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Ductile Shear Zone (),Textures, and Transposition ()

Jyr-Ching Hu, Department of Geosciences

National Taiwan University

Moine Thrust in Scotland

http://jaeger.earthsci.unimelb.edu.au/Im

ages/Geological/Structural/mylonites/my

lonite.jpg

http://www.uwsp.edu/geo/projects/geowe

b/participants/Dutch/VTrips/Scot75May25

-28.HTM

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Ductile Shear Zone

A tabular band of definable width in which there is

considerably higher strain than in the surrounding rock.

The total strain within a shear zone typically has a large

component of simple shear( ), where rocks on

one side of the zone are displaced relative to those onthe other side.

In its ideal form, a shear zone is bounded by two parallel

boundaries, outside of which there is no strain. In realexamples, shear zone boundaries are gradational.

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Ductile Shear Zone

The adjective ductile is used because the strainaccumulates by ductile process, which range fromcataclasis () to crystal-plasticity ()to diffusion.

A shear zone is like a fault in the sense that it

accumulates relative displacement of rock bodies, butunlike a fault, displacement in a ductile deformation

mechanisms and no throughgoing fracture is formed.

The absence of a single fracture is a consequence of

movement under relatively high temperature conditions

or low strain rates.

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Sibson-Scholz fault model

Brittle process

and cataclasticflow

Geothermal

Gradient of

20oC/km-30oC/km

Crustal strength

450oc

Brittle-plastic

transition

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Change in fault character with depth for a

steeply dipping fault

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Changes in the deformation behavior of quartz

aggregates with depth

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Distribution of the main types of fault rocks with the

depth in the crust

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Synoptic model of a shear zone

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Brittle process, cataclastic flow, frictional

regime and plastic regime Brittle process (): Occur along the discontinuity

in the few kms below Earths surface which result in

earthquakes if the frictional resistance () ondiscrete fracture planes is overcome abruptly.

Cataclastic flow ():A ductile process thatdisplacement occurs by movement on many smallfractures.

Frictional regime (): Frictional processes dominatethe deformation at upper levels of discontinuity and this

crustal segment. This region is pressure sensitive.

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Brittle process, cataclastic flow, frictional

regime and plastic regime Plastic regime ():: With depth, crystal-plastic and

diffusional processes such as recrystallization and

super-plastic creep, become increasing important due toincrease of temperature.

Below a depth of 10-15 km for normal geothermalgradients (20oC/km-30oC/km) in Qtz-dominated rocks.

Deformation in plastic regime is mostly temperaturesensitive.

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Frictional-plastic (-) and

brittle-plastic transitions (-)

Frictional-plastic transition or brittle-plastic

transition: transition zone between a dominantly

frictional and dominantly plastic regime.

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Brittle-plastic transition (-) and

brittle-ductile transition (-)

Brittle-ductile transition is in common use, it is

technically not correct, because ductileprocesses (such as cataclasis) may occur in the

frictional regime.

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Mylonites ()

Rigid clasts of varied lithologies in a fine-grained, crystal-plastically deformed

marble matrix. (Grenville Orogen, Ontario, Canada)

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Mylonites ()

A fault rock type with a relatively fine grain size

as compared to the host rock and resulting fromcrystal-plastic processes.

Dynamic recrystallization occurs at different

temperaturesCalcite 250oC

Quartz

300o

CFeldspar 450oC

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Types of Mylonites

Mylonite: 50-90% matrix

Protomylonite (): < 50% matrix

Ultramylonite (): 90-100% matrix

Blastomylonite ()(blastos meaning growth)and clastomylonite () (klastos meaning

broken): Describe mylonites containing large grainssurrounded by a fine-grained matrix and grew duringmylonitization or remained from original rock.

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Shear-sense indicators

Ductile shear zones concentrate displacement at

deeper levels in the crust, where recognizedmarkers that determine offset are often absent.

Sense of displacement: describes the relative

motion of opposite sides of the zone (left-lateralor right-lateral).

Magnitude of displacement: distance over which

one side moves relative to the other.

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Plane of Observation

Mylonitic foliation

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Internal reference frame

Most mylonites contains at least one foliationand lineation which we use as an internalreference

In the field we look for outcrop surfaces (or cut an

oriented sample in the lab) that are perpendicularto mylonitic foliation and parallel to the lineation.

We assume that the lineation coincides with themovement direction of the shear zone.

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Plane of Observation

From the opposite side:Left-lateral, why?

The displacement sense is the same in geographiccoordinates, it is a good habit to analyze surfaces in the

same orientation.

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Types of Shear-sense indicators

(1) Grain-tail complexes(2) Disrupted grains

(3) Foliations(4) Textures (or crystallographic fabrics)

(5) Folds

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Grain-Tail Complexes

A K-feldspar clast with a tail of fine-grained plagioclase ofthe -type complex (California, USA)

http://d/course/Earth%20Structure/2008/Structural%20Analysis/sigmaGrain.swf

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Grain-Tail Complexes: Rotated

porphyroblasts ()

-type: characterized by

wedge-shaped tails that

do not cross the

reference plane when

tracing the tail away

from the grain

-type: the tail wraps

around the grain suchthat if cross cuts

reference plane when

tracing the tail away

from the grainRotate the Greek letter over 90o

http://d/course/Earth%20Structure/2008/Structural%20Analysis/deltaGrain.swf

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Snowball garnet

http://d/course/Earth%20Structure/2008/Structural%20Analysis/trails.swf

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Diagnostic forms of porphyroblasts

Se: solid lines, external foliationSi: dashed lines,

internal foliation

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Progressive development of snowball textures

How do we preserve a spiral pattern in garnet and what

can it tell us?

Metamorphism is synkinematic.

Assignment: Reading 13.4

Deformation and metamorphism

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Evolution of a -type complex to -type

grain-tail complex

Mixed occurrence of -type

complex to -type:

Rate of recrystallization or

neocrystallization and rotation of

grain

1. Rail formation is fast

relative to rotation:

-type

2. The rotation of grain, the

tail is dragged along and

wrap around the grain : -type

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Fractured Grains and Mica Fish

Synthetic fractures (bookshelf-type or domino-type):

Fractures oriented at low angles to the mylonitic

foliation have a displacement sense that is consistentwith the overall shear sense of the zone.

http://d/course/Earth%20Structure/2008/Structural%20Analysis/domino.swf

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Fractured Grains and Mica Fish

Antithetic fractures: Fractures at angles greater than

45o to the mylonitic foliation show an opposite sense

of movement.

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1. Large phyllosilicate

grains: Mica and biotitein quartzo-feldspathic

rocks and phlogopite

in marbles

2. Micas are

connected by a

mylonitic foliation andtheir basal planes

(0001) oriented at an

oblique angle to

mylonitic foliation

Stair-stepping geometry

Basal planes of mica

Formation of mica

fish: Fish flash

http://d/course/Earth%20Structure/2008/Structural%20Analysis/micaFish.swf

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Characteristic geometry of C-S and C-C

structures in a dextral shear zone1. Most mylonites show at least one well-developed

foliations at low angle to the boundary of shear zone.

2. S-foliation: S comes from French word for foliation,schistosit.

3. C-foliation: C comes from French word for shear,

cisaillement.

http://d/course/Earth%20Structure/2008/Structural%20Analysis/SC.swf

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Characteristic geometry of C-S and C-C

structures in a dextral shear zone

3. C-foliation: Discrete shear displacement that is

oblique to the shear zone boundary.

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Summary

diagram of shear-sense indicators

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Strain in Shear Zone: Rotated Grains

Snowball granets: -type grain-tail complexes; in particular

the mineral garnet show this behavior , in which trapped

matrix grains eventually produce a spiraling trails.

Analog Experiment

= tan = = tan =

: mechanical coupling between

matrix and grain

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Strain in Shear Zone: Rotated Grains

Analog Experiment

= tan = =1, full coupling (clean ball bearing)

=0, no coupling

0<

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Homogeneous and Heterogeneous strain in

shear zone

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Deflection of the mylonitic foliation

Drenville Orogen, Ontario Canda. Width of view is ~20 cm.

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Strain in Shear Zone: Deflected foliations

Shear zone is characterized by a mylonitic foliation (S-

foliation) that is at ~45o to the shear-zone boundary

Wk = 1Kinematic vorticity number Prefect shear zone

Angular relationship

() between foliation

and shear-zone

boundary, and shear

strain

= 2/tan2 Progressive simple shear

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Strain in Shear Zone: Deflected foliations

Nonperfect simple shear (or general shear)

General shear with a shortening component is calledtransperssion and an extensional component is

called transtension

Component of pure shear

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Non-commutative nature of strain tensor

Superimposing simple

shear on pure shear

Superimposing pure

shear on simple shear

Simultaneously adding

simple and pure shear

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Development of a crystallographic-preferred

orientation by dislocation glide:C-axis

ABCD: crystallographic glide planes

: angle of shear along glide plane

: angle of finite extension axis

: rotation angle of the c-axis withrespect to an external ref. system

: rotation angle

of material line

BC

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The Symmetry Principle: Curie Principle

Pierre Curie

1859-1906

Orthorhomic: 3 two-fold axes or 3 symmetry planes

Coaxial strain: Incremental and finite strain

ellipsoids differ only in shape, not in orientation

Monoclinic: 1 two-fold axis or 1 symmetry plane

cube

cube

http://upload.wikimedia.org/wikipedia/commons/3/3a/Pierrecurie2.jpg

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Relationship between shape,

crystallographic, S and C

Randomly oriented

C: shear plane

S: mylonitic foliation

When shearing the aggregate, a pattern emerges in which the

majority of c-axes rotate toward an orientation perpendicular to

the bulk shear plane.

Simple shear

A dimensional-preferred fabric is formed that

define the mylonitic foliation (S-foliation).

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Foliation in shear zone and associated

crystallographic fabrics

S-foliation deflection: Angular

relationship between S and C

decreases with increasing shear strain(S and C approach parallelism).

C-axis girdle

C-axis girdle

Corresponding a-axis

patterns show n change.

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Asymmetric c-axis fabrics

E-twinning dominate calcite deformation Basal slip occurred

Reference: Mylonitic foliation S

F ld T iti i l k th t d

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Fold Transposition in a layer rock that undergoesnon-coaxial, layer-parallel displacement

Foliation-parallel shear

With increasing shear, the oblique (short) limb of the

asymmetric fold rotates back into a foliation-parallel orientation.

F ld T iti i l k th t d

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Fold Transposition in a layer rock that undergoesnon-coaxial, layer-parallel displacement

Foliation-parallel shear

The resulting perturbation gives rise to a new fold that is

superimposed on the original structure.

Continued shear reorients the fold pattern back into a layer

reorients the fold pattern back into a layer-parallel

orientation.

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Fold Transposition

Highlight two aspects of folds in shear zone:

(1) Fold symmetry may be representative for the

sense of shear:

Z-vergence: Right-lateral shear zone.

S-vergence: Left-lateral shear zone

A possibility, not a rule.

At high sear strains the vergence of small foldsmay actually reverse.

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Fold Transposition

Highlight two aspects of folds in shear zone:

(2) Folding is a progressive process, resulting in

complex patterns of folding and refolding.

Fold transposition occurs at all scales, from

microfolds to kilometer-scale folds.

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Transposed mafic layer in granitic gneiss:Snake outcrop

Mafic (dark) layer is traced

as a single bed refoldednumerous times

Reversal in fold vergence (from S shape to Z

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Reversal in fold vergence (from S-shape to Z-

shape) with increasing shear strain in a right-

lateral shear zone

Z-vergence: Right-lateral

shear zone.

S-vergence: Left-lateral

shear zone.

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Fold Transposition:

Folds in areas of high strain

are often disrupted,

preserving only isolatedhinges or fold hooks.

Competent layer

Progressive shortening: thinning of limbs

And locally hinges become detached.

Fold hooks

boudinage

Are there criteria to recognize

transposition?

Clues:

1. Regular repetition of lithologies;

2. Parallel between foliation and bedding;

3. Occurrence of minor isoclinal folds and

fold hooks.

A l f l t f

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An example of an early stage oftransposition

Newfoundland, Canada

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Sheath Folds

()

Restricted to regions of high shear, it can define

shear sense in ductile shear zones.

A special type of double-plunging folds (

), where the hinge line is bent around by as

much as 180o.

Layering in a sheath fold is everywhere at a high

angle to the profile plane (), which givethe characteristic eye-shaped outcrop pattern.

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Sheath Folds

()

Formed when the hinge line () of a foldrotates passively into the direction of shear,

while the axial surface () rotates towardthe shear plane .

The location of nose of sheath folds points in thedirection of movement, but this can be determined

only when the folds are fully exposed.

Most commonly, sheath folds define the direction

of shear rather than shear sense, with the hinge

line approximately parallel to the shear direction.

Conical geometry of a sheath folds

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Conical geometry of a sheath folds

Lowest amount ofshear at the left

highest shear

strain at the right

Stretching lineation

Lower-hemisphere

projections

Shear plane

Hinge line measurements

Shear direction

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Assignment 1

Reading: 12.3.5

Drawing and explain the Figure 12.12 Summarydiagram of shear sense indicators in a sinistral

shear zone

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Assignment 2

Structural Analysis: An interactive course for EarthScience Student by Declan G. De Paor

Chapter 14: shear sense indicators

(1) Offset markers; (2) Riedel shears;

(3) Domino fault; (4) Inclusion trail;(5) grains; (6) grains; (7) Mica fish;(8) Sheath folds; (9) Asymmetric folding;(9) Bedding/foliation; (10) Restraining bends;

(11) Releasing bends; (12) Terminations;(13) En echelon array; (14) S-C foliation