AS Structural Geology Flashcards

1
Q

Bed

A

a unit of sedimentation, which can vary considerably in thickness.

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2
Q

Bedding planes

A

mark a break between beds. They represents a break in sedimentation, a change in the composition or grain size, or a change in colour of the sediments.

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3
Q

Dip

A

the maximum inclination of a bed measured from the horizontal using a clinometer.

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4
Q

True dip

A

the actual angle of dip measured at right angles to the strike.

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5
Q

Apparent dip

A

a dip that is measured to be less than the maximum inclination.

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6
Q

Strike

A

the horizontal line on a bedding plane, measured as a bearing from north using a compass. It is at right angles to the dip.

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7
Q

Fault

A

a fracture in a rock along which there has been an observable amount of displacement.

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8
Q

Fault plane

A

a plane of fracture, along which the rocks have been displaced.

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9
Q

Upthrow

A

the side of the fault where the movement is upwards, in relation to the other side.

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10
Q

Downthrow

A

the side of the fault where the movement is downward, in relation to the other side.

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11
Q

Throw

A

the vertical displacement of rocks along the fault plane.

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12
Q

Footwall

A

the side of the fault that lies below the fault plane, if the fault is not vertical.

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13
Q

Hanging wall

A

the side of the fault that lies above the maximum inclination of the fault plane as measured from the horizontal.

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14
Q

Slickensides

A

the striations and polishing found on a fault plane indicating the direction of relative movement.

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15
Q

Fault breccia

A

composed of fragments produced by rocks fracturing during faulting. It is found along the fault planes.

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16
Q

Fault gouge

A

composed of very finely ground particles, produced by grinding of rock during faulting. These are often fused together due to frictional heat and found along fault planes.

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17
Q

Mylonite

A

a rock produced by dynamic recrystallisation of minerals on a fault plane.

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18
Q

Anticline

A

an upright fold with the oldest rocks in the core.

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19
Q

Antiform

A

an upward closing fold.

20
Q

Syncline

A

a downward pointing fold with the youngest rocks in the core.

21
Q

Synform

A

a downward closing fold.

22
Q

Fold limb

A

the section of a fold between one hinge and the next, forming the side of a fold.

23
Q

Hinge

A

the line along which there is a change in the amount and/or direction of dip, forming the most sharply curved part of the fold.

24
Q

Crest

A

the highest point of a folded bed.

25
Q

Trough

A

the lowest point of a folded bed.

26
Q

Axial plane

A

a plane that joins the hinges of all the beds. It bisects the fold.

27
Q

Axial plane trace

A

the outcrop of the axial plane at the Earth’s surface.

28
Q

Plunge

A

the angle of dip of the axial plane from the horizontal.

29
Q

Dome

A

an anticline which dips away from the centre in all directions.

30
Q

Basin

A

a syncline which dips towards the centre from all directions.

31
Q

Overfolds

A

folds which have both fold limbs dipping in the same direction but by different amounts.

32
Q

Recumbent folds

A

have axial planes and fold limbs which are close to horizontal and always less than 30 degrees.

33
Q

Nappes

A

recumbent folds that are broken along thrust planes.

34
Q

Isoclinal

A

folds have parallel limbs that are nearly vertical and in very tight folds.

35
Q

Stress

A

the force applied to rocks.

36
Q

Strain

A

a change in the shape or volume of a body as a result of applied stress.

In cases where vertical stress is applied to perfectly elastic materials:

Strain = the change of length of line ÷ the original length of line

37
Q

Competent

A

rocks are strong and brittle and tend to joint and fault.

38
Q

Incompetent

A

rocks are weak and plastic and tend to fold and develop cleavage.

39
Q

Tension

A

the force trying to pull rocks apart, the Earth’s crust will be lengthened.

40
Q

Compression

A

the force trying to push rocks together, the Earth’s crust will be shortened.

41
Q

Shear forces

A

are those which act along a plane in the rock and promote sliding along that plane.

42
Q

Joint

A

a fracture in competent rocks along which no observable movement has occurred.

43
Q

Tectonic joints

A

form as a result of folding and cause tension joints parallel to the axial plane and cross joints on the limbs.

44
Q

Cooling joints

A

form as a result of contraction on cooling of igneous rocks.

45
Q

Unloading joints

A

are often horizontal as well as vertical and form as a result of lower pressure near to the surface.

46
Q

Cleavage

A

planes of weakness in incompetent sedimentary rocks (e.g. shale, made from clay minerals) and low-grade metamorphic rocks (e.g. slate) along which these rocks will split. It forms perpendicular to o max in folds, so is at an angle to bedding planes.