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CET/UWA Microstructure Course

Shear Zones

Shear zones are planar zones of localised deformation.

shear zones are weaker than country rock.

• deformation induced localisation
• different starting material

Typically, there is a strong tendency towards non-coaxial (simple shear) deformation within the shear zone. This because of compatibility of strain rate and stress: normal stress across a plane equal parallel stretching across a plane equal simple shear component can be partitioned

Localisation causes (a) Changes in grain size, with grain size sensitive deformation mechanism or mechanism switch (b) Influx of fluids Enhanced permeability during deformation (dilatancy) Shear zones often associated with retrograde metamorphism Hydrolitic weakening in quartz (micro-cracking, H20 in lattice) (c) Previous brittle faulting or ductile shear in extension of brittle fault zone (d) Shear heating giving thermal weakening (e) Transformation plasticity: metamorphic reaction enhances deformation and reaction softening where a new metamorphic mineral is softer or finer grained than the old one (f) Geometric weakening Development of CPO Development of grain shape foliation & domainal fabric Rearrangement of mineral distribution & shape (frame-work collapse) (g) Recrystallisation

Crustal levels & mechanisms
 
 

Deformation mechanisms & localisation behaviour varies with P,T, etc. Shear zones may have long history and cut through large section of the crust. In general from upper to lower crust: brittle brittle-ductile transition ductile   Brittle: displacement variation discontinuous Ductile: displacement variation continuous Scale is important: small scale brittle can be large scale ductile

At same conditions one rock type or mineral may have brittle behaviour and another ductile. Within one shear zone, deformation structures may vary over time and in space. Different parts of the same shear zone (hanging wall / foot wall) may have different histories of deformation conditions. Shear along shear zone may vary within one deformation event and even more when shear zone is reactivated (opposite senses of shear very well possible) Result can be a wide variety of structures and overprinting relationships

Main types of shear zone rocks from shallow to deeper levels

Brittle fault rocks

fault breccia (cohesive / incohesive): >30% angular fragments

cataclasite (cohesive / incohesive): <30% angular fragments

fault gouge: very few isolated fragments in very fine matrix
 
 

A cohesive cataclasite with quartz fragments in a cohesive epidote-rich matrix.

General reduction in grain size due to brittle processes (breaking of grains, "milling"). At very shallow levels the fault rocks can be incohesive. Dissolution/precipitation processes can contribute to the deformation and cement grains together: cohesive fault rocks.

Associated (micro-) structures:

• fault planes, shear fractures (at variety of scales)
• foliations (mica alignment, compositional, shear fractures)
• polished surfaces, grooves, striations
• veins, stylolites, slickensides

Pseudotachylite
 
 

	Pseudotachylite: melting on fault plane during very rapid movement. (dark) glass or divitrification structures & spherulites injection veins Sharp contact with wall rock evidence for melting in the form of corrosion or complete melting of certain minerals Dark very fine grained cohesive cataclasites & gouges can look very similar to pseudotachylites

Mylonites are zones of strong to extreme localisation of ductile deformation.

• Localisation tends to decrease with metamorphic grade:
• upper extent ductile regime: narrow, extremely fine grained mylonites
• deeper: wide shear zones, gneisses to striped gneisses (coarse grained)

• fine grained due to deformation (recrystallisation, not brittle 'milling')
• normally well foliated (S-mylonite)
• often with linear shape fabric or stretching lineation (L-/LS-mylonite)
• containing remnants of coarser protolith: porphyroclasts
• 10-50% matrix: protomylonite
• 50-90% matrix: (meso-) mylonite
• >90% matrix: ultramylonite
• planar zones, straight or anastomosing
• can occur in mylonite zones with several or many mylonites

• monoclinic symmetry of structures
• transposition of structures towards foliation & lineation

Shear zone microstructures & kinematic indicators

Brittle structures     Riedel shears (Y, P, R, R' - shears) veins, vein arrays, fibres stepped faults

passive markers layers/dykes, etc larger than shear zone. Important to know shear direction (lineation) and take into account orientation of marker relative to shear zone vein arrays (folding & boudinage). Rotation, stretching (boudinage) and shortening (folding) depends on orientation w.r.t. deformation. Shortening and stretching is possible during progressive deformation, but also take into account variations over time of applied deformation sheath folds. Extreme strains can shear out any irregularity in layering into a tubular fold. Fold axes, like most other fabric elements get transposed towards flow plane (S) and flow direction (L). Active buckling and rotation enhances formation of sheath folds. Fold asymmetry can indicate shear, but be careful: original orientation of plane & different asymmetry on limbs of larger scale fold.

Shear bands & oblique foliations

Shear bands are small micro- or sub-shear zones in larger scale shear zone, i.e. small scale localisation or strain partitioning structures. They may look similar but are not the same as crenulation cleavages, which normally develop normal to shortening. (-> extensional crenulation cleavage (=shear bands) versus compressional crenulation cleavage).
 
 

S = main foliation (schistosité), often oblique to shear zone boundary (SZB) C = shear bands (cisaillement) parallel to SZB. C'= shear bands oblique to SZB C"=counterpart of C' in conjugate set; C" at high angle to SZB (rare)

 

S-C fabric (Berthé et al. 1979, type I) often good shear sense indicator. Develops early in shear zone formation. C' (and C") are often late structures and may develop due to shear zone parallel shortening or stretching. Note: also look outside shear zone for this, since such shortening/stretching must also occur there (stress & strain rate compatibility)

Oblique (grain shape) foliation mostly in rocks with one dominant mineral (e.g. micaceous quartzite). Angle <45^o with flow plane. Combination of:

• instantaneous stretching in 45^o direction (ISA)
• rotation towards flow plane
• recrystallisation

Oblique foliation + mica fish: type II S-C fabric of Lister & Snoke (1984).

• S = mica-fish & grain shape foliation oblique to flow plane (SZB)
• C = micaceous trails from fish parallel to SZB

Domainal fabrics: different areas within rock that display contrasts in grain size, grain shape foliation, crystallographic preferred orientation or some other microstructure

Porphyroclasts

(semi-) rigid objects cause local disturbance of stress field:

• higher differential stress inside object
• highest differential stress & pressure on 2 sides of object
• lowest differential stress & pressure on 2 other sides of object (pressure shadow)

Range of possible results in object-matrix system: (a) loss of cohesion with extensional failure between object and matrix: pressure fringes within object: boudinage (b) loss of cohesion with shear failure (c) no loss of cohesion; deflection of deformation around lens including pressure shadow (strain dependent rheology, anisotropy / micas) in simple shear no or little rotation of object (d) no loss of cohesion; flow of matrix around object. in simple shear rotation of object

pressure shadows & fringes, very useful, see lecture 5.b

Fractured objects & tiling
 
 

Fractured objects: antithetic and synthetic fractures (unreliable shear sense indicator). Fracture orientation depends on relative orientations of:

• shape of object
• deformation
• crystallography

Winged objects / mantled porphyroclasts. Porphyroclasts (typically feldspars) can have fine-grained mantles. The mantle forms by recrystallisation of the object rim, possibly with mixing in of matrix material (by grain boundary sliding and diffusion).

-> Mantle is not the same as pressure shadow or pressure fringe

With high strain, mantles can get stretched to form wings. Flow field and amount of mantle material determine geometry of mantled porphyroclasts.
Two basic flow fields, defined by shape of separatrix (planes that separate distinct flow fields)

• eye shape flow pattern:
• one field with closed flow loops around rotating object
• two fields where material flows past object
• bow-tie or double-bulge flow pattern:
• one field with closed flow loops around rotating object
• two fields where material flows past object
• two fields where flow lines turn back from object

d-objects (delta): mantle is partly outside of separatrix: part remains close to object, part flows away in long thin wings with embayments

f-objects (phi): long wings, but no embayments; mantle extended outside of separatrix; orthorhombic symmetry

sa-objects (sigma): long wings, but no embayments; mantle extended outside of separatrix; monoclinic symmetry. s-objects should not be confused with pressure shadows.

stair-stepping: the wings on either side of the object are parallel, but do not lie in the same plane. Upward step is in shear direction. Only use term (no) stair stepping when wings are long and parallel.

complex objects: objects with more than one pair of wings.
 

sb-objects (sigma): mantled porphyroclasts in S-C fabric.

Quarter structures

Monoclinic symmetry of structures at porphyroclasts without mantles:
 

quarter folds quarter mats (strain caps) asymmetric myrmekite (shape and distribution) V-pull apart structures

Lattice preferred orientations

see lecture 3