Lecture 13 // Crustal Deformation - Folds

Question 1

Strike

Answer 1

A line made by the intersection of the inclined surface with the horizontal Earth’s surface.

Question 2

Dip

Answer 2

The direction of incline (from the Earth’s surface downwards) plus the dip angle (angle between the inclined bed and the Earth’s surface).

Question 3

Geologic map symbols

Answer 3

(including strike and dip) Are used on geologic maps to indicate the orientation beneath the Earth’s surface of layers (strata), planar features (such as faults), and folds.

Question 4

Folds

Answer 4

Form from ductile deformation, typically at great depth under compressional stress.

Question 5

Hinge

Answer 5

The tightest part of the fold.

Question 6

Axial plane

Answer 6

An imaginary plane that runs through the hinge of the fold.

Question 7

Axis

Answer 7

The imaginary line on the Earth’s surface made by the axial plane.

Question 8

Limbs

Answer 8

The sides (‘arms’) of a fold.

Question 9

symmetrical anticlines and symmetrical synclines

Answer 9

• Upturned parts of folded rock.

- Downturned parts of folded rock.

Question 10

monoclines

Answer 10

A slight bend in parallel layers of rock.

Monoclines commonly form due to reactivation of old buried faults.

Question 11

asymmetrical anticlines and synclines

Answer 11

Axial plane is not vertical, thus dip angles differ (limbs are not symmetrical) as do the width of beds on the surface.

Question 12

overturned anticlines and synclines

Answer 12

Axial plane is even more off vertical and limbs now point in the same direction (one limb is completely overturned).

Question 13

recumbent folds

Answer 13

These are folds so compressed that anticlines and synclines become stacked upon each other.

The axial plane and limbs are
horizontal or near horizontal.

Question 14

plunging anticlines and synclines

Answer 14

When plunging anticlines & synclines are uplifted and eroded, a V pattern is revealed on the Earth’s surface.

Rule of V for uplifted & eroded plunging folds:

• Vs point in the same direction as tilt for plunging anticlines.
• Vs point in the opposite direction as tilt for plunging synclines.

Question 15

domes and basins

Answer 15

Rock is deformed upwards (domes) or downwards (basins) in all directions. Uplifted and eroded domes and basins show circular patterns.
Domes: The oldest rock is at the centre of an eroded dome.
Basins: The youngest rock is at the centre of an eroded basin.