Understanding an Orogenic Belt: Structural Evolution of the Himalaya

405 Pages·2014·24.651 MB·English

by  Ashok Kumar Dubey (auth.)

Springer Geology
Ashok Kumar Dubey
Understanding
an Orogenic
Belt
Structural Evolution of the Himalaya
Springer Geology
For further volumes:
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Ashok Kumar Dubey
Understanding an
Orogenic Belt
Structural Evolution of the Himalaya
1 3
Ashok Kumar Dubey
Wadia Institute of Himalaya
Dehradun
India
ISBN 978-3-319-05587-9 ISBN 978-3-319-05588-6 (eBook)
DOI 10.1007/978-3-319-05588-6
Springer Cham Heidelberg New York Dordrecht London
Library of Congress Control Number: 2014941098
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To my teachers:
John G. Ramsay
Peter R. Cobbold
John W. Cosgrove
Mike P. Coward
Preface
Structural geology is the study of deformation features in rocks from micro-
scopic to map scale. The required data are collected in field and supplemented
by laboratory studies, aerial photographs, satellite imageries, and subsurface data
(mostly obtained by geophysical methods). A geologist must explore an area with
an open mind and interpretations and hypothesis must be based on actual field
observations. Any evidence, however small, contradictory to a popular belief or
hypothesis must not be ignored and should be taken into account in the final inter -
pretation. Geophysical investigations are useful but some of the techniques are
based upon certain assumptions that allow alternate explanations for the same
data, in many cases. One such example can be cited from the Himalaya where
deep seismic sounding (DSS) studies were performed under an International
Geodynamic Project and investigations were carried out along the Tien Shan-
Pamirs-Karakoram-Himalaya geotraverse. A number of lithospheric sections were
drawn and these sections show very well-marked steep or vertical faults that reach
to a depth of ~90 km and displace the Moho (Kaila et al. 1978). However, later
subsurface sections that emerged after popularization of the thin-skinned thrust tec-
tonics model illustrate a marked basal decollement with a number of splay thrusts
(Allegre et al. 1984). The maximum possible error in case of dip measurements
can be 90° and that can be seen in the available sub-surface sections!
Recently, a problem has cropped up because of the modern evaluation system
of measuring the quality of research. A certain scale is definitely needed to meas-
ure the importance of scientific contributions but with increasing importance of
certain Indexes and factors, a negative trend is emerging. Field observation or lab-
oratory data, which is contradictory to a widely accepted hypothesis is sometimes
ignored by researchers to be in the frontline of research. It must be kept in mind
that because of the heterogeneous nature of the earth’s crust, different stress condi-
tions, and different temperature–pressure conditions during deformation, structural
evolution of each area is unique and each hypothesis must be modified with addi-
tional parameters to highlight this uniqueness. It is really painful if field data is fit
into a particular theory of tectonics without considering the possible alternatives.
One of the objectives of the present book is to bring out the limitations of some
of the modern concepts, related interpretations and propose an alternate model for
evolution of the Himalaya.
vii
viii Preface
In the modern times of super specialization, a field geologist may not be very
well conversant with certain aspects of recent developments in a particular topic.
This has been observed especially in case of the Himalaya where fieldworks
have been done mainly for understanding the regional stratigraphy and structure.
The problem is aggravated by absence of continuous rock exposures (especially
in the NE Himalaya) and lack of sub-surface data for most of the regions. Hence
the cross-sections are approximate and can be extrapolated to fit into a previously
conceived model. Since there are several aspects in earth sciences that cannot be
proved for want of suitable data, it is always better to work with multiple models
so that with gradual increase in database, the final model can withstand the test
and verification from different approaches.
Structural evolution of the Himalaya is largely attributed to collision followed
by subduction of the Indian plate below the Tibetan plate. The model explains
large-scale tectonics of the region but while doing so some of the important
structural features, e.g. the early rift phase in the Himalayan region, occurrence
of younger rocks on the thrust hanging wall, superposed deformation, etc., are
ignored by many. The history of Himalayan evolution does not initiate from the
Tertiary but from the Proterozoic when the normal listric faults initiated during the
rift phase. Relicts of some of these faults can be observed in older rocks.
The object of the book is to provide structural evolution of the Himalaya with
relevant basic information so that earth scientists of other specializations will find
it easy to comprehend. Hence the book is divided into two parts. Part I describes
the basic principles of structural geology and Part II describes the structural evo-
lution of the Himalaya. The book is not intended to be a substitute for an under-
graduate coursework because many of the structural geology topics (e.g. joints,
unconformities, plotting of field data, etc.) have not been incorporated. However,
the book can be used as a supplementary teaching aid.
Dehradun Ashok Kumar Dubey
References
Allegre CJ et al (1984) Structure and evolution of the Himalaya–Tibet orogenic
belt. Nature 307:17–22
Kaila KL, Krishna VG, Roy Chowdhury K, Narain H (1978) Structure of the
Kashmir–Himalaya from deep seismic soundings. J Geol Soc India 19:1–20
Acknowledgments
I am indebted to my teachers Professors J. G. Ramsay, P. R. Cobbold, J. W. Cosgrove,
and M. P. Coward for igniting my interest in structural geology. Their inufl ence can be
seen throughout the book. Many of the ideas discussed in the book have emerged as a
result of joint efi ld studies and innumerable discussions with M. I. Bhat. Constructive
discussions with V. C. Thakur and N. S. Virdi were very helpful. I am also thankful to
my colleagues especially, R. Jayangondaperumal, S. S. Bhakuni, S. J. Sangode, N. S.
Gururajan, S. K. Ghosh, Rohtash Kumar, Koushik Sen, P. Banerjee, Upasana Devrani,
Kavita Tripathi, and A. D. Selokar for continuous support in efi ld and in laboratory
Special thanks to Mr. Rambir Kaushik who helped at different stages of book writ-
ing and its organisation. Mr. Tajinder Ahuja and Mrs. Rama Pant helped in all com-
puter related matters. Mr. Saeed Ahmad and Mr. S. S. Bhandari helped in collection of
relevant scienticfi publications.
I am indebted to J. G. Ramsay, K. S. Valdiya, V. C. Thakur, D. H. Tarling,
B. E. Hobbs, J. Suppe, V. Raiverman, and S. K. Paul for permission to reproduce
their diagrams. I am also thankful to American Geophysical Union, Blackwell
Publishing Asia, Cambridge University Press, Elsevier, Geological Society of
America, Geological Society of London, John Wiley & Sons, NRC Research Press,
Royal Society of London, Royal Society of Edinburgh, Seismological Society of
America, and Taylor & Francis Group for reproduction of some of their gfi ures.
Finally but not least I thank my wife Usha, and son Chinmay for their constant
help and support.
Dehradun Ashok Kumar Dubey
ix
Contents
Part I Basic Concepts
1 Stress and Strain ............................................ 3
1.1 Stress ................................................. 4
1.2 Strain ................................................. 5
1.2.1 Simple Shear ..................................... 6
1.2.2 Homogeneous and Inhomogeneous Strain ............... 7
1.2.3 Extensional and Compressional Structures .............. 8
1.2.4 Strain Ellipsoid .................................... 10
1.2.5 Strains Associated with Homogeneous Deformation ....... 11
1.2.6 Pure Shear. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.3 Flinn Diagram .......................................... 13
1.4 Determination of Finite Strain: Precautions .................... 14
References .................................................. 15
2 Anisotropy of Magnetic Susceptibility .......................... 17
2.1 Different Forms of Magnetization ........................... 18
2.2 Anisotropy of Magnetic Susceptibility. . . . . . . . . . . . . . . . . . . . . . . . 19
2.3 Equipment ............................................. 20
2.4 Collection of Samples .................................... 21
2.4.1 Importance of Sampling in an Area .................... 23
2.5 Magnetic Anisotropy ..................................... 24
2.5.1 Mean Susceptibility (Km) ........................... 25
2.5.2 Magnitude of Anisotropy ............................ 25
2.6 Different Types of AMS Fabrics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.7 Plotting of Magnitude and Shape of Susceptibility Ellipsoid ...... 26
2.7.1 Jelinek Plot (Shape Plot) ............................ 28
2.7.2 Plotting of the Principal Axes ......................... 30
2.8 Hrouda Diagram ......................................... 31
References .................................................. 33
xi
xii Contents
3 Folds and Folding ........................................... 35
3.1 Fold Geometry ......................................... 36
3.2 Fold Classification ...................................... 43
3.2.1 Class 1, Convergent .............................. 43
3.2.2 Class 2, Folds with Parallel Isogons (Similar) .......... 45
3.2.3 Class 3, Divergent ............................... 45
3.3 Why Folds Develop ..................................... 46
3.4 Selection of Wavelength .................................. 48
3.5 Zone of Contact Strain ................................... 51
3.6 Development of Multilayer Folds .......................... 52
3.7 Development of Sinusoidal Buckles in Experiments ............ 54
3.7.1 Construction of Multilayer Plasticine/Modeling
Clay Models .................................... 54
3.7.2 Biaxial Press .................................... 55
3.8 Development of Flexural Slip Folds ........................ 59
3.9 Development of Asymmetric Folds ......................... 65
3.10 Development of Noncylindrical Folds ....................... 67
3.10.1 Importance of the Culmination Point ................. 71
3.10.2 Interference Patterns of Simultaneously Developing
Fold Complexes ................................. 72
3.11 Development of Polyharmonic Folds ........................ 76
3.12 Structures Developing on Fold Surfaces at Late Stages
of Fold Development .................................... 81
3.13 Superimposed Folding ................................... 85
3.13.1 Crossing Orogenic Belts .......................... 85
3.13.2 Successive Deformation Phases in One Orogenic Cycle ... 86
3.13.3 Successive Folding During a Single Progressive
Deformation .................................... 86
3.13.4 Simultaneous Folding in Several Directions During
One Deformation ................................ 87
3.14 Interference Patterns ..................................... 88
3.14.1 Type 0: Redundant Superposition ................... 88
3.14.2 Type 1: Dome-basin Pattern ........................ 89
3.14.3 Type 2: Dome-crescent-mushroom Pattern ............ 90
3.14.4 Type 3: Convergent-Divergent Pattern ................ 90
3.15 Creep ................................................ 96
3.16 Concluding Remarks .................................... 96
References .................................................. 97
4 Thrust Fault ................................................ 101
4.1 Reverse and Normal Drags ............................... 102
4.2 Classification of Thrust Faults ............................. 102
4.2.1 Basal Decollement ............................... 107
4.3 Flat and Ramp Model .................................... 107
4.4 Problems with the ‘Rocky Mountain’ Thrust Model ............ 109