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Answers to Review Questions in the Book Answers to review questions in the book On these pages you find the author’s answers to the review questions presented at the end of each book chapter. Hopefully they will be of help if you are uncertain about how to answer some of those questions. Any- way, some of the questions can be answered in different ways, so do not consider them as an absolute key. In general, when answering questions in structural geology (and most other fields within geology) sketches that in simple terms explain what you mean tend to be useful. A few are presented here, more could be added for more complete answers. Chapter 1 6. What is kinematic analysis? Kinematic analysis is the analysis of particle movement 1. What is structural geology all about? without considering the forces or stresses that caused it A big question that can be answered in many ways, but (involving forces makes our analysis dynamic). It can here is one attempt: Structural geology is about the be performed at any scale, from finding the sense of structures, such as faults, shear zones and folds that shear from a thin section image to determining nappe form as the lithosphere deforms; and about the forces, translations from kilometer-scale folds. stresses and processes that cause lithospheric deforma- tion. 2. Name the four principal ways a structural geologist can learn Chapter 2 about structural geology and rock deformation. How would you rank them? 1. What are the flow parameters discussed in this chapter? Field work, physical experiments, remote sensing (in- Flow parameters describe the flow pattern at an in- cluding seismic data), and numerical methods. Differ- stance, and are the flow apophyses, ISA (Instantaneous ent geoscientists would rank them differently (depend- Stretching Axes) and the kinematic vorticity number ing on their interest and experiment, but I would put (Wk). field work first, and perhaps keep the order given in the previous sentence. Everything should somehow build 2. What is the deformation called if flow parameters are constant throughout the deformation history? on or relate to field studies. Steady-state deformation. 3. How can we collect structural data sets? Name important data types that can be used for structural analysis. 3. Are ISA equal to stress axes? Through conventional field work, from remote sens- Not exactly, but the two are closely related. ISA tells us ing data (satellite images, Lidar data sets and Google how a rock instantaneously reacts to stress. For defor- Earth), seismic data (3-D cubes are great!) and ex- mation involving small strains and for simple bound- perimental data. Magnetic and gravity data are usu- ary conditions, such as in a deformation rig in a labo- ally used together with other types of data, particularly ratory, the two can be considered to be identical. This field data and seismic data. will also be the case for a homogeneous medium that is exposed to linear-viscous (Newtonian) deformation 4. What are the advantages and disadvantages of seismic reflec- (Chapter 6). tion data sets? 4. What is the difference between angular shear and shear strain? Seismic data give the unique opportunity to study and map subsurface structures in three dimensions, but has The angular shear, ψ, describes the change in angle be- a resolution issue (structures and beds below a certain tween two originally perpendicular lines. The shear limit are invisible). strain (γ) is simply the tangent to the angular shear strain (ψ): γ = tan ψ. 5. What is a scale model? 5. What is plane strain and where does it plot in the Flinn dia- A scale model is one where essential parameters, such as gram? geometry, model size, gravity, friction, viscosity, strain rate etc. have proportionally been scaled down. Plane strain is where there is no shortening or extension tion. Two related sets of linear markers, such as fossil perpendicular to the plane containing the maximum symmetry lines of brachiopods, can give us angular and minimum strain axes (X and Z), i.e. Y = 1. Hence shear strain. (X/Y)/(Y/Z) = 1 and X/Z = 1 for constant volume plane 3. What is the effect of a viscosity (competence) difference be- strain, which plots along the main diagonal in the Flinn tween strain markers and the matrix? diagram. If volume change is involved plane strain plots along an offset diagonal. Viscosity contrast may cause objects to deform different- ly from the matrix. Hence, the strain that we get from 6. Give examples of plane strain. the objects is not representative for the bulk strain. Simple shear, pure shear, subsimple shear. 4. How can we deal with pre-deformational fabrics, for example 7. What is meant by particle paths? in conglomerate pebbles? Paths or traces that particles outline over a time interval If all the objects (pebbles) had the same orientation be- during the deformation history. It could be a portion of fore deformation we have a problem. We can only use the history or the entire history of deformation. the Rf/φ-method if there are some pebbles that initial- ly had orientations different from the majority of the 8. What happens to the principal strain axes during pure shear- clasts. We then have to rely on independent knowledge ing? about the pre-deformational fabric, such as observa- They remain fixed in space while they change length ac- tion outside of a shear zone. cording to the equation X/Z = 1. 5. What is needed to find shear strain in a rock? 9. What is meant by the expression non-coaxial deformation his- We need two lines that were orthogonal prior to the de- tory? formation to define shear strain. The shear strain is the It means that some or all of the three principal strain axes tangent to the angular shear strain, which is the change rotate during deformation. in angle between the two originally perpendicular 10. What is the kinematic vorticity number? lines. 6. Give some serious concerns (pitfalls) regarding strain analysis. The kinematic vorticity number describes the ratio be- tween the rate of rotation of strain axes during the • There must be no viscosity contrast (see Question 3.3) deformation and the rate at which these axes change • There may be an initial fabric that must be accounted lengths. for, which is not always easy (Question 3.4). • Strain must be homogeneous at the scale of data collec- 11. What set of material lines do not rotate or change length dur- tion (a prerequisite) – a condition that is rarely fulfilled ing simple shear? to the full extent. Those that lie within the shear plane (parallel to the shear • Measuring the orientation of section(s) relative to prin- zone boundaries). cipal strain axes involves an uncertainty. In most cases, two-dimensional strain analyses is done in the section to represent one of the principal strain planes. Chapter 3 7. How can we find three-dimensional strain from a deformed conglomerate? 1. What is meant by the term strain markers? Give examples. By finding strain from at least two of the three principal planes, or by finding strain from at least three arbitrarily Strain markers are any visual expression in a deformed oriented planes (at relative high angles to each other). rock that allow us to identify changes in shapes and orientations caused by strain. They can be linear or 8. Shear zones are an expression of heterogeneous strain. How may represent areas or volumes. For strain markers can we perform strain analyses in shear zones? to be useful we must know or make some assumption about their pre-deformational shape, length, or orienta- We must look at areas or volumes that are small enough tion. that strain can be treated as homogeneous. 2. What information can we get out of linear or planar strain 9. What is meant by passive and active strain markers? markers? Passive strain markers have no viscosity contrast with the Linear markers may express changes in length (e.g. matrix, while active ones do. See Question 3. boudinaged minerals, belemnites etc.) and/or orienta- 2 – Structural geology 10. What is meant by strain partitioning in this context? That the coordinate axes of our coordinate system paral- When viscosity contrast causes objects to deform differ- lel the principal stress axes (or vice versa). ently from the matrix, the strain is partitioned between the matrix and the strain objects. 7. If the diagonal entries in a diagonal stress matrix are equal, what does the stress ellipsoid look like, and what do we call this state of stress? The stress ellipsoid will be a sphere, and the state of stress Chapter 4 is isotropic. 1. When is it appropriate to use the term pressure in geology? 8. If we apply a stress vector at various angles to a given surface, at what angle is the shear stress at its maximum? How does Pressure should be used when considering a fluid, where that compare to applying a force (also a vector) to the same there is no shear stress. Pressure is correctly used about surface, i.e. at what orientation would the shear component of pore fluids and magma, but also about salt, which over the force be maximized? geologic time behaves more or less like a fluid. Meta- morphic petrologists also use pressure when describing The shear stress is at its maximum when oriented at 45° the general pressure resulting from rock overburden in to the surface, while the shear force is at its maximum the crust, thus implicitely relating to the hydrostatic or when parallel to the surface. See Figure 4.2b. lithostatic model. 2.
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