Term 1 - Progressive deformation, sense of shear & structural symmetry

Question 1

Pure & simple shear

Answer 1

Homogeneous pure shear
Shorten // Z
Extend // X
No strain // Y
∆V=0
Simply “squash the rock” 

Homogeneous simple shear
o	Translate upper block relative to lower block in a direction // to the shear zone margins 
(= shear plane)
o	No change in length 
// shear zone margins; 
o	No strain // Y

Question 2

Kinematic vorticity

Answer 2

Wk
Kinematic vorticity number, Wk, is the ratio of the total angular velocity of material lines to the strain rate

Pure shear
Angular velocities of material lines cancel out
Wk=0

Simple shear
Total angular velocity > 1
Wk=1

wk to infinity: completely rotated square

Question 3

Shear zones

Answer 3

Shear zones are tabular volumes of high strain
Ideal shear zones have parallel sides and deform by simple shear, ∆V=0
The strain increases towards the centre of the shear zone

Question 4

Structural symmetry

Answer 4

Pure shear (Wk=0) tends to produce symmetric structures
Simple shear (Wk=1) tends to produce asymmetric structures

Question 5

Stereonet Poles to Planes

Answer 5

o Represent the orientation of a plane by the pole to the plane, i.e. a line
o 90 degrees to plane

Question 6

Shear indicators

Answer 6

Super simple shear:
•	P-type porphyroclast
•	Mica fish
Sub simple shear:
•	 δ type porphyroclast