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Earth structure and tectonics - 33740 > Lecture 3 - Folds, Principles, Geological mapping > Flashcards

Lecture 3 - Folds, Principles, Geological mapping Flashcards

1
Q

What are folds the product of?

A

ductile deformation and the resulting bending of a planar structure

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2
Q

What are the different parts (elements) of fold?

A

Hinge
Limb
Hinge line
Fold axis
Axial plane

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3
Q

What is the hinge in folding?

A

point/ zone of greatest curvature

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4
Q

What is the limb in folding?

A

connects 2 hinge points

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5
Q

What is the hinge line in folding?

A

Line through hinge along 1 layer

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6
Q

What is the fold axis in folding?

A

any line parallel to the hinge line

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7
Q

What is the axial plane in folding?

A

divides two limbs, passing through the hinge lines of overlying layers

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8
Q

How are folds classified?

A

axial plane and plunge of hinge line

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9
Q

What are the most common folds?

A
  • Upright
  • Plunging upright
  • Horizontal inclined
  • Recumbent
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10
Q

What are the different fold tightness’s and their matching interlimb angle?

A

Gentle 180-120
Open 120-70
Close 70-30
Tight 30° - 0
Isoclinal 0°

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11
Q

What are monoclines?

A

folds with 1 limb

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12
Q

How are monocline produced?

A

By deeper faults that have not reached the surface (blind faults)

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13
Q

What are kink bands?

A

sharp angular folds bounded by planar surfaces (normally small scale)

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14
Q

How do chevron folds form?

A

in layers with a regular alternation of contrasting (soft – rigid) competences (e.g. claystones and sandstones)

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15
Q

What are the characteristics of chevron folds?

A
  • “V” shape
  • straight limbs
  • sharp hinges
  • 60° interlimb angle
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16
Q

What are the characteristics of concentric folds?

A
  • Rounded shape
  • Curved limbs
  • Broad hinges (hinge zone)
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17
Q

What are the characteristics of box folds?

A
  • Box shapes
  • Three limbs
  • Two hinges and axial planes
  • ~90° interlimb angle
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18
Q

What are sheath folds?

A

Highly non-cylindrical folds formed in high-strain shear zones, hence deep levels crust
where rocks deform plastically

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19
Q

Where do parasitic folds form?

A

in layers with different competence (soft and rigid layers alternate)

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20
Q

What are the different types of parasitic fold?

A

Z-shaped
S-shaped
M-shaped

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21
Q

What do Z shape parasitic folds indicate?

A

fold core is located to the right

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22
Q

What do S shaped parasitic folds indicate?

A

fold core is located on the left

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23
Q

Where do M shaped parasitic folds form?

A

Near fold hinge

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24
Q

What is the difference between antiform and synform?

A

Both fold form:
Antiform =convex up
Synform =convex down

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25
Q

What is the difference between anticline and syncline?

A

Describe the age distribution in the fold:
* Older rocks in fold core = anticline
* Younger rocks in fold core = syncline

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26
Q

What is flexural slip?

A

When a layered rock unit is folded, adjacent
layers within each limbs experience a simple shear, which has opposite sense of slip in the two limbs.

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27
Q

What are slickenlines?

A

Scratches parallel to the main stress responsible for the folding may form above the layers surfaces

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28
Q

What is flexural flow?

A

Essentially the same as slip but with deformation distributed more evenly
within the limbs.

29
Q

What conditions does flexural flow more commonly occur in?

A

It commonly occur in more plastic conditions (higher temperature)

30
Q

What 3 ways can folding occur?

A

Buckling
Bending
Passive folding

31
Q

What is buckling? (folding)

A

Layers parallel shortening

32
Q

What is bending? (folding)

A

forces applied at a high angle to the
layers

33
Q

What is passive folding?

A

Produced by simple shear

34
Q

What is the basis for the principle of Uniformitarianism?

A

Present is the key to the past

35
Q

What are the uses of the principle of uniformitarianism?

A

look at modern geological systems (sedimentary, mountain-building, volcanoes etc.) explain past geological processes

can look at ancient events (e.g., climate changes) to understand current processes

36
Q

What is an example of an age where modern tectonic processes occurred?

A

The Jurassic (170 Ma) i.e. Fossil oceanic detachment fault 130 Ma (South Tibet)

37
Q

What is the principle of superposition?

A

Younger sediments are deposited horizontally over older sediments in a “layer cake” style

38
Q

What is the principle of cross cutting relationship?

A

Younger rocks cut across older rocks (faults)

39
Q

What are the different types of unconformities?

A

Angular unconformity
Non-conformity
Disconformity

40
Q

What is an angular unconformity?

A

Older rocks below were tilted and eroded before the deposition of the younger layers above. Angle between the beds

41
Q

What is non-conformity?

A

Younger layers were deposited over metamorphic/igneous basement where layering is absent

42
Q

What is a disconformity?

A

Uplift and erosion of layered
sedimentary rocks is followed by down thrown and new deposition: all beds are parallel

43
Q

What do unconformities indicate?

A

time gap in deposition (or hiatus) i.e. deformation, uplift, burial, erosion, and metamorphism

44
Q

What are contour lines?

A

lines connecting points of equal elevation

45
Q

What is an individual contour line?

A

the intersection between a 3D object (i.e., topography) and a horizontal plane with a given elevation (in meters)

45
Q

What are the lines dividing areas of different colours called on a geological map?

A

Unit boundaries

46
Q

How will lines (unit boundaries or faults) usually be shown on a geological map?

A

As curved lines (curve controlled by dip and topographic profile)

47
Q

What is true thickness of rock unit?

A

visible only along a section perpendicular to its layering/bedding.

48
Q

What is apparent thickness of a rock unit?

A

when observing rock thickness from ant perspective then perpendicular to its bedding

49
Q

What is the apparent thickness like compared to true thickness?

A

always larger

50
Q

What is apparent thickness controlled by?

A

angle at which the topographic surface cut the the beds

51
Q

How can true dip of a rock unit be observed?

A

along a section normal to the strike (or
parallel to the dip direction)

52
Q

What is apparent dip of a rock unit?

A

Observing a rock unit dip on any angle other then parallel to dip direction

53
Q

What will apparent dip be like compared to true dip?

A

Shallower

54
Q

what 3 geometries can faults and unit boundaries appear as on geological maps?

A

Vertical plane - straight cross cut line

Horizontal plane - curved parallel to specific contour

Inclined plane - Curved line cutting across contour

55
Q

How will dip be shown on a geological map?

A

It will be a T like shape with the long side being strike and the short being dip with an accompanying angle (drawn on cross section with protractor)

56
Q

How are folds in geological maps represented?

A

a repetition of units, which are symmetric
with respect to the hinge line

57
Q

What will an upright fold produce on a geological map?

A

map unit boundaries (lines) that are parallel to each other

58
Q

What will plunging folds produce on topographical maps?

A

V-shaped unit boundaries

59
Q

What will the strike be like on the 2 limbs of a plunging fold?

A

Will be different

60
Q

What are the 2 outcomes if a plunging fold forms chevron shaped unit boundaries?

A

Plunging antiform: plunge is in the direction of the chevron

Plunging synform: plunge towards the core of the chevron

61
Q

What will faults do in geological maps?

A

cut and offset unit boundaries

62
Q

What are structure contours?

A

connect points of equal elevation on a geological surface (bedding plane etc)

63
Q

What do structure contours run parallel to?

A

Strike

64
Q

How can structure contours tell you if a surface in planar?

A

if the contours are straight, parallel and equidistant

65
Q

What does it mean if the structure contours are curved?

A

the strike of the bed varies
If spacing of structure contours vary so does bed dip

66
Q

What is the formula to calculate dip?

A

a=tan*-1(VI/HE)
VI= vertical elevation interval between contours
HE= horizontal spacing between selected structure contours

67
Q

How can structure contours be used to find true thickness? (equation)

A

t = HE *sin(a)

HE= total map distance horizontal meters between equal elevation structure contours
(a)= known dip of structure

Earth structure and tectonics - 33740 (7 decks)

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