Week 3

Question 1

Types of shear in homogeneous PLANE strain (k=1)

Answer 1

PURE

SIMPLE

Question 2

Pure shear

Answer 2

Decrease z
Increase x
Same y

/\V=0

Fixed principle axes = coaxial

Question 3

Pure shear structures

Answer 3

Symmetric

Strain ellipse looks squished

Material lines rotate towards x-axis

Question 4

Simple shear

Answer 4

No length change
No strain

TRANSLATE upper block // to shear zone margins (SHEAR PLANE)

Principal axes rotat = non-coaxial

Question 5

Simple shear structures

Answer 5

Asymmetric

Strain ellipse ‘skewiff’

Material lines rotate in same sense as x-axis i.e. towards the shear plane

Question 6

Angular shear, Ψ =

Answer 6

change in angle between 2 originally perpendicular lines after deformation

Question 7

Shear strain, γ‎ =

Answer 7

tanΨ

Question 8

Kinematic vorticity, W(k) =

Answer 8

total angular velocity of material lines:strain rate

Question 9

Kinematic vorticity in pure shear

Answer 9

= 0

• angular velocities of material lines cancel

Question 10

Kinematic vorticity in simple shear

Answer 10

= 1

• angular velocities > 1

Question 11

“Spectrum of plane strain deformations”

Answer 11

Pure –> simple –> rigid rotation

increasing W(k) –>

Question 12

Shear zones

Answer 12

Strain highest at centre

Domains homogeneous

Zone itself heterogeneous

Question 13

What plane must you observe shear zones in?

Answer 13

THE X-Z PLANE

Question 14

Stereograph steps:

Answer 14

1. Count to strike and mark
2. Spin mark to nearest pole
3. Count IN to dip
4. Follow circle line

= plane for line

= dot for plunge/azimuth

5. Count 90’ from line = pole to plane

Question 15

Assumption with vorticity

Answer 15

The e.g. coral originally grew perpendicular to the bed

Question 16

How must we view shear sense indicators in order to determine sense of shear?

Answer 16

In the XZ principal plane of finite strain

Question 17

δ-type porphyroclasts

Answer 17

Rotate more rapidly than they deform = supersimple shear

Question 18

σ-type porphyroclasts & mica fish

Answer 18

Deform more rapidly than/as rapidly as they rotate = simple to subsimple shear

Question 19

‘Toothpaste effect’

Answer 19

During pure shear strain, the boundaries between deformed/undeformed rocks must form discontinuities = strain compatibility problem (continuity across shear zone boundaries is lost)
- shear zone has to extrude laterally while wall rock remains undeformed

= CRUSTAL DEFORMATION ZONES SHOULD BE DOMINATED BY SIMPLE SHEAR, ALTHOUGH DEVELOPMENTS OF FAULTS AND OTHER STRUCTURES MAY DEPART FROM THIS

i.e. shear zone will parallel boundaries is not compatible with (cannot accommodate) pure shear

Question 20

Why doesn’t the ‘toothpaste effect’ occur during simple shear?

Answer 20

The shear plane is a surface of no finite strain

Also applies to heterogeneous strains - easy in simple shear to vary strain with no holes, overlaps developing

Question 21

What is required for the state of strain in two adjacent layers to be compatible?

Answer 21

The section through their respective strain ellipsoids parallel to their interface must be identical

Question 22

What do/do not stereographic projections preserve?

Answer 22

DO: angular relationships between lines and planes

DON’T: spatial relationships