding

5.4  Lineations associated with faults

5.5  Mapping linear structures

6  Faults and shear zones

Faults

6.1  Classification and description of faults

6.2  Fault displacements

6.3  Extension faults

6.4  Contraction faults

6.5  Strike-slip and wrench faults

6.6  Fault rocks

Shear zones

6.7  Geometry of shear zones

6.8  Structures in shear zones

6.9  Mapping shear zones

7  Joints, veins and stylolites

7.1  Types of joints

7.2  Joints in fold and fault systems

7.3  Veins

7.4  Stylolites

7.5  Mapping joints, veins and stylolites

8  Polyphase deformation

8.1  Fold interference patterns

8.2  Lineations in polyphase terranes

8.3  Sub-areas

8.4  Mapping polyphase terranes

9  First steps in overall interpretation and analysis

9.1  Map patterns and map interpretation

9.2  Cross-sections

9.3  Report writing

References and further reading

Appendix I Real and apparent dip nomogram

Appendix II Thickness exaggeration with apparent dip

Appendix III Strain measurements

Copyright © 1987 K. R. McClay

Reprinted in September 1991 byJohn Wiley & Sons Ltd, Baffins Lane, Chichester,West Sussex P019 1UD, England

First published in 1987 by Open University Press

Reprinted September 1992

All rights reserved

No part of this book may be reproduced by any means, or transmitted, or translated into a machine language without the written permission of the publisher.

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Library of Congress Cataloging-in-Publication Data

McClay, K. R. (Kenneth R.)

  The mapping of geological structures.

  1. Geology, Structural—Maps

  2. Geological mapping.

  I. Title

QE501.3.M42 1987        551′.022′2        86-9919

ISBN 0 471 93243 4

British Library Cataloguing in Publication Data

McClay, K. R.

  The mapping of geological structures.

  —(Geological Society of London handbook)

  1. Geological mapping

  I. Title    II. Series

  551.8    QE36

ISBN 0 471 93243 4

Acknowledgements

The completion of this Handbook has been possible through the generous support and encouragement of the staff and students of the Department of Earth Sciences at Goldsmith’s College. In particular I thank Dr D. Helm for his critical reviews and advice. T. Easter ably assisted with the photographic reproductions, M. Insley assisted with the drafting of the figures and Carol Sutton assisted with the word processing. Dr M. de Freitas, the first Geological Society of London Handbook Series Editor and Ms R. Dixon of the Open University Press are thanked for their advice, patience and encouragement.

A first draft of this handbook was prepared whilst the author was on sabbatical leave at the Geological Survey of Canada, Vancouver, supported by the Royal Society and the Geological Association of Canada. Dr R. I. Thompson is thanked for his support and encouragement during this period. M. de Freitas, P. Ransom, P. Ellis and M. Insley gave critical reviews.

Illustrations in this Handbook were adapted from the following sources to whom grateful acknowledgement is made:

Fig. 2.2: RAGAN D. M. (1985): Structural Geology; an Introduction to Geometric Techniques. 3rd ed., John Wiley and Sons, New York.

Fig. 3.5: HUDDLESTON, P., (1973): Tectonophysics 16, 1–46. Amsterdam, Elsevier.

Fig. 3.6 & Fig. 3.7: WILLIAMS G. D. & Chapman, T. J. (1979), Journal of Structural Geology, 1, 181–186, Oxford, Pergamon.

Fig. 3.9 & Fig. 3.10: BELL, A. M., (1981), Journal of Structural Geology, 3, 197–202. Oxford, Pergamon.

Fig. 6.23b: Ramsay, J.G., (1980). Journal of Structural Geology, 2, 83–99, Oxford, Pergamon.

Table 6.4: SIBSON, R. H., (1977). Journal of the Geological Society of London, 133, 191–214. Oxford, Blackwells.

Fig. 9.1: RAGAN, D. M., (1985). Structural Geology; an Introduction to Geometric Techniques. 3rd ed., New York, John Wiley and Sons.

Photographs for Figs. 4.1d and 7.7 were kindly supplied by John Ramsay.

1

Introduction

1.1 Objectives

This book is designed as a basic guide to the field mapping and interpretation of geological structures. Emphasis is placed upon the identification of structures and the systematic recording of structural data, as both should be a fundamental part of any mapping programme. The identification and description of structures, together with an understanding of their development, i.e. their movement patterns (Kinematic analysis) and an appreciation of the forces and stresses responsible for them (Dynamic analysis) are extremely useful for interpreting particular structures, and for knowing what geometry to expect whilst mapping in a particular terrane.

Structural data cannot be recorded or used in a vacuum. They must be accompanied by full lithological, sedimentological, petrological and palaeontological descriptions for their complete interpretation.

The following aspects are emphasised in this Handbook;

1  Recognition of structures.

2  What to measure and what to describe.

3  How to analyse the data collected.

4  How to interpret the data and incorporate it into the stratigraphy, interpretation and regional syntheses for an area.

In all cases emphasis is placed upon systematic field observations, accurate measurements of the orientations of structural elements, careful recording of the data in the field notebook, sketching and photographing the structures, and analysis in the field using the stereographic projection. Above all, structural geology requires the appreciation of the three-dimensional nature of structures. Think in 3D and learn to extend your view of structures above and below the map sheet.

1.2 Fieldwork

The importance of careful, accurate and systematic fieldwork cannot be overemphasised. Basic geologic mapping techniques are described in Barnes (1981), and the field descriptions of sedimentary, metamorphic and igneous rocks are outlined in the companion Handbooks by Tucker (1982), Fry (1984), and Thorpe and Brown (1985) respectively.

This Handbook describes the field techniques for mapping geological structures and for the identification and mapping of particular types of structure. It also gives a brief summary of the interpretation and analysis of structures.

Remember the following points:

1  Accurate measurement, observation and recording of all structural elements is essential. Avoid data selection in the field, otherwise you may find that upon further interpretation in the laboratory, you have failed to measure a key structural feature.

2  Carry out an ongoing interpretation whilst in the field (draw sketch cross-sections and maps). This will help you recognise key areas where further work may be necessary. Your interpretation will be governed by your experience and knowledge of regional structure, but only accurate and well-recorded data will have a permanent value and permit continuous reinterpretation.

3  Data should always be plotted on maps and cross-sections whilst in the field. Only in these circumstances can an effective, ongoing interpretation be achieved.

4  Structural data must be collected in conjunction with other lithological, petrological and palaeontological data.

Conduct and safety in the field

Fieldwork frequently puts geologists in hazardous situations. Structural geologists commonly work in rugged and exposed terrain where 3D exposure is good. Be safety conscious and aware of the possible dangers, particularly from loose rock underfoot, and from rock falls. Barnes (1981) outlines fieldwork safety, and in addition to reading this the reader should also consult the safety checklist on p. 16 of this Handbook before commencing fieldwork. Always carry out fieldwork in compliance with the Geologists’ Association Code of Conduct (see Barnes, 1981).

1.3 Tectonic and structural regimes

It is beyond the scope of this Handbook to describe regional structural relationships in detail, but it is useful to identify the dominant features associated with particular tectonic settings, as they provide a useful guide to the structures that may be found whilst mapping (Table 1.1). Characteristic families of structures may be expected to occur in a particular environment, e.g. shallow thrust faults and parallel folding in frontal regions of foreland fold and thrust belts, and this knowledge can greatly aid any ongoing interpretation. Table 1.1 is neither exhaustive nor exclusive in its contents and you should always be prepared for other structures to occur and record all the structural information from the outcrops in your mapping area.

Table 1.1 Structures associated with particular tectonic regimes.

1.4 Bedding

In sedimentary and many metamorphic rocks, bedding surfaces (surfaces of primary accumulation) are our principal reference frame (or datum). There are many possible bedforms in sedimentary sequences (see Tucker, 1982 for details) and the structural geologist must be aware that significant departures from layer-parallel stratigraphy can occur in certain sedimentary environments, e.g. deltas; thus structural data must always be collected in conjunction with sedimentological and stratigraphic data.

Bedding is one of the most important structural elements and the structural data that should be collected for bedding are outlined in Table 1.2. The spatial distribution of bedding or compositional banding (e.g. ingneissic terranes), will define the major fold and fault structures within your mapping area.

1.4.1 Way-up/younging and facing

Way-up/younging is the direction in which stratigraphically younger beds/units are found. (The term tops is also sometimes used in this context.)

The stratigraphic way-up is of fundamental importance in determining the structure of an area. It is based upon a knowledge of stratigraphy and of small-scale sedimentary structures which indicate the stratigraphic way-up and the sequence of deposition. Sedimentary structures which indicate way-up are discussed in Tucker, 1982 and are summarised in Fig. 1.1. Always look for and record way-up features when mapping.

The structural way-up refers to the bedding/cleavage relationships that indicate the position within a major fold structure (e.g. on the overturned limb of a recumbent fold). This may have no relationship to stratigraphic way-up. Take care to distinguish the two — see Chapter 3 for greater detail.

Facing is the direction within a structure i.e. along the fold axial plane or cleavage plane, in which younger beds/units are found. This term is generally applied to folds, or cleavage relationships.

1.5 ‘Synsedimentary’ versus tectonic structures

In many areas of deformed sedimentary rocks it is difficult to distinguish between structures formed during deposition or early diagenesis when the sediment was unconsolidated, and those formed after lithification in response to tectonic forces. On cursory examination many ‘synsedimentary’ structures such as slump folds have superficial geometric similarities to ‘tectonic’ folds (Fig. 1.2a). Syndepositional faults are also common (Fig. 1.2b) and in some instances syndepositional cleavage fabrics have been observed (Fig. 1.2c). It is therefore extremely important when mapping to distinguish between syndepositional (prelithification) structures and post-lithification, ‘tectonic’ structures. In some situations e.g. active continental margins, sediments are deformed by tectonic forces very soon after deposition, before complete lithification. Hence you may find a complete spectrum of structures, from those formed during deposition to those formed deeper in the crust.

Table 1.2 Data to be collected from observations on bedding S0.

Fig. 1.1 Primary structures that may be used to determine the stratigraphic way-up of beds.

Fig. 1.2a Synsedimentary (Slump) fold in sandstone. Note the absence of fractures, veins, and cleavage.

Fig. 1.2b Listric syndepositional extensional fault (a) in siltstones and mudstones. Note the development of left-dipping antithetic faults (b and c). Scale, near (b), is 1 m.

Fig. 1.2c Recumbent Synsedimentary (Slump) fold in siltstones with the development of a weak flat-lying cleavage (cl).

Fig. 1.2d Syndepositional extensional faults (f) in sandstones. Note that the faults are curved in plan and are consistently downthrown to the right. Field of view is 4 m.

1.5.1 Discrimination of prelithification (‘synsedimentary) from post-lithification (tectonic) folds

‘Synsedimentary’ folds or ‘slump’ folds have many geometric similarities to the shapes, wavelengths and size distributions of ‘tectonic folds’. Slump folds are generally tight to isoclinal with variable shapes at low fold amplitudes. Their fold axes are commonly dispersed in the plane of the slump sheet and recumbent folds are dominant (Fig. 1.2c). The fold axial surfaces may be slightly imbricated (stacked up like shingles) and the folds face and verge down the inferred palaeoslope. Axial-planar cleavages are sometimes developed, particularly in the hinge regions (probably due to later compaction during burial). Lineations and grooving are sometimes produced by the motion of the slump sheet, and these may be refolded along with other minor structures. Slump sheet contacts may be gradational. Their upper boundaries may exhibit sharp erosional truncations. Synsedimentary fractures within slumped sequences are generally not sharp, and fracture openings are not maintained. Veining is absent although the fracture plane may be infilled with mobilised sediment slurry. In general, slump folds have no genetic or geometric relationship to large macroscopic folds.

Syndepositional folding is commonly associated with disturbed sedimentary sequences—synsedimentary extensional faulting, convolute laminations, ball and pillow structures, dewatering structures, sand and mud volcanoes. Remember that slumps are characterised by extensional structures at the rear, whereas the front of the slump sheet will be marked by localised compression, with the development of folds, thrust faults and imbrications. The characteristic features of syndepositional folds are listed in Table 1.3 and are compared with the features of ‘tectonic’ folds (see Chapter 3).

1.5.2 Discrimination of synsedimentary faulting from tectonic faulting

The attributes of tectonic faults are described in Chapter 6. They are characterised in particular by their geometric relationships with associated structures, folds, fractures and veining, and most importantly, by the development of fault rocks along the fault planes (Chapter 6.6). The presence of faults bounding major basin margins will be revealed by regional mapping, by the associated facies distributions of coarse-grained fault-derived sediments adjacent to the fault scarp, by increased sediment thicknesses adjacent to the fault, and by associated smaller syndepositional faults and slumps indicating active tectonism during sedimentation. Here, attention is focussed on outcrop scale features indicative of synsedimentary faults.

Table 1.3 Criteria used to distinguish between synsedimentary folds and ‘tectonic folds’. A is the most reliable criterion and C is the least reliable. Note that several criteria must be used in conjunction in order to determine the origin of a particular fold.

1  Synsedimentary faults typically do not affect all of the stratigraphic sequence and are overlain by unfaulted sediments in depositional contact.

2  The faults are typically listric in shape (Fig. 1.2b).

3  The faults are typically irregular in plan — often curved (Fig. 1.2d).

4  The down-thrown side of the fault is commonly infilled with a triangular wedge of sediments (Fig. 1.3a) which in some cases may be coarser grained than the surrounding sediments.

5  There is an absence of veining and fault rocks typical of brittle deformation (Chapter 6.6).

6  The fault planes are not generally smooth planar fractures, but are often irregular on a small scale (Fig. 1.3b), commonly with injected sediment slurry along the fault plane.

7  The faults are often associated with syndepositional slumping and disturbed sedimentary sequences — convolute laminations, dewatering structures and sand volcanoes.

Fig. 1.3a Syndepositional extension faults (f) showing rotation of fault blocks and infilling of the half-graben with a wedge of sediment (w).

Fig. 1.3b Small syndepositional fault in sandstones, showing an irregular fault plane and injection of sediment slurry along the fault plane.

This is commonly found in the deformed mudstones of slumped sequences. The foliations are planar and parallel to the sheet dip of the sediments, and typically have the appearance of a slaty cleavage or very closely spaced, fine fracture cleavage (Fig. 1.4). They are axial-planar to recumbent slump folds (Fig. 1.2c) and generally do not penetrate sandstone layers but are restricted to mudstones. Slight fanning of the cleavage may occur but the strong refraction of cleavages commonly found in lithified rocks (Chapter 4.3) does not usually occur.

If there is evidence of synsedimentary deformation, then great care has to be taken in recognising and mapping cleavage features. In such circumstances careful examination of all of the field relationships is required before a cleavage can be ascribed to syndepositional processes or to later tectonism.

Fig. 1.4 Slump fold with a flat-lying syndepositional axial-planar cleavage (cl). Field of view 2 m.

1.6 Basic References

BADGELY, P.C., (1959) Structural Methods for the Exploration Geologist. New York, Harper, 280 pp.

BARNES, J.W., (1981) Basic Geological Mapping. Geological Society of London Handbook Series, No. 1. Open University Press, 112 pp.

DAVIS, G. H., (1984) Structural Geology of Rocks and Regions. New York, Wiley, 492 pp.

FRY, N., (1984) The Field Description of Metamorphic Rocks. Geological Society of London Handbook Series, No. 3. Open University Press, 110 pp.

HOBBS, B.E., MEANS, W. D. & WILLIAMS, P.F., (1976) An Outline of Structural Geology. New York, Wiley, 571 pp.

PARK, R.G., (1983) Foundations of Structural Geology. Glasgow, Blackie, 135 pp.

PRICE, N.J., (1966) Fault and Joint Development in Brittle and Semi-Brittle Rock. Oxford, Pergamon, 176 pp.

RAGAN, D., (1985) Structural Geology. An Introduction to Geometrical Techniques. 3rd ed. New York, Wiley, 393 pp.

THORPE, R.S., & BROWN, G. C., (1985) The Field Description of Igneous Rocks. Geological Society of London Handbook Series, No. 4. Open University Press, 162 pp.

TUCKER, M., (1982) The Field Description of Sedimentary Rocks. Geological Society of London Handbook Series, No. 2. Open University Press, 124 pp.

WILSON, G. (with COSGROVE, J.) (1982) An Introduction to Small-Scale Geological Structures. Allen and Unwin, 128 pp.

1.7 Safety

1.   Do not run down hills.

2.   Do not climb rock faces unless you are a trained climber and you have a friend present.

3.   Do not enter old mine workings or cave systems except by arrangement, and always in company.

4.   Wear easily seen clothing.

5.   Always wear a safety helmet in quarries, under steep cliffs and scree slopes, and underground, and wear goggles when hammering rocks.

6.   Note weather forecasts in mountainous country and if you are going into a remote part of an area leave with a responsible person your route map and the time you expect to return.