Kristallingeologie

lecture 8

Shear zones

Summary lecture on foliations

• Rocks can contain foliations and lineations

• Some important foliations

• Primary foliation (S0), Axial planar & crenulation cleavage

• Cleavage // axial plane // XY-plane of finite strain

• But, refraction changes orientation of cleavage

• Important lineations:

• Intersection lineation -> often indicates fold axes

• Stretching lineation -> indicates shear direction (X-axis of

strain)

• This lecture: zones of very high strain: shear zones

Strength of the crust

• Near surface: Low P & T

• Brittle failure (Mohr-Coulomb behaviour)

• ! = !(P)

+P

+T

"Brittle-Ductile transition"

surface stress

• Deeper: higher P & T

• Ductile flow

• ! = !(T) at given strain rate

Strain

rate

Faults and shear zones

• Near surface: Low P & T

• Brittle failure (Mohr-Coulomb behaviour)

• Discrete narrow zones of deformation

+P

+T • Deeper: higher P & T

• Ductile deformation behaviour

• Wider zones of deformation:

Shear zones (Scherzonen)

"Brittle-Ductile transition"

Shear zone

• Narrow shear zones in schists (Cap de Creus, Spain)

Mylonite (mylonit)

• A mylonite is a foliated and usually lineated rock that

shows evidence for strong ductile deformation

• A mylonite has a tectonically reduced grain size

Mylonite zone

• Thin-section cut parallel to lineation shows monoclinic

symmetry of mylonitic fabric elements

Sense of shear (Schersinn)

• One important aspect of a shear zone is the sense of

shear (Schersinn)

• Many structures occur in shear zones, which tell the

sense of shear:

• Shear sense indicators (kinematic indicators)

• Best viewed in plane:

• Parallel to stretching lineation

• Perpendicular to shear foliation

= XZ-planexz

y

Marker and foliation deflection

• Rotation of planar passive

markers usually indicate the

SOS well.

• Dextral shear zone in migmatite (Finland)

Characteristics of shear zones

• Foliations• mylonitic foliation

• asymmetric folds

• oblique grain shape foliation

• shear bands

• Lineations• stretching lineation

• Porphyroclasts• fracturing, rotation and boudinage of porphyroclasts

• sigma & delta porphyroclasts

Mylonitic foliation

• Extreme stretching and flattening inside the shear

zone produces a new foliation // to the shear plane

• Grains are converted into ribbons by the strong

stretching and recrystallisation

Oblique grain shape foliation

GSFGSF GSFGSF

• Oblique grain shape foliation forms by competition of

• shearing: stretches grains towards shear plane

• recrystallisation: reverts shape towards equidimensional

Asymmetric folds

• Stretched pegmatite and quartz veins in dextral shear zone, Punta dels

Farallons, Spain

SC-fabric

• S-foliation

• Passive shearing of fabric elements

• C-bands

• Parallel to shear zone

• Subordinate "mini" shear zones

• Normally long and straight

• Synthetic

Also called extensional crenulation cleavage: extends S

C'-type shear bands

• C'-type shear bands

• Oblique to shear zone

• Normally short and wavy

• Synthetic

• C"-type

• High angle to shear zone

• Antithetic

• Rare

• Conjugate partner of C'

• C and C' type shear bands are difficult to distinguish

• No problem: you still get the right sense of shear!

C'-type shear bandsC!-type shear bands

shear zone boundaryshear zone boundary

• Micrographs of sinistral shear bands in granites (w.o.v. 4 mm)

Type 2 SC-fabric

• S foliation:

• Oblique grain shape foliation

• C foliation:

• Bands of mica

• // shear zone

• Most (not all)

micas lie at a

small angle to the

C-foliation

mica fish

Stretching

lineation

• Extreme stretching

produces strong

lineation in suitable

rock types

• granite

• pegmatite

• coarse sandstone

/ arkose

• Fold axes rotate

towards stretching

lineation

stretching

lineation

fold

axis

Extension structures

• Boudinage of:

• Sphene porphyroclasts

• Aplite layer

Strong stretching: L-tectonite

• Section // lineation

• Statically recrystallised

quartz ribbons

• Section " lineation

• Only very weak foliation

visible

Porphyroclasts

• Porphyroclasts are relatively large crystals or rock fragments

that float in a fine grained matrix in a mylonite

• ~10-50% matrix: protomylonite

• ~50-90% matrix: mylonite

• >90% matrix: ultramylonite

Fragmented porphyroclasts

• Porphyroclasts are stronger than their ductile matrix

• They may deform by brittle failure

• Typically: Feldspar in deformed granite

• Failure can be:

• Syntethic (with SOS)

• Antitethic (against SOS)

• Microfaults in porphyroclasts are not good SOS indicators• Micrograph of synthetic microfaults in hornblende porphyroclasts in fine-

grained quartz matrix. Hidden Valley, S. Australia (w.o.v. 4 mm)

Both antithetic and synthetic

microfaults can occur

• Porphyroclasts in shear zone from Cap de Creus, Spain

Mantled porphyroclasts

• Rims of porphyroclasts often recrystallise and get sheared

away from the original object, creating wings

complex objects (several sets of wings)

# - objects (with embayment)

! - object (no embayment)

stair-stepping no stair-stepping

$ - object (no embayment)

% - object (no wings)

!!

!!

!! !!

!!

&&

&&

Formation of winged clasts

• Example of the development of an experimental#-clast

• Clast = camphor

• Matrix = octachloropropane (OCP)

• Experiment by Coen ten Brink (Utrecht University)

Formation of winged clasts

• The shape of the wings depends on:

• The flow field around the object

• The amount of mantle material

• The finite strain

• Development of a $-clast

Formation of winged clasts

• The shape of the wings depends on:

• The flow field around the object

• The amount of mantle material

• The finite strain

• Development of a #-clast, without stair stepping

Formation of winged clasts

• The shape of the wings depends on:

• The flow field around the object

• The amount of mantle material

• The finite strain

• Development of a !-clast, with stair stepping

• Micrograph of feldspar “fish” in mylonitised pegmatite from Cap de

Creus, Spain

!-type porphyroclast

• Dextrally sheared pegmatite, Cap de Creus, Spain

!-type porphyroclast

• Dextrally sheared limestone, Cap Norfeu, Spain • Hornblende #-clast in sinistral shear zone from Hidden Valley, South

Australia (w.o.v. 4 mm)

#-type porphyroclasts

• Retrograde ultramylonite, SW Finland

(courtesy Pietari Skyttä)

Summary

• Deformation often localises in shear zones

• With increasing depth (T), they become more diffuse

• To determine the sense of shear (SOS)

• Determine shear plane (mylonitic foliation)

• Determine stretching lineation

• Look on XZ-plane for kinematic indicators

• Main SOS-indicators:

• Shear bands

• Mantled porphyroclasts

• Off-set foliations