Shear Zones

Shear ZonesStructural GeologyUniversity of Glamorgan22/03/2012

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. FaultingBut 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 displacementFault zone cores are thinner than a standard shear zoneVariation of strain in a shear zone is more gradual than in faultsShear zones can occur under pure shear and simple shear conditionsMicroscale deformation can occur either plastically or ductily.Shear Zones vs. Faults

Geometry and ThicknessGraph 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)

Shear Zone Mechanisms

Simple shear vs. pure shearRemember, pure shear is not rotational.

Simple shear is rotational

Shear Zone Brittle vs. DuctileIdeal shear zones = perfectly ductile and involve simple shear with or without additional compaction/dilationNo discontinuities/slip surfaces should exist under this criteriaPassive markers (i.e. something that indicates displacement) can be traced continuously through a perfectly ductile shear zone

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-ductileExamples:

Note flattening and rotation of grains as enter the main shear zoneThis indicates that the feldspar crystals are becoming progressively more strained into the shear zone.

(and satisfies the shear zone definition)Direction of ShearLineations can be used!

Kinematic Indicators:

Direction of ShearDirection of ShearAsymmetric structures

Direction of ShearAsymmetry 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)Microscale FoliationsMica fish and S-C structures in mylonite:

Tails indicate sense of shear on mica fish!Sense of Shear - Porphyroclasts type do not cross reference line type do cross reference line type symmetric about reference line

Sense of Shear - Porphyroclasts types and 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).

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

Sense of Shear - PorphyroclastsSynthetic 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 porphyroclastTiling

Antithetic fractureSense of Shear - Porphyroclasts

Sinistral or dextral?Sense of Shear - Porphyroclasts

Sinistral!

Sense of Shear - Porphyroclasts

Sinistral or dextral?Sense of Shear - Porphyroclasts

Dextral!

Quarter Structures

1133Local thickening and thinning can occur around a clast in a shear zone in accordance with the extensional and contractional quadrants, respectivelyThis 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 Tension GashesExtensional veins (small fractures) can occur in the shear zones and are often filled in due to mineralisationThese align parallel to 1, because the fractures open parallel to 3 (the instantaneous stretching axis)Thus they can be used to determine shear senseOver time, the shear sense will cause tension gashes to fold as in (b)Form en echelon patterns

Tension Gashes

Sinistral or dextral?Tension Gashes

Sinistral or dextral?Opening direction11Tension Gashes

Dextral!11If 1 is in this orientation then the shear arrow must be in the direction that 1 pointsTension Gashes

Dextral!11

Pitfalls

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