Shear Zones Lecture

Tension Gashes
Sinistral or dextral?

Opening direction

σ1
σ1
Sense of Shear - Porphyroclasts
σ type – do not cross reference line
δ type – do cross reference line
φ type – symmetric about reference line
Microscale Foliations
Mica fish and S-C structures in mylonite:
Tails indicate sense of shear on mica fish!
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:
Newly formed foliation S is cut by shear surface C.
Continued deformation rotates S so that it is close to parallelism with C, known as CS-foliation.
New and oblique shear bands C' form and back-rotate the CS-foliation, called S in (b)
Direction of Shear
Asymmetric structures
Kinematic Indicators:
Direction of Shear
Direction of Shear
Lineations can be used!
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)
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).
φ types occur during coaxial deformation (pure shear; no rotation)

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
Tension Gashes
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

Quarter Structures
σ1
σ1
σ3
σ3
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
Dextral!
Sinistral!
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
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

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

Simple shear is rotational

Shear Zones
Structural Geology
University of Glamorgan
22/03/2012
Pitfalls
An extension fracture might rotate during deformation, and show an incorrect sense of shear
Tension Gashes
Dextral!

σ1
σ1
Tension Gashes
Dextral!

σ1
σ1
If σ 1 is in this orientation then the shear arrow must be in the direction that σ1 points
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.
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)
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

Shear Zones vs. Faults
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Shear Zones Lecture

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