Shear zone Flashcards
What is a zone?
A localised area of deformation
Define fault
Where almost all deformation is accommodated within a narrow zone around the fracture surface
Define shear zone
A region of highly localised deformation
Why does the character of localised deformation zones change with depth?
Mechanical properties of rocks change with temperature and pressure
What shear zones are found in the brittle upper crust?
Faults and deformation bands
What shear zones are found at greater depths, where rocks flow rather than fracture?
Ductile shear zones
What are shear zone walls and how is their orientation measured?
The margins that separate undeformed rock outside the deformation zone from the deformed rock inside it, measured with strike/dip
How is shear zone thichness related to displacement?
More displacement usually results in a thicker shear zone.
What does the geometry of the rock fabrics in the shear zone indicate?
Sense of displacement (normal/reverse, sinistral/dextral)
How are shear zones classified? (Kinematic classification)
Using the relative movement between the two shear zone walls
What three factors combine to give the relative movement of a shear zone?
Pure shear, simple shear, volume change
Define transtension of a shear zone
Extension
Define transpression of a shear zone
Contraction
What type of movement during shear can be seen in low porosity rocks?
Simple shear zones, transtension, transpression
What type of movement during shear can be seen in high porosity rocks?
Within deformation bands of these rocks, the full range of compaction, dilation, and simple shear deformation may occur
Describe the progressive simple shear zone model
Across the zone the strain is heterogeneously distributed, it is zero at the shear zone boundaries and maximum in the midle of the zone (but never becomes parallel with the shear zone wall)
How can displacement across a shear zone be shown graphically?
It is the area under a curve of shear strain vs distance across the shear zone
How can shear strain a points be calculated?
When the orientation of the long axis of a strain ellipse is known at several places and simple shear is assumed
What can be seen in ductile shear zones as strain accumulates?
Foliation and stretching lineation
What grade is considered a ductile shear zone?
Greenschist metamorphic grade and above
What happens to the fabric in a shear zone?
Objects in the protolith are stretched and rotated into the shear zone, the fabric is intensified in the centre of the shear zone
What can indicate the sense of shear in a ductile shear zone?
Curvature of foliation (must be foliation formed during the shearing, not pre-existing)
Define mylonite
The banded rock formed by the transposition of pre-exisiting tecures due to a large strain in the middle of the shear zone, can be several kilometres thick
Give the formal definition of mylonites
Fault rocks containing a well-developed foliation that arises during tectonic grain size reduction
What does foliation indicate?
The principal flattening plane (XY plane) across the shear zone
Describe foliation at the shear zone walls and at the centre of the shear zone
Foliation is ~45deg to the shear zone walls and rotates towards parallelism at the centre (never becomes parallel
Describe stretching lineation in shear zones formed by progressive simple shear
Parallel to the maximum extension direction (X direction)
Where should the stretching lineation (X) and poles to the foliation (Z) be on a stereogram?
On the same great circle (XZ plane)
What should the stretching lineation do towards the centre of the shear zone on a stereogram?
Reorient towards the slip vector
What should the pole to the foliation do towards the centre of the shear plane?
Reorient to the pole of the shear plane
Where should the pole to the XZ plane (Y) be on a stereogram?
Within the shear plane
What can deflected markers indicate?
Shear sense
What should happen to the layers that cross the shear zone close to the maximum extension direction?
They should be stretched/boudinaged
What can pre-exisiting lineations and planar features be used for?
To determine the orientation fo the shear plane and slip vector
Where is the slip vector on a stereogram?
Where the great circle of the shear plane intersects with the great circle with the lineations on
Where are the poles of a pre-existing plane feature on a stereogram?
Along a great circle towards the pole to the shear plane, the pole of this great circle (fold axis) lies in the shear plane
When do markers fold?
When they are oriented close to the maximum shortning direction of the incremental strain ellipse
When do markers start to stretch?
When shear increases the folded markers can rotate into an extensional filed of the incremental strain ellipse and start to stretch
Describe the geometry of markers that are passively folded
Class 2 (similar) geometry with thickened hinges and attenuated limbs
Describe intrafolial folds
Folded foliation in mylonitic shear zones, the folds are commonly isoclinal with axial planes in orientations that are indistinguishable from the main mylonitic foliation
What causes intrafolial folding?
Local distortions in the flow field during shearing
What indicates the sense of shearing in intrafolial folds?
Asymmetry of the folds when views in the XZ plane
Describe sheath folds
Where the tubular shapes of intrafolial folds is amplified, the hinge lines become parallel witht he X direction as the strain increases
Describe curtain folds
Folds with hinges that are parallel but are not tubular
Describe C-surfaces
Set of shear bands formed as strain increases, they are parallel to the shear zone walls, they deflect the main foliation (S-surface) in the same sense as the shear sense
What happens to the C and S surfaces as strain increases?
The angle becomes very small and C’-surfaces are formed, 15-35deg to the shear zone walls
Describe C’-surfaces
Sense of deflection of the main foliation is extensional and in the same general direction as the shear sense, this is extensional crenulation cleavage
What are C-surfaces indicative of?
Inhomogeneous simple shear
When are C-surfaces and C’-surfaces formed?
C-surfaces develop early in the formation and are often straight and continuous, C’-surfaces develop late in deformation and are commonly short and wavy
What are the three types of object seen in mylonites that can indicate shear sense?
Single crystals, porphyroclasts with rims, and polycrystals
Describe ϕ-type mantled porphyroclasts
The wings look symmetric because of being views in the YZ plane, they cannot be used to infer sense of movement
What type of mantle porphyroclasts can be used to infer sense of movement?
σ - type and δ - type viewed in the XZ plane
What is the difference between mantled porphyroclasts and sigmoids?
Mantled porphyroclasts have a central single crystal and sigmoids don’t, they have been entirely recrystallised
Describe quarter structures
The way in which foliation wraps around a porphyroclast that can be used to infer shear sense
Describe strain fringes
Can be formed due tot eh oblique orientation of teh incremental strain ellipse, is asymmetric becuase the the fringes develop preferentially in the direction of the maximum extension
Describe low angle fractures
Occur in rigid components within shear zones, they have a C’ orientation and slip synthetically to shear sense, creating shear-band boudins
Describe high angle fractures
Occur in rigid compenents within shear zones, they slip antithically to the shear sense and have back rotation, creating domino boudins
Describe mineral fish
Elongate single crystals that mostly lie with their longest dimension at a small angle to the shear plane and stepping up in the direction of shear, most commonly white micas
What indicates the sensse of shear in mineral fish?
The relationship of the cleavage to the external form of the crystal
What three ways can be used to define the foliations in a mylonite?
By the preferred orientation of micas, by a mineralogical layering, by a grain shape preferred orientation
How do the S-surfaces (principal foliations) relate to the grain shape preferred orientation?
They both track the orientation of the finite strain ellipse
How can shear sense be inferred from the asymmetry of the lattice preferred orientation (LPO) in ductile shear zones?
At the deformation conditions in which ductile shear zones form, the grain shape fabric is usually accompanied by LPO of the individial grains, this is an alignment of the crystallographic c-axes
When do upper crustal environemnts show both ductile and brittle deformation (semi-brittle shear zones)?
When there are fluids present
What can be seen in brittle deformation shear zones?
Fractures and veins
Describe the fractures in semi-brittle shear zones
Typically in en echelon arrays, form at 45deg to the shear zone wall and are parallel with the max. shortning direction of the incremental strain ellipse (can be used to infer shear sense)
What type of fabric is a foliation?
Pressure solution fabric, forms are ~90deg to tension gashes
When can a second generation of veins develop?
When veins widen due to continguing deformation, they rotate to become sigmoidal, the second generation develops and transects the initial set
How do veins initiate during transpression?
At >45deg to the shear zone walls
How do veins initiate during transtension?
At <45deg to the shear zone walls
What leads to the formation of a vein?
Fracturing in the presence of a solute-rich fluid that precipitates into the fracture
Describe the crystals in veins?
Have a fibrous morphology, the fibres grow parallel to the maximum extension direction of the incremental strain ellipsoid, curving as the deformed rock rotates with respect to the strain ellipsoid
Describe syntaxial veins
Where the fibres grow inwards, often into an open crack
Describe antiaxial veins
The fibres grow outwards from the fracture surface (median line) where the fibres have been initiated
Describe stretching/antaxial veins
Where the fibres are built up by repeated small opening and cementation episodes, they indicate the overall opening direction but not the strain increments because the location of the events is irregular
Describe a fault breccia (brittle deformation zone)
Tectonite formed by localised zone of brittle deformation, more than 30% clasts that are over 2mm in size
What are the three classifications of fault breccia and what are they based on?
Crackle, mosaic, chaotic, based on how well the clasts fit together
Define cataclasites (brittle deformation zone)
Non-foliated fault rocks with less than 30% clasts over 2mm
What are the three division of cataclasites?
Procataclasites (0-50% matrix), mesocataclasites (50-90% matrix), ultracataclasites (90+% matrix)
Describe pseudotachylytes
Cataclasites that contain dark glass, formed by localised melting during earthquakes, can be injected into the rock wall
Describe fault gouges
Fault rocks with less than 30% clasts over 2mm that are in cohesive at the present outcrop
Give four fabric elements that can indicate shear sense in clay-bearing fault gouges (brittle deformation zones)
P-foliation, R1 Riedel shears, Y-shears, B-shears
what does P-foliation depend on?
The alignment of platy minerals
Describe R1 Riedel shears
Slip synthetic to shear sense
Describe Y-shears
Parallel to shear zone walls
Describe B-shears
Run along shear zone walls
What fabrics intersect in the Y-direction of clay-bearing fault gouges (brittle)?
Any two planar fabrics, such as P foliation and Riedel shears
Describe the effect of deformation bands in very porous rocks
Are a result of pure and simple sheer components, can affect the flow in oil/gas/water reservoirs
