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RESOLUÇÃO DE QUESTOES DO LIVRO FOSSEN, Exercícios de Geologia

Universidade Federal do Oeste do Pará (UFOPA)Geologia

GEOLOGIA ESTRUTURAL, ESTRUTURAS, RESPOSTAS E PERGUNTAS

Tipologia: Exercícios

2019

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bg1
Answers to review questions in
Fossen 2016
On these pages you find the author’s answers to the review questions
presented at the end of each book chapter. Several of the questions
can be answered in different ways, so do not consider these answers
as absolute. In general, sketches may be useful when answering ques-
tions in structural geology. A few figures are presented here, more
could be added for more complete answers.
a resolution issue (structures and beds below a certain
limit are invisible).
5. What is a scale model?
A scale model is one where essential parameters, such as
geometry, model size, gravity, friction, viscosity, strain
rate etc. have proportionally been scaled down.
6. What is kinematic analysis?
Kinematic analysis is the analysis of particle movement
without considering the forces or stresses that caused it
(involving forces makes our analysis dynamic). It can
be performed at any scale, from finding the sense of
shear from a thin section image to determining nappe
translations from kilometer-scale folds.
Chapter 2
1. What are the flow parameters discussed in this chapter?
Flow parameters describe the flow pattern at an in-
stance, and are the flow apophyses, ISA (Instantaneous
Stretching Axes) and the kinematic vorticity number
(Wk).
2. What is the deformation called if flow parameters are constant
throughout the deformation history?
Steady-state deformation.
3. Are ISA equal to stress axes?
Not exactly, but the two are closely related. ISA tells us
how a rock instantaneously reacts to stress. For defor-
mation involving small strains and for simple bound-
ary conditions, such as in a deformation rig in a labo-
Chapter 1
1. What is structural geology all about?
A big question that can be answered in many ways, but
here is one attempt: Structural geology is about the
structures, such as faults, shear zones and folds that
form as the lithosphere deforms; and about the forces,
stresses and processes that cause lithospheric deforma-
tion.
2. Name the four principal ways a structural geologist can learn
about structural geology and rock deformation. How would
you rank them?
Field work, physical experiments, remote sensing (in-
cluding seismic data), and numerical methods. Differ-
ent geoscientists would rank them differently (depend-
ing on their interest and experiment, but I would put
field work first, and perhaps keep the order given in the
previous sentence. Everything should somehow build
on or relate to field studies.
3. How can we collect structural data sets? Name important data
types that can be used for structural analysis.
Through conventional field work, from remote sens-
ing data (satellite images, Lidar data sets and Google
Earth), seismic data (3-D cubes are great!) and ex-
perimental data. Magnetic and gravity data are usu-
ally used together with other types of data, particularly
field data and seismic data.
4. What are the advantages and disadvantages of seismic reflec-
tion data sets?
Seismic data give the unique opportunity to study and
map subsurface structures in three dimensions, but has
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Baixe RESOLUÇÃO DE QUESTOES DO LIVRO FOSSEN e outras Exercícios em PDF para Geologia, somente na Docsity!

Answers to review questions in Fossen 2016 On these pages you find the author’s answers to the review questions

presented at the end of each book chapter. Several of the questions can be answered in different ways, so do not consider these answers as absolute. In general, sketches may be useful when answering ques tions in structural geology. A few figures are presented here, more could be added for more complete answers. -

5. A scale model is one where essential parameters, such as What is a scale model?^ a resolution issue (structures and beds below a certain^ limit are invisible). geometry, model size, gravity, friction, viscosity, strain 6. Kinematic analysis is the analysis of particle movement What is kinematic analysis?^ rate etc. have proportionally been scaled down. without considering the forces or stresses that caused it (involving forces makes our analysis dynamic). It can be performed at any scale, from finding the sense of shear from a thin section image to determining nappe translations from kilometer-scale folds.

Chapter 2 1. Flow parameters describe the flow pattern at an in What are the flow parameters discussed in this chapter? stance, and are the flow apophyses, ISA (Instantaneous Stretching Axes) and the kinematic vorticity number-

2. Steady-state deformation. 3. What is the deformation called if flow parameters are constant throughout the deformation history?Are ISA equal to stress axes?^ (Wk). Not exactly, but the two are closely related. ISA tells us how a rock instantaneously reacts to stress. For defor mation involving small strains and for simple bound ary conditions, such as in a deformation rig in a labo---

Chapter 1 1. A big question that can be answered in many ways, but What is structural geology all about? here is one attempt: Structural geology is about the structures, such as faults, shear zones and folds that

2. (^) about structural geology and rock deformation. How would you rank them?Name the four principal ways a structural geologist can learn^ form as the lithosphere deforms; and about the forces,^ stresses and processes that cause lithospheric deforma^ tion. - Field work, physical experiments, remote sensing (in cluding seismic data), and numerical methods. Differ ent geoscientists would rank them differently (depend ing on their interest and experiment, but I would put field work first, and perhaps keep the order given in the previous sentence. Everything should somehow build--- 3. Through conventional field work, from remote sens How can we collect structural data sets? Name important data types that can be used for structural analysis.^ on or relate to field studies. ing data (satellite images, Lidar data sets and Google- 4. What are the advantages and disadvantages of seismic reflec tion data sets?^ Earth), seismic data (3-D cubes are great!) and ex^ perimental data. Magnetic and gravity data are usu^ ally used together with other types of data, particularly^ field data and seismic data. --- Seismic data give the unique opportunity to study and map subsurface structures in three dimensions, but has

2 – Structural geology

4. What is the difference between angular shear and shear strain?^ ratory, the two can be considered to be identical. This^ will also be the case for a homogeneous medium that^ is exposed to linear-viscous (Newtonian) deformation^ (Chapter 6). The angular shear, 5. What is plane strain and where does it plot in the Flinn dia gram? tween two originally perpendicular lines. The shear strain ( strain ( ψγ ) is simply the tangent to the angular shear): γ = tan ψ ψ , describes the change in angle be. -- Plane strain is where there is no shortening or extension perpendicular to the plane containing the maximum and minimum strain axes ( ( strain, which plots along the main diagonal in the Flinn diagram. If volume change is involved plane strain X / Y )/( Y / Z ) = 1 and X / Z = 1 for constant volume plane X and Z ), i.e. Y = 1. Hence

  1. Give examples of plane strain. Simple shear, pure shear, subsimple shear. 7. What is meant by particle paths?^ plots along an offset diagonal. Paths or traces that particles outline over a time interval 8. What happens to the principal strain axes during pure shear ing? during the deformation history. It could be a portion of the history or the entire history of deformation. - They remain fixed in space while they change length ac 9. What is meant by the expression non-coaxial deformation his It means that some or all of the three principal strain axes tory? rotate during deformation. cording to the equation X / Z = 1. --
  2. What is the kinematic vorticity number? The kinematic vorticity number describes the ratio be tween the rate of rotation of strain axes during the deformation and the rate at which these axes change lengths. -
  3. What set of material lines do not rotate or change length dur Those that lie within the shear plane (parallel to the shear ing simple shear? zone boundaries). -

Chapter 3

  1. Strain markers are any visual expression in a deformed What is meant by the term strain markers? Give examples. rock that allow us to identify changes in shapes and orientations caused by strain. They can be linear or may represent areas or volumes. For strain markers to be useful we must know or make some assumption
  2. Linear markers may express changes in length (e.g. What information can we get out of linear or planar strain markers?^ about their pre-deformational shape, length, or orienta^ tion. -
  3. What is the effect of a viscosity (competence) difference be tween strain markers and the matrix?^ boudinaged minerals, belemnites etc.) and/or orienta^ tion. Two related sets of linear markers, such as fossil^ symmetry lines of brachiopods, can give us angular^ shear strain. -- Viscosity contrast may cause objects to deform different 4. How can we deal with pre-deformational fabrics, for example in conglomerate pebbles? ly from the matrix. Hence, the strain that we get from the objects is not representative for the bulk strain. - If all the objects (pebbles) had the same orientation be fore deformation we have a problem. We can only use the Rf/φ-method if there are some pebbles that initial ly had orientations different from the majority of the clasts. We then have to rely on independent knowledge about the pre-deformational fabric, such as observa---
  4. We need two lines that were orthogonal prior to the de What is needed to find shear strain in a rock?^ tion outside of a shear zone. formation to define shear strain. The shear strain is the tangent to the angular shear strain, which is the change-
  5. • There must be no viscosity contrast (see Question 3.3) • There may be an initial fabric that must be accounted Give some serious concerns (pitfalls) regarding strain analysis.^ in angle between the two originally perpendicular^ lines.
  • Strain must be homogeneous at the scale of data collec • Measuring the orientation of section(s) relative to prin^ for, which is not always easy (Question 3.4). tion (a prerequisite) – a condition that is rarely fulfilled to the full extent. cipal strain axes involves an uncertainty. In most cases,--
  1. How can we find three-dimensional strain from a deformed conglomerate?^ two-dimensional strain analyses is done in the section^ to represent one of the principal strain planes.

4 – Structural geology

There could be many reasons for this. Here are some: Many orogenic belts result from slightly oblique con vergence, which is partitioned into pure contraction and localized strike-slip motion (strike-slip faults). Strike-slip faults may also result from blocks escap ing laterally during convergence. Elevated parts of the--

  1. What stress regime(s) would we expect along strike-slip In general, a strike-slip regime, but extensional in releas faults, such as the San Andreas Fault?^ orogen may collapse gravitationally to form normal^ faults. -
  2. Why does the differential stress increase downwards in the Because it is more difficult to fracture a rock under con brittle crust?^ ing bends and compressional regime in restraining^ bends (see Chapter 19). -
  3. If we increase the fluid pressure in a sandstone unit, will the^ fining pressure than an unconfined rock. The deeper^ down in the upper crust, the more stress can build up^ before it fractures (yields). Thus the strength increases^ downwards through the brittle crust, until crystal-plas^ tic mechanisms kick in at the brittle-plastic transition.- If we look at Equation 5.2 it is obvious that increasing p^ effective stress increase or decrease? reduces the effective stress. This is because an elevated fluid pressure helps lifting the overburden so that the stress across grain contacts becomes smaller. f

Chapter 6 1. Rheology has more to do with how rocks flow over geo What is the difference between rheology and rock mechanics? logic time as a continuum, while rock mechanics also deals with how rocks fracture along sharp discontinui--

  1. A constitutive law is a mathematical equation that relates 3. What is a constitutive law or equation?What does isotropic mean?^ ties. stress and strain. An isotropic medium is one that has the same mechanical 4. An elastic medium is one that reacts to stress by accumu What is an elastic material? properties in all directions, so that it reacts identically to stress regardless of its orientation. - lating recoverable strain, which means that it returns to its pre-deformational shape once the stress is removed.

It is a state of stress controlled or prescribed by a given strain condition, which is that of compaction. When a rock compacts under the weight of the overburden without being able to expand in the horizontal direc tion, stresses arise. -

  1. What physical factors controls the state of stress in a rock that There are two important effects: The thermal and the is being uplifted through the upper crust? Poisson effects, both of which tend to decrease the horizontal stress. The thermal or cooling effect is re lated to the cooling of rocks as they ascend. Cooling- causes the rock to contract and potentially fracture. The Poisson effect is related to the change in pressure from removal of overburden: Poisson effect is the ratio of the horizontal strain and vertical extension, or the “reluctancy” of a rock to contract perpendicular to the vertical extension. The relevant equation in the book

σ H^ is (5.6): = σ h = 1 ν− ν σ v + 1 − E^ ν α T

where the first term is the Poisson effect and the second is 5. Why does sandstone fracture more easily than shale when up A sandstone has different elastic properties than shale, lifted? the thermal effect. -

  1. How can we define tectonic stress? Tectonic stress is the deviation from the reference stress^ and it reacts as a stronger and more brittle rock. or expected stress caused by tectonic processes.
  2. What conditions must apply for Anderson’s classification of Anderson’s assumptions: One of the principal stresses is 8. What is the differential stress at 5 km depth for continental tectonic stress to be strictly valid? crust if we have a perfect lithostatic state of stress? vertical and strain is coaxial. There is no differential stress at any depth for perfect 9. What forces related to plate tectonics can cause tectonic stress? Slab pull, ridge push, collisional resistance and basal drag lithostatic stress, since all the principal stresses are al ways equal. -
  3. Why do we find evidence of strike-slip and normal fault stress regimes in addition to the thrust-fault regime in active (con tractional) orogens such as the Himalaya and Andes?^ (frictional drag along the base of the lithosphere). -

Answers to review questions in Fossen 2016 – 5

  1. An incompressible medium maintains its volume dur What is an incompressible medium, and what is its Poisson’s ratio? ing deformation. Poisson’s ratio is the ratio between the extensions normal and parallel to the axis of major-
  2. Some media are easier to elastically bend, stretch or shorten than others – we could say that there is a difference in stiffness. What constants describe the stiffness of an elastic material or its resistance against elastic deformation?^ compressive stress. Young’s modulus, which is simply stress over strain. 7. The yield stress is the stress level (elastic limit) at which What is the yield stress and what happens if it is exceeded? a rock accumulates permanent strain, either through brittle or plastic deformation (or both).
  3. Linear elastic means that the stress-strain curve is linear What is the difference between linear elastic and linear vis cous? so that, at any point on the curve, a given increase in stress yields a fixed increase in strain. Linear viscuous means that stress is linearly related to strain rate, so-
  4. Fluids are truly viscous. The asthenospheric mantle is What types of materials are truly viscous? What parts of the Earth can be modeled as being viscous?^ that higher stress means faster flow and strain accu^ mulation. -
  5. What does it mean that a rock layer is more competent than It means that it has a higher viscosity than its surround its neighboring layers?^ considered to be viscous. ings and therefore is more resistive to flow. -
  6. What could cause strain softening and strain hardening in a Strain softening is promoted by growth of platy miner deforming rock? als, reduction in grain size, increased fluid pressure, recrystallization into newer and weaker minerals and an increase in temperature. Strain hardening may be-
  7. What is the difference between plastic deformation and creep?^ caused by accumulation of dislocations, cementation,^ geometric interlocking of grains or structures and^ much more. Plastic deformation is not strain-rate sensitive. Creep 13. What controls the locations of brittle–plastic transitions in the lithosphere? does not involve strain hardening or softening.

This is the transition between brittle and plastic deforma tion mechanisms at the microscale, which is controlled by temperature, to a lesser extent by pressure, and to a large extent by mineralogy. Also, “dry” rocks will behave in a brittle manner at higher temperatures than “wet” rocks. Strain rate also has some influence, where-

Chapter 7^ rapid strain accumulation more easily results in brittle^ deformation.

  1. Granular flow involves frictional sliding along grain con What is the difference between cataclastic and granular flow? tact surfaces and grain rotation and preserves grains from internal deformation. Cataclastic flow is destruc tive because it also involves extensive microfracturing of grains. --
  2. Frictional sliding is mechanical sliding along a surface What is frictional sliding? where frictional forces must be overcome. Another type of sliding, known as grain boundary sliding, oc curs by means of diffusion in the crystal-plastic (non- frictional) regime. -
  3. The process zone ahead of shear fractures is a zone or What is the process zone that is located at the tip of shear frac tures? volume of microcracks that soften the rock and eases fracture propagation. -
  4. Deformation bands (tabular discontinuities) are thicker What is the difference between fractures and deformation bands? than fractures with comparable offsets (sharp disconti nuities). Bands do not accumulate more than cm-scale offsets while fractures can show meter-scale offsets or-
  5. Why do not shear fractures form at 45° to σ solved shear stress is at its maximum?^ more. Fractures show a loss of cohesion, while most^ deformation bands preserve cohesion or are mechani^ cally stronger than the host rock. 1 , where the re-- This is explained in Box 7.2, where it is illustrated how shear fracture initiation or reactivation is controlled not only by shear stress (which is at its maximum at the plane oriented at 45° to σ stress (high normal stress counteracts slip). The ideal balance between these two effects is found at an angle 1 ), but also by the normal
  6. What is a wing crack and how do they form?^ higher than 45°.

Answers to review questions in Fossen 2016 – 7

Chapter 9^ hance macro-permeability and production, which is^ good.

  1. Shear fractures are single structures, commonly referred What is the difference between shear fractures and faults? to as surfaces but with a very small thickness (gener ally <1 mm). A fault is a more composite structure with a thicker zone of strongly deformed rocks (the fault core) in which there may be one or more slip surfaces.-
  2. While the shear stress is greatest at 45° to a fracture, the Why do normal faults tend to be steeper than reverse faults?^ A fault also contains a damage zone of deformation^ bands and/or fractures where strain is much lower than^ in the fault core. balance between shear and normal stress components makes fractures slip most easily at around 30° to Since becomes ~ with σ σ 1 1 being horizontal, the dip should be around 30°. is vertical for normal faults the expected dip 60°. Similarly, for reverse faults formed σ 1.
  3. A mylonite forms as a result of predominantly crystal- What is the main differences between a mylonite and a cata clasite? plastic deformation (see Chapter 11), while a cataclas ite is the result of cataclasis (grain fracturing, frictional sliding and grain rotation). Hence, the two form in the--
  4. This characterizes a reverse fault. Isoclinal folding would A vertical well drills through a part of the stratigraphic section twice (repeated section). What type of fault can we infer, and why can we not explain this by folding?^ plastic and brittle (or frictional) regimes, respectively.
  5. The damage zone tends to grow during faulting due to Why would the damage zone grow during faulting?^ also represent a repeated stratigraphy, but the stratigra^ phy in the inverted limb would be inverted. geometrical complications such as interlocking of-
  6. In general we are not able to see any seismic response Would the damage zone be visible on good 3-D seismic data?^ faults, fault bends, lense formations in the fault, exten^ sive cementation and more. from or image of the damage zone, although the noise- generated along faults may overshadow such signals. Only wide damage zones with deformation structures
  7. What is the difference between syntaxial and antitaxial veins? Syntaxial veins form by mineral growth in the central median line (the crack). Antitaxial veins grow from a median zone toward the walls, meaning that the vein has two growth walls; one along each wall. Fibrous mineral growth can occur in antitaxial veins, not in
  8. Joints typically form during exhumation of rock units. Why? For two reasons: the release of pressure from the overbur^ syntaxial veins. den, and the contraction associated with cooling. Both effects create tensional stress within rock, particularly-
  9. During extension, why do we sometimes get joints (and Joints and extension fractures in general form when one veins) and other times shear fractures and faults?^ in stiff rock layers. of the principal stresses is negative (= tensile). This
  10. What structures can be found on joints that can reveal their growth history?^ can occur near the surface, or if the fluid pressure in a^ crack exceeds the remote minimum stress (hydrofrac^ turing). - Plumose structures with hackles that outline the propaga 12. Are joints good or bad structures in the field of petroleum geology, and why?? tion direction, and ribbs (also called arrest lines or hes itation lines), that outline the joint tip line as various times. Plumose structures are perpendicular to ribs. -- They can be both. If they affect a top seal (cap rock) so that hydrocarbons leaks out of the structure it is bad. However, if they are limited to the reservoir they en-

σn

σs

a b a c Hybridfracture Joint T Shearfracture

Figure Q20.10. Joints form when the minimum stress is tensile, i.e. to theb c left of origin of the Mohr diagram.

8 – Structural geology

Conjugate faults are faults whose acute angle bisects σ1. They form at the same time in the same stress field. Hence, they give the orientation of σ1, σ3 (their line of intersection) and therefore also of σ1 (perpendicular to the others).

  1. What does the Wallace The Wallace 6. What is the reduced stress tensor? planar fracture occurs along the greatest resolved shear stress on that plane. – Bott hypothesis postulates that slip on a – Bott hypothesis postulate? The reduced stress tensor is one that contains informa tion about the orientations of the principal stresses and their relative (but not absolute) magnitudes. Hence, the reduced stress tensor gives us the orientation and shape of the stress ellipsoid. -

Chapter 11 1. What is the difference between a slip plane in a plastically de A slip plane in the brittle crust is a plane along which forming crystal and a slip plane associated with brittle fault ing? --

there is or has been frictional sliding so that the rock is permanently destroyed. A slip plane in a plastically deforming crystal is much smaller (it occurs within a grain at the atomic scale) and heals continuously with out loosing strength. Intercrystalline plastic slip im plies movement of a dislocation front within a plane in--

  1. What are the main principal differences between brittle and The two differ in terms of microscale deformation pro plastic deformation?^ the crystal, and such a slip plane is usually the plane in^ a crystal that has the highest density of atoms. -
  2. Why is intracrystalline fracturing so common in brittle defor mation of highly porous rocks?^ cesses^ through brittle (frictional) processes (cataclasis, rigid^ rotation and frictional sliding) while plastic deforma^ tion occurs by diffusion and dislocation creep.^ (mechanisms):^ Brittle^ deformation^ occurs-- Intracrystalline means within (rather than between) crys tals. In highly porous rocks the stress that builds up across grain forces are distributed across very small areas (stress is force per area), which makes them fracture internally – grain contacts become large because the-
  3. Dipmeter data can show the abrupt changes in strike and How can dipmeter data help identifying faults?^ that strongly alter the velocity structure of the rock^ may yield a seismic response strong enough that we^ may hope to see it on (future) high-resolution images.
  4. (^) and production?Is fault sealing good or bad in terms of petroleum exploration^ (particularly) dip that occur as we drill through faults.^ In some cases dipmeter tools may pick up fracture and^ deformation band orientations and show a concentra^ tion of those in the damage zone. - Large sealing faults are generally good because they can contribute to a trap. Many traps rely on sealing faults. However, small sealing faults within an oil field can cause trouble during water injection and hydrocarbon production because they compartmentalize the reser voir and necessitate more wells and better knowledge-

Chapter 10^ of smaller faults and their properties. Such knowledge^ is extremely difficult to get if the faults are subseismic.

  1. Why must we be careful when interpreting lineations as dis Lineations may not be representative of the displacement placement indicators? we are after, because they may represent only the last minor displacement of the fault, possibly a late reacti vation that has nothing to do with the main fault move---
  2. What are the premises for successful paleostress analysis? Paleostress analysis is based on the assumptions that all^ ment. the faults included in the analysis formed in the same stress field, that the rock is homogeneous, the strain is
  3. What is meant by the expression “slip inversion”? Slip inversion means reconstructing the orientation and^ low and the structures did not rotate since their forma^ tion (or that the rotation can be accounted for). shape of the stress ellipsoid (finding the stress matrix)-
  4. What are conjugate faults, and what stress information do they give?^ from measured fault slip data (the orientation of fault^ planes, their lineation and sense of slip for a number of^ differently oriented fault planes).

10 – Structural geology

The dip isogons are perpendicular to layering for buck 15. Why do we get asymmetric folds on the limbs of lower-order folds? le folds and parallel to the axial trace for shear folds (looking at sections perpendicular to the axial surface).- Because there tends to be a component of flexural shear on each side of the hinge zone. This flexural shear transforms small folds from symmetric to asymmetric structures.

Chapter 13 1. Primary foliations are primary planar structures such as What is the difference between primary, secondary and tec tonic foliations? bedding and magmatic layering. Secondary foliations-

  1. How are primary foliations recognized in deformed and meta morphosed sedimentary rocks?^ form later, and tectonic foliations are secondary folia^ tions that are related to tectonic processes. Almost all^ secondary foliations are tectonic. -- Primary sedimentary structures are not always easy to distinguish from secondary structures. One would, mentally or quantitatively (if possible), account for the strain in the rock to restore the structure to its pre- deformational state. Cross-stratification is a character istic feature, and the angular relations of cross-stratifi-- cation will change during deformation, depending on the type and quantity of strain. Be aware of pseudo cross-stratification created by sheared fold hinges (see Figure Q13.2). Look for remnants of fold hinges along the structure (see red circle in Figure Q13.2).
  2. A fracture zone lacks cohesion (unless cemented by sec What separates a fracture zone from a cleavage or foliation? ondary minerals) and rarely shows the number and distribution of fractures sufficient to define a foliation (fabric). -
  3. Transpression which is a simultaneous combination of 5. The very fine grain size characteristic of very low-grade What type of strain can produce transected folds?What favors the formation of continuous (dense) cleavage? shear and shortening across the shear plane. metamorphic rocks (lower greenschist facies and be low). -
  4. Shear folds are similar (Class 2) folds with no compe A shear fold?^ thin ones. Buckle folds are parallel (Class 1B) folds^ with a neutral surface. tence contrast between folded layers. Shear folds tend to be harmonic. -
  5. Parallel folds (Class 1B) have constant layer thickness, 7. What is the difference between a parallel and a similar fold?What fold types are parallel? similar folds have constant thickness parallel to the axial trace and therefore have attenuated limbs. Ideal buckle folds, orthogonal flexure folds, flexural slip 8. Similar folds are common in strongly sheared quartzite Where can we expect to find similar folds? folds and flexural flow folds. and evaporates. They can also occur in softly deformed
  6. Monoclines can form in sedimentary packages above a In what settings do monoclines typically form?^ sediments. reactivated normal, vertical or reverse fault in the sub strate. Gentle monoclines can form due to differential-
  7. If we compress a thin layer and a thick layer so that they start The thin layer starts buckling first. buckling, which layer do you think starts to buckle first?^ compaction across the crests of major fault blocks.
  8. Which one do you think thicken the most prior to folding? 12. Which of the two layers would form the largest folds? The thick layer must thicken more prior to folding be cause it experiences a longer period of shortening be fore it starts to fold. -- The thick layer forms the largest folds (longest wave 13. What conditions give more or less concentric and parallel Ideal buckle folds tend to be concentric and parallel. Dé folds? length, largest amplitudes). --
  9. How can dip isogons help us separate between buckle folds and shear folds?^ collement folds are commonly found to be concentric^ where there is a large competence contrast between the^ folded layer and its neighboring layers.

Answers to review questions in Fossen 2016 – 11

  1. Only lineations restricted to a slip plane or extension What types of lineations form in the brittle regime?^ shortening and both develop a characteristic wave^ length that is related to the viscosity contrast fracture form in the brittle regime: Mineral lineations-
  2. What lineations mark the X-axis of the strain ellipsoid?^ (fiber lineations), striations (slickenlines) and geomet^ ric striae defined by corrugated slip surfaces. Also, in^ tersection lineations occur where subsidiary fractures^ intersect the main slip surface. -- Stretching lineations 4. Fiber lineations, unless they have been rotated. Fiber What type of lineation can be related to ISA1? lineations can also grow perpendicular to fracture walls, in which case they do not indicate the maximum
  3. They indicate the local slip vector, but do not reveal the 6. How do striations or slickenlines relate to kinematics?What is the difference between crenulation lineations and in^ stretching direction. sense of slip. - A crenulation lineation is made up by small crenulations^ tersection lineations? or microfolds and therefore reflect layer-parallel short ening. An intersection lineation can be the intersection between any two sets of planar structures, and does not have to have this relation to the strain ellipsoid. -
  4. Boudins lie in the field of finite extension. How do boudins relate to the strain ellipsoid?
  5. Wet diffusion (pressure solution). 7. A QF-domain is dominated by quartz and feldspar, while What is the most important mechanism for cleavage forma tion?What is meant by QF- and M-domains? - (^8) Cleavage refraction is the symmetric fanning of cleavage What is cleavage refraction and how can we explain it?^ an M-domain is dominated by phyllosilicate minerals. associated with folds in layers with contrasting com petence. A way of explaining this is that incompetent- (^9) This is easy if we consider the cleavage as the axial plane How can we use the angle between cleavage and a pre-cleav age foliation to predict large-scale fold geometry?^ layers experience (more) flexural slip that rotates the^ cleavage in these layers during folding. -
  6. Because of the anisotropic loss of volume by pressure so late strain ellipsoids)? and try to draw the fold (see Figure Q13.9). lution across the cleavage.Why is cleavage always associated with flattening strain (ob--
  7. What is the difference between shear bands (extensional Shear bands are sets of parallel small-scale shear zones crenulation cleavage) and ordinary crenulation cleavage? while ordinary crenulation cleavage forms by shorten ing and pressure solution perpendicular to the cleav age. --

Chapter 14 1. They both form in or along a competent layer embedded What makes mullions and buckle folds similar? in a less viscous matrix, they form by layer-parallel NO!

Figure Q13.9. YES!

Figure Q13.2. Pseudo cross-stratification created during discrete shear ing along the axial surface of tight folds. Easily mistaken for primary stratification, such structures are seen in sheared quartzites.^ a)b) -

Answers to review questions in Fossen 2016 – 13

  1. What assumption do we have to make for the last increment of strain to be representative for earlier strain increments in the zone?^ or less during the entire history, although this does not^ always have to be the case. We have to assume steady-state deformation, where flow parameters are the same throughout the deformation history. If, for example, we have a change from simple shear to subsimple shear the last increment would just reveal the subsimple shear part of the history. (^5) S–C structures are composite structures consisting of a What is meant by the term S–C structures? foliation (S) and a set or sets of small-scale shear bands with cm-scale offset that back-rotate the foliation rela tive to the shear direction. The shear bands form dur ing shearing, and the foliation is formed or modified--
  2. Deflected markers are perhaps the best indicators. Many What do you think are the most and least reliable shear sense structures described in this chapter?^ during the same shearing history (although in many^ cases at a somewhat earlier stage). S–C structures are good indicators, but may only be representative of the last part of the shearing, and the same goes for oblique microscale foliations formed during dynamic recrystallization of quartz grains and aggregates. difficult to interpret in practice, and fold asymmetry Porphyroclast geometry may be more
  3. How could you get information about the strain path (strain evolution) of a shear zone?^ should be used with care. Importantly, shear sense in^ dicators should always be used together, and if they are^ not in conflict they are making a good case. - The strain path can be approached by comparing strain in the marginal, intermediate and central parts of the zone. This would reflect a path if the shear zone grew from the central part outward. Hence, this type of con sideration requires a model for shear zone growth, which may be difficult to establish. -

Chapter 17 1. Branch lines are the lines where a fault splits into two. What are branch lines and how could they be useful? In cross-section the branch line is reduced to a branch point.

Chapter 16 1. A shear zone is wider relative to the offset, it has main What makes a typical shear zone different from a typical fault? tained cohesion, markers are still (mostly) continuous for most shear zones (ductile shear zones), they do not-

  1. Can you draw the upper shear zone margin on Figure 16.9? Is The upper margin is, by definition, located at the transi it easily definable?^ show a core–damage zone anatomy, and most shear^ zones involve plastic deformation mechanisms. - tion from unstrained to (weakly) strained amygdales. This is quite difficult to identify because the initial amygdales are irregular and with slightly different shapes. It may perhaps seem like there is a weak pre ferred orientation of amygdales outside the shear zone boundary as drawn in Figure Q16.2. It is difficult to-
  2. What type of data support the idea that shear zones grow in width as they accumulate displacement, and where would we look for the last increment of strain?^ know if this is a weak but pervasive strain on which^ the shear strain was superimposed or if it is related to^ the shear zone. There is a positive correlation between shear zone thick ness and displacement, indicating that shear zones grow in width as they accumulate displacement. This implies that the low-strain margins of the shear zone record the last part of the deformation history while the more central parts have been accumulating strain more-

Figure Q16.2. Interpretation of the upper margin of the shear zone.

14 – Structural geology

The shape of an orogenic wedge is controlled by the basal 9. The basal friction can change due to changes in fluid How can those conditions change during an orogenic event? friction, the internal strength of the material within the wedge, and erosion at the surface of the wedge. pressure and fluid availability. Fluids decrease the basal friction, and dryer conditions may occur once metamorphic reactions have depleted the rocks in flu ids. The strength of the rocks within the wedge may decrease if many faults and fractures form within the wedge, and erosion may change with climate changes-

  1. Why is it likely to have pre-existing normal faults in an oro Because most orogens form at the locations of older rifts genic belt?^ (wet climate causing more erosion than dry climate). (in agreement with the Wilson cycle). -

Chapter 18 1. A reverse fault can be regarded as extensional if rotated How can a reverse fault in some special cases also be exten sional? bedding or some other layering is used as reference. In this case it can (for some combinations of fault and-

  1. What is unrealistic about the domino fault model?^ layer orientations) extend the layer(s) at the same time^ as being reverse, as shown in Figure Q18.1.
  2. We would look for a structure that can trap oil, and an If you were looking for oil, what type of contractional structure covered in this chapter would you look for? anticlinal stack of thrust sheets, such as the example shown in Figure 17.15, would be excellent. It contains several closures in which oil could be trapped, provid- ed there is an impermeable rock (shale) that can act as a cap rock. Fault-bend folds (Figure 17.16–18) would also be good for the same reasons. Even décollement folds could trap some oil if the folds are large, although they are seldom large enough to contain large volumes of hydrocarbons.
  3. A mylonite nappe is a nappe that is made up of mylonitic 4. What is meant by a mylonite nappe?What is the difference between a fault-propagation fold and a fault-bend fold? rocks, i.e. the nappe has a pervasive mylonitic fabric. A fault-propagation fold forms as the fault propagate through an overlying layer so that a ramp forms. Fault- propagation folds move together with the tip. In con trast, a fault-bend fold is stationary at the location of a ramp, and affects rocks as they enter the ramp. Once they leave the ramp they are (technically) unfolded.-
  4. Back-thrusts form at ramps, as shown in Figure 17.10. Where could we expect to find back-thrusts?^ The exception is the forelimb which is steep and moves^ with the upper allochthonous unit (Figure 17.17).
  5. Detachment folds form where there is a very weak layer What is the ideal setting for detachment folds to form? in a contracting rock sequence that can accommodate and localize shear, and a competent layer above that can buckle.
  6. They can form where contraction thickens the nappe stack How can extensional faults form in a thrust-and-fold belt? or wedge so that it collapses under its own weight. It does so by extending laterally through the formation of normal faults. Such overthickening can occur where a nappe is ripped off the basement and incorporated
  7. What conditions determine the shape of an orogenic wedge?^ into the wedge as shown in Figure 17.13 and 17.25b.^ Lowering the basal friction could also create normal^ faulting in the overlying orogenic wedge. Figure Q18.1. Reverse faults extending the layering.

16 – Structural geology

They could extend into the plastic field where they would become plastic shear zones. There is evidence that some strike-slip structures extend through the entire crust and into the lithosphere. In other cases strike-slip faults may be connected with a low-angle detachment, for instance in subduction zone settings (see Figure

  1. Flattening strain in a (strike-slip) shear zone forms when How would you explain flattening strain in a shear zone with a clear strike-slip offset?^ 19.7). there is shortening across the zone. This can be the re-
  2. Like Death Valley, the Dead Sea is a place where you can walk on dry land below sea level, and an apparently unmotivated^ sult of anisotropic volume loss, but the shortening is^ more likely to be compensated by extension in a per^ pendicular direction, in which case we have transpres^ sion. -- The Dead Sea is a releasing bend or a stepover between 9. “hole in the ground” along a strike-slip fault. How do you think^ it formed?How can strike-slip faults accommodate large-scale pure two strike-slip fault segments. Strike-slip faults arranged in conjugate sets can accom^ shear? modate large-scale pure shear. -

Chapter 20 1. The overburden is generally too strong, so the overburden Why does diapirism not initiate as a result of density inversion alone? must fracture or fault before diapirism can initiate.

  1. Reactive diapirism means salt rising as a diapir due to tec What is meant by reactive diapirism and in what tectonic regime(s) can it occur? tonic deformation. This occurs most easily during ex tension, in which case space is easily provided so that salt can ascend. Compression can modify (squeeze)--
  2. Classic centrifuge models model both the salt and the What is it about classic centrifuge models that in some ways makes them unrealistic?^ existing diapirs but is less likely to initiate diapirs. overburden as fluids, while the overburden should be modeled as a frictional material.

Chapter 19 1. Most large-scale strike-slip faults (shear zones) are steep What are the characteristics of large-scale strike-slip faults or shear zones? or vertical structures that are relatively straight in map

view, although they may contain stepovers or bends where extensional structures or contractional struc tures form. Some of the largest strike-slip faults are plate boundaries and large strike-slip faults are associ ated with major earthquakes. --

  1. Transfer faults transfer displacement between two exten What is the role of a transfer fault and in what settings do they occur? sional or two contractional structures, for instance be tween two normal faults, two graben segments or two thrust faults. --
  2. It depends on the kinematics of the fault and geometry What type of structures form where a strike-slip fault makes a stepover or an abrupt bend? of the bend. Releasing bends create extensional struc tures, notably normal faults, and restraining bends gen erate contractional structures (folds and reverse faults).--
  3. A profile across restraining and releasing bends may show What would a profile look like across a restraining bend? Re leasing bend? that faults connect at depth to form something simi lar to a positive and negative flower structure, which generally means an extensional graben structure and--
  4. It is the type area for a releasing bend or stepover. What type of setting does Death Valley represent?^ a contractional structure, such as is shown in Figure^ Q19.4.
  5. What could happen to strike-slip faults at depth? Figure Q19.4.^ Profile acrossreleasing bend Profile acrossrestraining bend

Answers to review questions in Fossen 2016 – 17

  1. During active diapirism the salt forces its way upward What is the difference between active and passive diapirism and how can we distinguish between them? through the overburden, driven by some sort of load ing, for example differential loading (a heavier load next to the diapir making salt flow into the diapir). In- passive diapirism diapirs continually rise as sediments are deposited around them. The crest of passive dia pirs are at or close to the surface while active diapirs may be deeper. Active diapirs show evidence of force ful intrusion by means of rotated flaps of what was once roof layers, while layers around passive diapirs--
  2. Downbuilding is the expression sometimes used about What is meant by the expression “downbuilding”?^ are only gently rotated due to compactional effects that^ may create minibasins next to the salt structure. sedimentation around passive diapirism, where sedi- mentary layers are being added as the diapir grows. If our reference is the top of the diapir, which is more or less at the surface (sea bottom), a sedimentary se quence and the source salt layer will build down under the addition of new strata during passive diapirism. -
  3. There is no or too little loading that can drive diapirism. Why do we not see diapirs in the upper part of a gravity-driven décollement setting such as shown in Figure 20.24? The load increases down dip due to gravity and the tapering of the sedimentary wedge overlying the salt.
  4. The strength of the overburden is important. If the What determines whether a salt wall or salt diapir forms? strength is reduced by means of a long fault or fracture zone we can get a salt wall. Salt walls are also common where folding is involved (contractional deformation). However, if weak structures are shorter and more ran-
  5. A salt sheet is a single salt structure that has flowed lat What is the difference between a salt sheet and a salt canopy?^ domly or evenly distributed in the overburden, diapirs^ are more common. erally to obtain a width ≥5 times the thickness of the-
  6. It lowers the basal friction and therefore creates a lower What is the effect of a basal salt layer in an orogenic wedge?^ original diapir or its underlying stem. A canopy forms^ when several (three or more) such sheets grow into a^ connected sheet-like structure. height deformation to extend farther into the foreland, pro – length ratio of the wedge. It also causes the- 10. Can you do a rough interpretation of Figure 1.6 and identify the stratigraphic level of the salt, pencil in salt structures and interpret the folds?^ moting a very wide zone of thin-skinned tectonics in^ orogenic wedges. This line is full of interesting details that can be studied. The large-scale features are a Cretaceous rift (from the time when Gondwana rifted apart) and early postrift sediments underlying an evaporitic section. The evap orites are marked in blue, while purer salt of the lower part of this section is uncolored. The salt has mobilized- into diapirs, although their vertical extent seems to be confined by the top of the evaporite layer. Salt move ments has caused most of the folding. Some of the ax ial traces have been drawn in on Figure Q20.10. Most of them are upright folds and the folds are open. Fold 2 looks tight, but remember that the vertical scale is in-- seconds, so to evaluate interlimb angles the interpreta tion must be depth converted. At present, the section appear to be a bit squashed. Fold 2 is clearly a result of salt withdrawal. The history of salt withdrawal and di apir growth is reflected in the thickness variations seen in the evaporite section and some of the higher strata.-- Fold 1 is a more complex inclined fold, with the main^ In fact, this syncline and the one in the right-hand part^ of the section define salt minibasins. axial surface dipping to the left and with some of a box-fold geometry in the hinge zone. This fold is the tightest one and clearly the result of upwarping during Folds 3 is a monocline developed atop a rift-related nor^ the evolution of the adjacent salt diapir. mal fault. Such monoclines may be due to differential compaction (or fault-propagation faulting). In this case it can be related to the down-warping of the evaporite section above. - Fold 4 is a shallower monocline that seem to have formed in a similar way, marking the flank of the minibasin to the left.

Chapter 21 1. The strain must be plane strain or at least close to plane What are the two most basic conditions that must be fulfilled for section balancing to make sense? strain, and the section must be oriented in the transport

direction, i.e. the section must contain the two princi pal strain axes X and Z. -

Answers to review questions in Fossen 2016 – 19

  1. Can you list other methods and criteria that can be used for One useful supplementary method is radiometric age dat this purpose?^ metamorphism must have occurred. The relationship^ must be consistent within the study area. - ing, where ages of individual minerals associated with the mineral paragenesis are dated. Another is the use of kinematic indicators. If the kinematics associated with the two different fabrics are opposite or at least differ ent, this may indicate two different phases rather than one progressive phase. -
  2. How can we distinguish between pretectonic (intertectonic), Posttectonic porphyroblasts are easy, because they simply syntectonic and posttectonic porphyroblasts? overgrow the existing fabric. Pretectonic or intertec tonic ones occur enveloped in a later fabric, typically with inclusion trails oblique to the external and later- foliation. In other words they occur as porphyroclasts. Syntectonic porpyroblasts grew during deformation. Because porphyroblasts grow outward and because the external foliation tends to rotate with respect to the porphyroblast the inclusion trails commonly become deflected towards the margins of the porphyroblasts. In extreme cases spiral-shaped patterns may occur, particularly in garnets. It may however be difficult to distinguish between syn- and intertectonic porphy roblasts in cases where intertectonic ones overgrow a crenulated fabric. -

Ductilie strain is most easily (and therefore most com 5. In cross-section a folded layer can be restored by means How could we restore a folded layer? monly) modeled as penetrative (simple) shear, either vertical or inclined (usually antithetic). - of vertical and inclined shear. A common example is the restoration of a rollover or reverse drag above a listric fault. An alternative model is the flexural shear model often applied to fault-bend folds (see Chapter 17). We have not discussed this method in any detail in this book, but the mathematics are available in Suppe (1983) and the method is integrated into most commer cial balancing software. In map view we sometimes have to deal with domes and basins, which can still be restored by means of shear. Vertical shear is equal to projecting particle points onto a horizontal surface, implying a change in area of the folded surface. -

  1. Map-view restoration gives information about the trans What information can map-view restoration give? port or strain pattern in the horizontal plane. Specifi cally it outputs the displacement field, which reveals whether strain is plane (parallel displacement vectors) or not. It also gives information about block rotations--

Chapter 22^ about vertical axes and reveals overlaps and gaps that^ may indicate the quality of the interpretation.

  1. How can metamorphic petrology help evaluating whether a If the overprinting structures have metamorphic miner set of overprinting structures formed during progressive de formation or as separate deformational events? als distinctly different (in terms of pressure and tem perature stability field) than those associated with the--- previous fabric, then this indicates that we are dealing with two different deformation phases. An example could be porphyroclasts of kyanite or hornblende in a reworked foliation made up of minerals consistent with greenschist-facies metamorphism (chlorite, al bite, mica). Another is localized eclogite-facies shear- zones in granulite-facies metamorphic rocks. Thermo barometric methods may be useful to characterize the metamorphic conditions associated with each meta morphic mineral assemblage. The change in P ditions between the two phases must be large enough that it can be assumed that a time period of no tectono – T con----

200

3.02. Pressure (GPa)1.00.0 AFT (~180 Ma) Jadeite + QuartzAlbite Temperature (°C) 400 Kyanite^ CoesiteQuartzAndalusite 600? Sillimanite 409±8 Ma423±4 Ma 800 ~930 Ma Figure Q22.5. P–T–t data given in question 21.5 plotted in a P–T diagram. A part of a clockwise path is indicated during the Paleozoicand Meso zoic. -

20 – Structural geology

  1. In what tectonic environment can we expect clockwise P–T A clockwise P–T path implies rapid increase in pressure paths to form? followed by heating and then decompression. This is difficult to achieve except in subduction settings, where rocks are buried during subduction and later ex-
  2. Make a P–T-t diagram based on this information from the eclogite province of the Bergen Arcs (for references, see Bin gen et al, 2004): (1) Granulite facies: almost 1 GPa and 800–850 °C at ~930 Ma (U–Pb ages of zircon). (2) Eclogite facies: 1.8–2.1 GPa and ~700 °C at 423^ humed. ± 4 Ma (U-Pb zircon rim crystallization- The data and path are shown in figure Q22.5. The old^ age). (3) Amphibolite facies retrogressive shearing: 0.8–1.2 GPa^ and ~690 °C at 409^ amphibolite facies shearing: 422^ and 418^ indicate time of cooling through 150 °C.^ ±^ 9 (U-Pb). Apatite fission-track ages around 180 Ma^ ±^ 8 Ma (Rb/Sr). Other ages: Dikes predating^ ±^ 6 Ma to 428^ ±^ 6 Ma (Rb/Sr) (930 Ma) granulite fabric is not directly related to the Caledonian ages of eclogite and amphibolite facies metamorphism and must therefore be considered sepa rately (it may or may not be that the rocks were at or close to the surface between the granulite and eclog ite facies tectonometamorphic events). A path can be-- drawn between the 423 the 409 date. The latter date indicates burial depths of around 6 km and thus low pressures. The path seems to repre sent the retrograde part of a clockwise P sistent with a subduction zone setting. ± 8 Ma amphibolite facies event and the AFT ± 4 Ma eclogite-facies event, – T path, con--
  3. What characterizes pre-, syn- and posttectonic sedimentary Pretectonic sedimentation in a half-graben is not related sequences in a halfgraben setting? to the graben formation. It is expected to show no thickness change across the graben-bounding fault. Syntectonic sediments show thickening towards the graben-bounding fault and rotation so that the layers dip toward the footwall. Unconformities may exist in the high part of the rotated block, while evidence of continuous sedimentation is expected in the lower part (close to the fault). Posttectonic sediments fill in the relief created by the graben without any other evidence of fault movement than that related to differential com paction across the fault. -