Term 1 - Progressive deformation, sense of shear & structural symmetry Flashcards
1
Q
Pure & simple shear
A
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
2
Q
Kinematic vorticity
A
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
3
Q
Shear zones
A
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
4
Q
Structural symmetry
A
Pure shear (Wk=0) tends to produce symmetric structures Simple shear (Wk=1) tends to produce asymmetric structures
5
Q
Stereonet Poles to Planes
A
o Represent the orientation of a plane by the pole to the plane, i.e. a line
o 90 degrees to plane
6
Q
Shear indicators
A
Super simple shear: • P-type porphyroclast • Mica fish Sub simple shear: • δ type porphyroclast