Week 15

Shear zones I. Read pages 493-551 in Chapter 9: Shear Zones and Progressive Deformation.

• readings
• terms
• questions
• classnotes

You are expected to read all the sections listed below. Information from the sections in italics will be discussed in class. You are expected to read the other sections and you may be called on in class to answer questions based on that material.

The Nature of Shear Zones p.493-502

• General Characteristics
• Geometries
• Offset and Deflection of Markers
• Tectonic Settings

Types of Shear Zones p.503-508

• Brittle Shear Zones
• Ductile Shear Zones
• Between Brittle and Ductile: The Middle Ground
• Semibrittle Shear Zones
• Brittle-Ductile Shear Zones

Why Shear Zones Form, Thin, and Thicken p.509-511

• Softening Processes in Ductile Shear Zones
• Strain Hardening in Ductile Shear Zones

The Strain in Shear Zones p.511-515

• Coaxial or Noncoaxial
• The Instantaneous and Finite Strain Ellipses
• The Coaxial-Noncoaxial Distinction

Determining Sense of Shear p.515-536

• A Frame of Reference
• Offset Markers
• Deflection of Markers
• Foliation Patterns
• Shear Bands, S-C Fabrics, and Oblique Microscopic Foliation
• Mica Fish
• Inclusions
• Pressure Shadows
• Porphyroclasts and Porphyroblasts
• Foliation Fish
• Fractured and Offset Grains
• Veins
• Folds
• Orientation of Folded and Boudinaged Layers
• A Word About Consistency of Shear-Sense Indications

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You should become familiar with the following terms during this weeks lectures and readings:

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You should be able to answer the questions below following this week:

1. What are the differences between brittle, semibrittle, and brittle-ductile shear zones?
2. Draw a diagram that illustrates the potential shear sense indicators for a sinistral shear zone in the S₁S₃ plane.
3. Explain what process may allow shear zones to maintain a constant width but increase strain.
4. Explain how a shear zone could increase width with relatively low internal strains.
5. What is the difference between coaxial and non-coaxial deformation?
6. Define the following terms: shear zone, geometric softening, simple shear, pure shear, S-C fabrics, porphyroclasts, porphyroblasts, theta, phi, sigma, and delta objects.

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Shear Zones

The Nature of Shear Zones p.493-502

 Shear zone - tabular, planar zone composed of rocks that are more highly strained than those adjacent to the zone (localization of deformation into a narrow zone)

Fault - brittle shear zone

We want to determine - amount of displacement, sense of displacement, amount of strain

General Characteristics

• Strain varies from interior (high) to exterior of shear zone (low)
• Heterogeneous strain distribution (careful, depends on scale of observation)
• Continuous shear zone - decrease in strain is gradual
• Discontinuous shear zone - abrupt break in shear zone
• Pre-existing markers change thickness and orientation within shear zones
• Shear zone terminology mimics that of faults

Types of Shear Zones p.503-508 - 4 types

Brittle Shear Zones

• Form in upper crust in relatively rapid strain rates
• Characterized by breccia, fractures, faults, fault gouge
• Thickness increases with increasing displacement

Ductile Shear Zones

• Middle to lower crust and asthenosphere, mostly under metamorphic conditions
• Mylonites and tectonites formed

Semibrittle Shear Zones

• Dominated by brittle mechanisms but with some ductile aspects
• En echelon joints/veins - brittle joints with distributed deformation between veins

Brittle-Ductile Shear Zones

• Deformation by both brittle and ductile processes, typically have tectonite fabric
• Physical conditions may permit both brittle and ductile condition; e.g. marble with calcites with different crystallographic orientations; grain size may vary
• Different parts of the rock may have different mechanical properties; e.g. marble with calcite and dolomite, former is ductile while latter is brittle
• Shear zone may strain harden, leading to more abrupt zone margins
• Physical conditions change during deformation, e.g. increasing strain rate, increasing fluid pressure, both promote brittle response
• Shear zone reactivated under different conditions, easier to recognize ductile overprinted by brittle deformation

Why Shear Zones Form, Thin, and Thicken p.509-511

 Softening Processes in Ductile Shear Zones

For deformation to be localized in shear zones we must assume that strain softening occurs. Strain softening occurs by:

1. Grain size reduction: Deformation mechanisms are more effective in fine grained rocks
2. Geometric softening: grains rotate during deformation until slip systems are favorably oriented for slip. Results in crystallographic preferred orientation for rock.
3. Reaction softening: deformation accompanied by formation of new minerals that deform more easily than original mineral, e.g. formation of serpentine in ultramafic rocks
4. Fluid-related softening: fluids may dissolve resistant grains or introduce weak minerals to shear zone (hydrolitic weakening); fluids may promote some deformation processes

Strain Hardening in Ductile Shear Zones

Some shear zones are several km wide, for deformation to be so widely distribute, the shear zone must have widened to incorporate some of the surrounding country rock.

• Strain hardening must have occurred in the shear zone to make it more difficult to deform the zone than the surrounding protolith
• Due to deformation mechanisms that result in dislocation tangles and prevent climb (temperatures not high enough)

 Strain in Shear Zones p.511-515

 Typical to assume deformation is plane strain - strain restricted to a family of parallel planes.

• Maximum finite extension in one direction is accompanied by maximum finite shortening in a perpendicular direction within the plane of strain.
• No strain in third direction, perpendicular to plane of strain.
• Strain may be coaxial (pure shear) or non-coaxial (simple shear)

Coaxial or Non-coaxial

• In coaxial strain (pure shear), particles move toward the center of the strain ellipse on paths sub-parallel to S₃, and towards the edge on paths parallel to S₁ (overhead)
• In non-coaxial strain (simple shear), particles move parallel to the shear sense (overhead)

The Instantaneous and Finite Strain Ellipses

Instantaneous strain ellipse defines changes with one small increment of deformation.

• Incremental strain ellipse contain instantaneous stretching axes that exhibit shortening or extension
• Orientations of fields of instantaneous shortening and extension vary with coaxial vs. simple shear
• In coaxial strain, the finite stretching axes do not rotate and are aligned with the instantaneous stretching axes throughout deformation (hence, coaxial)
• In non-coaxial strain, the finite stretching axes are rotated so that S1 is rotated towards parallelism with the shear zone (hence, non-coaxial). The orientations of maximum finite extension and shortening do not coincide with directions of instantaneous finite shortening and extension

The Coaxial vs. Non-coaxial Distinction

• Particle motion: symmetrical for coaxial deformation, asymmetrical for non-coaxial deformation
• Instantaneous stretching axes: parallel & perpendicular to shear zone for coaxial deformation, inclined to shear zone for non-coaxial deformation
• Finite stretching axes: fixed for coaxial deformation, rotate for non-coaxial deformation

Question: How do we discriminate between coaxial and non-coaxial deformation?

Answer: By observing small-scale structures in the shear zone.

 Determining Sense of Shear p.515-536.

A Frame of Reference

• Sense of shear plane is cut perpendicular to the shear zone and parallel to stretching lineation

Offset Markers or Deflection of Markers

• Sense of offset or deflection illustrates sense of shear (be careful, the separation vs. slip discussion from faults arises again here)

Foliation Patterns

• Foliation is inclined to the shear zone for non-coaxial deformation
• Leans over in direction of shear
• Sigmoidal or curved shape with sense of deflection indicative of shear sense

Shear Bands, S-C Fabrics

Shear bands (C-surfaces) - discrete thin zones of high shear strain in the main shear zone

• Cross-cut foliation (S-fabric) within the shear zone
• C-surfaces are oriented subparallel to the shear zone boundary
• Foliation is deflected into parallelism with shear band generating a sigmoidal shape

Inclusions

• Mechanically distinct objects within, considered to be more rigid than the shear zone, e.g. xenoliths
• Rotation of inclusions will vary in coaxial shear zone but show uniform sense of rotation in a non-coaxial shear zone

Pressure Shadows

• Mineral fibers grow adjacent to grains in pressure shadows on flank of deformed grains
• Pressure shadows parallel the axis of instantaneous extension and will be parallel to the shear zone for coaxial deformation, oblique for non-coaxial deformation

Porphyroclasts and Porphyroblasts

Porphyroclast - relic grains from the protolith

Porphyroblast - grains that grew during (synkinematic) or after deformation (postkinematic)

• Inclusion trails in porphyroblasts
• Recrystallized mantles of porphyroblasts or porphyroclasts may generate "tails" that are deformed and that indicate the sense of shear

1. Theta (q): objects with round to elliptical mantles, no wings
2. Phi (f) - porphyroclasts with symmetrical wings around centerline
3. Sigma (s) - stair stepping pattern of asymmetric wings
4. Delta (d) - asymmetric, strongly curved wings that cross the centerline

Fractured and Offset Grains

• Rigid inclusions in the shear zone may be sliced up on faults
• If fractures are subparallel to the shear zone, faults are synthetic
• If fractures are at high angle to shear zone the faults are typically antithetic

Veins

• Form perpendicular to direction of maximum instantaneous extension (i.e. unlike other strain sense markers, veins "lean" in opposite direction to shear zone boundary)
• Older veins will be sheared into sigmoidal shape and cross cut by younger veins forming in accordance with instantaneous strain ellipse

Folds

• Sheath folds - wind-sock like folds that form lineation in direction of shear

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