what is shear zone

1. Shear Zones
Structural Geology
University of Glamorgan
22/03/2012

2. What is a shear zone?
Shear zones can be quite similar to faults and
thus confusing, but a simple definition for
shear zones:
A tabular zone in which strain is noticeably
higher than in the surrounding rock

3. Shear Zones vs. Faulting
But wait, that sounded like a definition for a fault as well?
It is, however, a fault is a special case of shear zone where
brittle deformation occurs:
A ‘traditional’ shear zone is one where ductile deformation
occurs (plastic). Some facts:
Shear zones have a thickness relative to displacement
Fault zone cores are thinner than a standard shear zone
Variation of strain in a shear zone is more gradual than in
faults
Shear zones can occur under pure shear and simple shear
conditions
Microscale deformation can occur either plastically or ductily.

4. Shear Zones vs. Faults

5. Geometry and
Thickness
 Graph above shows how
shear zones possess a
relatively thicker
deformation zone over
fault cores.
 However, thickness in
shear zones is comparable
to the entire damage zone
(fractured region
surrounding the fault core)

6. Shear Zone Mechanisms
Simple shear vs. pure shear
Remember, pure shear is not
rotational.
Simple shear is rotational

7. Shear Zone Brittle vs.
Ductile
Ideal shear zones = perfectly ductile and involve
simple shear with or without additional
compaction/dilation
No discontinuities/slip surfaces should exist under this
criteria
Passive markers (i.e. something that indicates
displacement) can be traced continuously through a
perfectly ductile shear zone

8. Shear Zone Brittle vs.
Ductile
A perfectly ductile shear zone
contains no internal
discontinuities
Many plastically deformed
shear zones show sharp
internal discontinuities in the
form of slip surfaces,
extension fractures, and veins
= these are called semi-
ductile

9. Examples:
Note flattening and rotation of grains
as enter the main shear zone
This indicates that the
feldspar crystals are
becoming progressively
more strained into the shear
zone.
(and satisfies the shear zone
definition)

10. Direction of Shear
Lineations can be used!

11. Kinematic Indicators:
Direction of Shear

12. Direction of Shear
Asymmetric structures

13. Direction of Shear
Asymmetry of mylonites
can be used to evaluate
the sense of shear and
sometimes also the
degree of coaxiality of a
mylonite zone
Development of S-C
structures:
1. Newly formed foliation S
is cut by shear surface C.
2. Continued deformation
rotates S so that it is
close to parallelism with
C, known as CS-foliation.
3. New and oblique shear
bands C’ form and back-

14. Microscale Foliations
Mica fish and S-C structures in mylonite:
ils indicate sense of shear on mica fish!

15. Sense of Shear -
Porphyroclasts
σ type – do not cross reference line
δ type – do cross reference line
φ type – symmetric about reference line

16. Sense of Shear - Porphyroclasts
σ types and δ types can indicate the sense or direction
of shear in a similar way to the mica fish (tails point in
direction of shear), this allows us to identify if the
shear zone is left-lateral (sinistral) or right-lateral
(dextral).

17. Sense of Shear - Porphyroclasts
φ types occur during coaxial deformation (pure
shear; no rotation)

18. Sense of Shear - Porphyroclasts
Synthetic fracture of porphyroclasts can also
define sense of shear (like a Riedel-Shear in strike-
slip faults these will be at low angle in relation to
the shear zone)
Antithetic fractures also occur [like antithetic R-
shears (R’ -shears), these will be at high angle to
the shear zone]
Tiling – an imbrication of clasts, note that stepping
occurs consistently in same direction.
Synthetic fracture
of porphyroclast
Tiling
Antithetic
fracture

19. Sense of Shear -
Porphyroclasts
Sinistral or dextral?

20. Sense of Shear -
Porphyroclasts
Sinistral!

21. Sense of Shear -
Porphyroclasts
Sinistral or dextral?

22. Sense of Shear -
Porphyroclasts
Dextral!

23. Quarter Structures
σ1
σ1
σ3
σ
3
Local thickening and
thinning can occur around
a clast in a shear zone in
accordance with the
extensional and
contractional quadrants,
respectively
This is due to the fact that
matrix shear zone material
must accelerate past the
clast on the thinning side
and as the material moves
to the extensional side, it
accretes as it decelerates

24. Tension Gashes
Extensional veins (small
fractures) can occur in the shear
zones and are often filled in due
to mineralisation
These align parallel to σ1,
because the fractures open
parallel to σ3 (the instantaneous
stretching axis)
Thus they can be used to
determine shear sense
Over time, the shear sense will
cause tension gashes to fold as
in (b)
Form en echelon patterns

25. Tension Gashes
Sinistral or dextral?

26. Tension Gashes
Sinistral or dextral?
Opening direction
σ
1
σ
1

27. Tension Gashes
Dextral!
σ
1
σ
1
If σ 1 is in this orientation then the shear arrow must be in
the direction that σ1 points

28. Tension Gashes
Dextral!
σ
1
σ
1

29. Pitfalls
An extension fracture might rotate during
deformation, and show an incorrect sense
of shear