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2022 | Buch

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Structural Geology

verfasst von: A.R. Bhattacharya

Verlag: Springer International Publishing

Buchreihe : Springer Textbooks in Earth Sciences, Geography and Environment

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik"

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Über dieses Buch

This textbook is a complete, up-to-date, and highly illustrated account of Structural Geology for students and professionals, and includes fundamentals of the subject with field and practical aspects. The book aims to be highly reader-friendly, containing simple language and brief introductions and summaries for each topic presented, and can be used both to refresh overall knowledge of the subject as well as to develop models for engineering projects in any area or region. The book is presented in 20 chapters and divided into 3 parts: (A) Fundamental Concepts, (B) Structures: Geometry and Genesis, and (C) Wider Perspectives. For the first time as full chapters in a textbook, the book discusses several modern field-related applications in Structural Geology, including shear-sense indicators, and deformation and metamorphism. Also uniquely included are colored photographs, side by side with line diagrams, of key deformation structures not seen in other books before now. Boxes in each chapter expand the horizons of the reader on the subject matter of the chapter. Questions at the end of each chapter, and detailed significance of the key structures, provide a better grasping to students. Glossary at the end of the book is a refreshing aspect for the readers. Though written primarily for undergraduate and graduate students, the text will also be of use to specialists and practitioners in engineering geology, petrology (igneous, sedimentary, and metamorphic), economic geology, groundwater geology, petroleum geology, and geophysics, and will appeal to beginners with no preliminary knowledge of the subject.

Inhaltsverzeichnis

Frontmatter

Fundamental Concepts

Frontmatter

1. Introduction to Structural Geology

Abstract

Standing on a rocky terrain, one may occasionally notice beautiful structures in rocks. The variety of geometrical shapes of the structures poses great inquisitiveness to an onlooker. What are these structures, how are these formed and what does their occurrence indicate are some of the questions that haunt his/her mind. Answers to these questions are hidden in a discipline of geology called structural geology, which encompasses the study of all aspects of the deformation structures such as their geometry, field relations, geographic distribution, genesis and related aspects. Since the structures in the context of structural geology are formed by deformation, these are also called deformation structures or tectonic structures. They occur on scales ranging from tens of kilometres to those that can be seen only under a microscope. Faults, folds and joints are some common examples of structures. Structural geology is closely related to another discipline of geology called tectonics, which includes deformation on a much larger scale, i.e. from regional to lithospheric. Structural geology and tectonics commonly go together and even overlap when studied on the same scale. Structures are sometimes associated with economic minerals and hydrocarbons. The type and orientation of structures are important in civil engineering work such as dams, tunnels and bridges. Structural geology has implications for academic, economic, societal and environmental aspects.

This chapter presents a panoramic view of the wonderful world of structural geology so that the reader systematically starts gearing up for an in-depth study of this discipline.

A. R. Bhattacharya

2. Attitudes of Structures

Abstract

While in field, the foremost thing a geologist should do is to note down the attitude of the rock or of any structure he/she intends to study. Attitude is a fundamental geometrical attribute that describes how a plane or a line is oriented on the surface of the earth. Attitude of any geological structure undoubtedly constitutes the starting point of any geological work of whatsoever nature. Measurement of the attitude of structures especially involves the concepts of direction, planar structures, linear structures, dip of beds, strike line, bearing and back-bearing. The direction system used by a structural geologist is either the conventional system or the azimuth system. The attitude of planar structures is defined by dip of beds and strike line, while those of linear structures by plunge and pitch. Bearing and back-bearing are used to locate any object or yourself in a topo-sheet. The most fundamental geological field instrument, clinometer compass, is described including its use in field. In general, the chapter provides the basic concepts of attitudes of structures, thus enabling a beginner to carry out independent field work in structural geology.

A. R. Bhattacharya

3. Stress

Abstract

A layman may laugh at you when you say that a rock feels the pinch of a force imposed on it! Yes, it does feel irrespective of the amount of force. In fact, the rock gets disturbed when a force is imposed upon it. The disturbance thus developed in a rock is called stress which is expressed as the force acting per unit area of a rock. If the applied stress persists uniformly till its amount exceeds the strength of the rock, the latter undergoes deformation, thus developing strain. Depending upon the specific conditions in the earth’s crust, stress can be of various types such as hydrostatic stress, differential stress, deviatoric stress and lithostatic stress. The stress that is locked in the rocks when they were formed in the geological past is called palaeostress that has implications for the amount of deformation (strain) in rocks as well as for the directions in which the stresses had acted upon the rocks. Precise knowledge of the present-day state of stress inside the earth’s surface is important in geology and in engineering geology, especially for various engineering and mining projects as well as for our preparedness for earthquakes. This chapter highlights some aspects of stress as relevant to structural geology.

A. R. Bhattacharya

4. Strain

Abstract

This chapter takes you to where one can visualize how a stress can produce its effects on an undeformed rock. The amount of deformation that a body has undergone in response to an externally applied stress is called strain. The various types of tectonic structures such as folds, faults and boudins noticed in the rocks are manifestations of the strain that the rocks have undergone. Strain may cause change in shape or volume or both. The state of deformation of a rock mass can be referred to three mutually perpendicular lines, and then we can imagine a sphere which after homogeneous deformation is changed to an ellipsoid, called strain ellipsoid. The latter in two dimensions can be described a strain ellipse. An undeformed body can take up different shapes by homogeneous progressive strain either by (a) pure shear or irrotational shear when the shape changes in such a way that the material lines do not change their angular relations or by (b) simple shear or rotational shear when the shape changes in such a way that the material lines have changed their original angular relations. The process of deformation by which a body continues to change its shape and size is called progressive deformation. Knowledge of strain is important in geology as, among others, it provides information on the physico-mechanical aspects of rocks. Identification of low-strain and high-strain zones in rocks has bearing in the localization of some mineral deposits.

A. R. Bhattacharya

5. Estimation of Strain

Abstract

Nowadays, a statement such as weakly, moderately or strongly deformed rock sounds rather vague to a structural geologist as it does not give any idea of by how much the rock is deformed. Over the last about half century, this vagueness has been mitigated to a great extent by estimation of strain, i.e. the amount of deformation, from deformed rocks. This is done by identifying some objects in a deformed rock, called strain markers, whose original shape or size is known. Strain estimated at the time of measurement is called finite strain that can be considered as a cumulative effect of several deformation events, and the amount of strain added in each deformation event may be described as incremental strain. A variety of methods have been developed for estimation of one-, two- and three-dimensional strain in deformed rocks. Some commonly used methods are described in this chapter.

A. R. Bhattacharya

6. Rheology

Abstract

Rocks flow! Sounds unrealistic to a non-geologist. But yes, rocks flow under certain physical conditions though we cannot see it. Study of deformation and flow under different physical conditions is called rheology. Rocks change their deformation behaviour under different physical conditions. Rheology thus deals with how the physical factors control the nature of deformation of a material in respect of flow. The rate at which a rock deforms is called strain rate. The relationship between strain rate and stress is called constitutive law for a substance. The physical parameters of rocks such as rigidity, elasticity and viscosity are called the intrinsic parameters. The latter depend upon external factors such as temperature, pressure and time that together are called the extrinsic parameters. The mathematical relations existing among all these factors constitute the rheological equations or constitutive equations. Mathematical expression that relates differential stress, temperature and strain rate is called flow law that is expressed by means of specific constitutive equations supported by experimental data. Rheological properties of lithospheric rocks depend much on the temperature gradient. Rocks down to the brittle-ductile transition (15–20 km) are under very high temperature gradient and thus behave as elastic material, while those below it behave as ductile material due to low temperature gradient. Although rheology is an important aspect of rock mechanics, the discussions contained in this chapter will show that rheology is also an important aspect of structural geology.

A. R. Bhattacharya

7. Concept of Deformation

Abstract

While loading a box of 20 kg weight on your head, have you ever noticed that your face looks distorted? With more weight, your face looks more distorted. A load of, say, 2 kg, on the other hand, will make practically no visible effect on your face. With progressive increase of load, your face progressively starts showing visible effects of distortion. In the language of structural geology with this analogy, the load of 20 kg will produce strain or deformation on your face because it exceeds your strength or loading capacity. Likewise, rocks too show signatures of deformation when they are under stress. Effect of stress is sometimes visible and sometimes not, depending upon the strength of a rock with respect to the applied stress.

This chapter takes the readers to explore the concept of deformation of rocks with the necessary elementary ideas so that their journey to structural geology may not become an uphill task!

A. R. Bhattacharya

Structures: Geometry and Genesis

Frontmatter

8. Folds

Abstract

Can rock layers bend? Yes, they can but only when they are ductile, and can thus produce spectacular structures called folds that are bends in rocks formed due to compressive stresses acting parallel to, or across, the originally flat surfaces of rock layers. Folds may occur in a single layer or in multilayers. They are developed on all scales ranging from microscopic to kilometric and occur in all types of rocks. They commonly occur in strongly deformed rocks of orogenic provinces. This chapter provides a description of various parts of folds, their classification and theories of their formation. The significance of folds is highlighted.

A. R. Bhattacharya

9. Faults

Abstract

This chapter takes you to the amazing world where a block of rock mass moves past another such block along a plane of separation called fault. A fault is an elongated zone of high shear stress along which two blocks of adjacent rocks have been offset. It is thus a discontinuity or anisotropy due to which the rock loses its cohesion. Being a plane, a fault can be described in geometric terms such as dip and strike. The orientation of stresses acting upon a fault surface controls the geometry of the fault. Rubbing of two blocks of rock masses not only leaves behind a variety of signatures on or in close vicinity of the fault surface but may also genetically modify the host rocks. During fieldwork, a geologist is sometimes surprised to note that in some areas a fault has concealed one or a few beds while in other areas it has got a few other beds repeated. In brief, a fault is able to do or undo many things to the rocks of an area that sometimes leave a field geologist baffled!

In this chapter, the reader may find a detailed description of faults in respect of their geometrical attributes, classification, recognition in field, common rock types found in fault zones and mechanics of faulting. Faults are highly useful structures in the exploration of hydrocarbon, minerals and groundwater. The reader can get all these and many more once he/she takes a plunge into this chapter!

A. R. Bhattacharya

10. Extensional Regime and Normal Faults

Abstract

Extensional regime refers to a state of tectonism developed by the extension of a layer or a part of the crust. Extension means lengthening and is caused by tension (in contrast to compression that causes shortening). Our knowledge on extensional tectonism increased in the 1980s when several normal faults of the Basin and Range Province of the western USA were reinterpreted as low-angle extensional structures, called detachment structures that have accommodated tens of kilometres of displacement. Extensional regime commonly develops along divergent or constructive plate boundaries and intra-plate regions where extensional regime may develop locally by the formation of rifts, domino faults, grabens and growth faults. Models of extensional tectonism are discussed in this chapter. Normal faults are the most common structures to cause extension. A normal fault is one in which the hanging wall has gone down, dominantly by dip-slip movement, relative to the footwall. Because of a downward movement, this fault is also called a gravity fault. Other structures developed in extensional regime include domino faults, detachment faults, growth faults, metamorphic core complexes, rifts, ring faults and calderas. Normal faults bear great academic and economic significance.

A. R. Bhattacharya

11. Contractional Regime and Thrust Faults

Abstract

Rocks travel? More than a century ago, a common man could not believe when a structural geologist said that in some mountain belts rocks have travelled for several, even tens of, kilometres in the geological past. Geological studies of several mountain belts revealed the occurrence of older rocks over younger ones, and the source area of the older rocks could be found at some distances ranging from a few kilometres to tens of kilometres. That rocks can travel along some weak planes, called thrust faults, was thus established. A thrust fault, also called a reverse fault, is one in which the hanging wall has moved up relative to the footwall along a surface that has low inclination (commonly less than 45^°). Thrust faults are able to transport rocks to long distances.

Thrusts and folds are characteristic structures of contractional regime, which is one in which a layer or a part of the earth’s crust is shortened. Various articulations of thrusts, known as thrust geometry, have come in a big way to help extract hydrocarbons and economic minerals from the earth’s interior. Thrust geometry is nowadays a highly sought-after aspect of thrust faults. This chapter takes the readers to explore contractional regime, various aspects of thrusts, thrust geometries, genesis of thrust faults and a special structure called salt diapir.

A. R. Bhattacharya

12. Strike-Slip Faults

Abstract

A strike-slip fault is one that shows horizontal slip parallel to the strike of a vertical or subvertical fault plane. Strike-slip faults are often associated with large displacements. The fault plane in most cases extends for great depth, and as such most of them show brittle behaviour on the upper part and ductile behaviour at depth. The trace of the fault may show curvatures where it shows some structural complexities. Due to motion along the strike-slip faults, the ground surface and the associated rock masses get rubbed against each other, thus producing earthquakes. In this chapter, we shall describe the various geometries and types of strike-slip faults and the structures associated with such faults.

A. R. Bhattacharya

13. Joints and Fractures

Abstract

Rocks crack! Yes, they do, provided that they are brittle. A look on some outcrops occasionally reveals that the rocks are riddled with cracks that are called joints along which there has been little or no transverse displacement of rock. A related structure is fracture, which is a general term for any kind of break or discontinuity in rock. Yet another related structure is shear fracture in which wall-parallel displacement is discernible. Joints may be open or filled with some minerals. Joints and fractures filled with minerals are called veins. A variety of joint types have been identified on the basis of geometries shown by joints. Joint surfaces show several types of features that provide information on the direction of propagation and mode of formation of the joints. Joints form when the tensile stresses of a rock exceed its tensile strength, and they propagate where the component of maximum effective tensile stress is normal to the plane of the crack. Joint mechanics and causes of formation of joints are discussed in this chapter. Fracture mechanics, that concerns the study of stress concentrations caused by sharp-tipped flaws and the conditions for the propagation of these flaws, has also been discussed. Since joints make a rock mechanically weak, their study is important for engineering purposes and civic construction work. Further, since joints and fractures are avenues for passage of fluids, their study is also important for migration of petroleum, water and economic minerals.

A. R. Bhattacharya

14. Foliation

Abstract

Foliation is a planar structure given by preferred orientation of minerals generally showing a platy or tabular habit. The preferred orientation is produced by deformation and is uniformly pervasive in a rock. Foliation is commonly developed in metamorphic rocks and includes cleavage, schistosity, gneissosity and gneissic banding. Cleavage or rock cleavage is a planar fabric along which rocks easily split or cleave into parallel or subparallel surfaces formed during metamorphism and deformation. Along the cleavage, a rock shows the ability to split or cleave into parallel or subparallel surfaces. Cleavage is generally developed in low-grade metamorphic rocks, while foliation is developed in rocks showing low- to high-grade metamorphism and is developed in all types of rocks. Foliation is assumed to represent the XY or the flattening plane of the strain ellipsoid. The foliation-forming processes can be considered to be those that give rise directly or indirectly to preferred orientation of components of a rock. In deformed rocks, study of foliation helps in tracing the deformation history.

A. R. Bhattacharya

15. Lineation

Abstract

Any fabric or orientation in a rock developed in a linear fashion as a result of tectonic deformation is called lineation or linear structure. Though lineation is commonly developed on the surface of rocks, it may also penetrate to a small extent, up to a few millimetres only. As such, lineation is broadly grouped as non-penetrative lineation such as slickensides, slickenlines and slickenfibres and penetrative lineation such as mineral lineation, crenulation lineation, intersection lineation, boudins, mullions, rods, pencil structure and pressure shadows. Lineation can form by processes that can be metamorphism dominated, deformation dominated or geometrically controlled, though it is difficult to draw any specific line of demarcation between these groups of processes. Study of lineation is important in structural geology as some lineation types constitute structural markers to relate smaller structures with the larger ones. Some other types, such as intersection lineation, given by a folded layer and an axial-plane foliation indicate the orientation of fold axis. This chapter describes the various types of non-penetrative and penetrative lineation, lineation as a tectonic fabric, genesis and significance of lineation.

A. R. Bhattacharya

Wider Perspectives

Frontmatter

16. Mechanisms of Rock Deformation

Abstract

The structures and microstructures of rocks that we see today are the culmination of several mechanisms that the rocks have undergone in the geological past. A mechanism of rock deformation broadly means the process that tends to accommodate large strains in rocks, thus leading to a stable structure/microstructure to the rock. The processes operate on different scales, but those operating on grain scales bring about significant changes to the structure and texture of rocks. The processes may operate singly or in combination with other ones. The various physico-mechanical conditions that prevailed during a particular mechanism may change with time and in turn may trigger another mechanism(s) to operate. This chapter is a survey of the various mechanisms of rock deformation that operate on microscale and mesoscale.

A. R. Bhattacharya

17. Shear Zones

Abstract

Shear zones are tabular bodies of rock that accommodate the bulk or whole deformation so that there is practically no or less deformation outside this zone. As such, they constitute anisotropy in a rock mass. An ideal shear zone is bounded by two parallel surfaces. Shear zones can occur in any rock type and can develop in various geologic settings, commonly in contractional, extensional and strike-slip settings. Most shallow-level faults continue at deeper levels where they form ductile shear zones containing rocks deformed under high temperature-pressure conditions and over a wide zone ranging up to tens of kilometres. Shear zones are characterized by the occurrence of some typical rocks because deformation pattern within a shear zone is different from that of the outside rocks. Common brittle shear zone rocks include breccia, cataclasite and gouge while those of ductile shear zones include mylonite. Large-scale ductile shear zones are characterized by progressive development of crystallographic preferred orientation (CPO) of their minerals, mainly quartz. Formation of shear zones depends upon the mechanisms that are able to localize strains in a narrow zone. Most shear zones grow in length and thickness over time. We take you through this chapter to explore the concept of shear zones, their geometry, rock types, classification, formation and significance.

A. R. Bhattacharya

18. Shear-Sense Indicators

Abstract

A common practice in rocks deformed by shear deformation is to know the sense of shear. This is done by identification of some small-scale structures, called shear-sense indicators that provide the sense of shear of the shear zone. The indicators are developed on all scales ranging from mesoscopic to lattice scales and in practically all rock types. In this chapter, we have described some common shear-sense indicators developed in rocks deformed by both ductile and brittle deformations. Although shear-sense indicators indicate the sense of movement during deformation, studies reveal that the practice may sometimes lead to misleading results by providing opposite sense also. This phenomenon is called opposite shear sense.

A. R. Bhattacharya

19. Deformation and Metamorphism

Abstract

It is believed that most metamorphic processes go together with deformational processes. While metamorphic processes involve several physico-chemical factors, operate in a complicated way and produce new minerals or alter some pre-existing minerals, the deformational processes provide suitable stresses that reorient the newly formed minerals and mineral assemblages to give rise to new fabric or microstructure to the rock. In other words, deformation promotes metamorphism and vice versa. Deformation can accelerate reactions in several ways, while metamorphism can promote deformation in several ways. In this chapter, we have described some common deformation structures in metamorphic perspective. The significance of porphyroblasts has especially been highlighted in this context. Also, a discussion on relative timing of deformation and metamorphism has been presented to elucidate how these two processes interact.

A. R. Bhattacharya

20. Superposed Folds

Abstract

Superposed folds are complex folds formed due to superposition of an early-developed fold set by some later fold set(s). The resultant fold geometry is said to show a fold interference pattern. Superposed folds may form during a single deformation event or during different deformation events of a single orogeny. These folds commonly occur in most orogenic belts. The classical grouping of superposed folds is in two dimensions. However, some later workers have also considered three-dimensional grouping of superposed folds. Although our understanding of superposed folding is broadly field based, experimental studies of both single-layer and multilayer folds have thrown significant light on their mode of formation. Reconstruction of three-dimensional shapes of the superposed folds, and thus outlining the stages of evolution of the superposed folds, is sometimes difficult. However, the study of the reorientation of lineations and cleavages or the relations between folds and cleavage or lineation can help in distinguishing the earlier and later generations of folds. Recently, the concepts of progressive deformation and deformation phase are being considered in the context of fold superposition.

A. R. Bhattacharya

Backmatter

Titel: Structural Geology
verfasst von: A.R. Bhattacharya
Verlag: Springer International Publishing
Electronic ISBN: 978-3-030-80795-5
Print ISBN: 978-3-030-80794-8
DOI: https://doi.org/10.1007/978-3-030-80795-5