Download lectures 9 polyphase structures lecture plan

LECTURES 9 POLYPHASE STRUCTURES
LECTURE PLAN
1) INTRODUCTION
2) CHARACTERISTICS OF POLYPHASE DEFORMATION
3) TERMINOLOGY USED IN POLYPHASE TERRAINS
4) FOLD INTERFERENCE PATTERNS
5) SUPERPOSED LINEATIONS
6) SUPERPOSED FOLIATIONS
7) SHEATH FOLDS
8) MAPPING OF SUB-AREAS
Undeformed cover
sedimentary rocks
Folded basement
metamorphic rocks
Time 1
Folded basement metamorphic rocks are
unconformably overlain by unfolded
cover sedimentary rocks. A second
phase of deformation folds the cover
rocks and re-folds the basement rocks to
form polyphase folds in the basement.
The polyphase folds have folded axial
planes.
1) INTRODUCTION
In many continental terrains the crust may be divided into a
basement which may contain folds, faults and fractures
acquired during early orogenic phases, unconformably overlain
by undeformed sedimentary or volcanic rocks which are
termed cover. When such an area undergoes a new phase of
orogenic activity, the cover rocks may develop relatively simple
structures such as the faults, folds fracture systems described
in earlier lectures.
However, the development of new structures in the basement
rocks which already contain structures leads to the
development of complex structures known as polyphase
structures such as superposed folds. The unconformity
between the cover and basement rocks may represent
substantial erosion which can only be produced during a long
period of time. Thus, two tectonic phases that produce
polyphase structures in basement rocks may be separated by
relatively long periods of time (50 - >500Ma).
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Time 2
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Practical 1 2 3 4 5 6 71 8 9 10
2) CHARACTERISTICS OF POLYPHASE DEFORMATION
If an area has undergone only one phase of deformation, then
poles to bedding are generally distributed about a great circle
when plotted on a stereonet (see stereonet practicals). Minor
fold axis are parallel to major fold axis, and cleavages are
generally axial planar to folds.
If an area has undergone more than one phase of deformation
then more complex structures develop which contain:1) A wide distribution of bedding attitudes whose poles do not
plot on a great circle on a stereonet.
Poles
to bedding
fan around
the fold
ld
Fo
axi
Poles
Fold axis
s
Apparently random
poles for a re-folded
fold
2) Fold interference patterns
3) Foliations and lineations of an earlier deformation which are
folded.
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4) Superposition of later fabrics
3) TERMINOLOGY USED IN POLYPHASE TERRAINS
Deformation
1st = D1
2nd = D2
3rd = D3
Fold Phase Axial-planar foliation Lineations
F1
S1
L1 (L1 in So)
F2
S2
L2
F3
S3
L3
Start of this Lecture
4) FOLD INTERFERENCE PATTERNS
Type 1
Three basic end-member of a continuous series of interference
patterns can be recognised.
Type 1- This pattern arises when both F1 and F2 fold axial
planes and fold axes are orthogonal to each other, with the fold
axes originally co-planar. The F1 axial planes remain unfolded,
but the F1 fold axes are themselves folded, running across
domes and basins. Often called an egg box or dome and basin
pattern.
F fold axis
1
F fold axial plane
1
F fold axis
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2
F fold axial plane
Type 3
2
F fold axial plane
These fold interference patterns only appear if the erosion
surface through the rock is in a certain orientation.
2
F fold axis
1
F fold axis
F1 fold axial plane
Start of this Lecture
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Type 2
2
Type 2- This occurs when the F1 and F2 fold axial planes are
orthogonal to each other, but the F2 fold axis lies within the F1
fold axial plane and at 90o to theF1 fold axis. The F1 fold axial
plane is re-folded. Often called the mushroom pattern because
some of the fold closures are attached to stalks unlike the
completely closed forms of Type 1.
Type 3- This occurs when the F1 and F2 fold axial planes are
orthogonal to each other, but the F1 and F2 axes are parallel.
F1 fold axial planes are folded. This is called the re-folded fold
pattern.
2
F fold axial plane
1
F fold axial plane
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5) SUPERPOSED LINEATIONS
6) SUPERPOSED FOLIATIONS
Linear structures have a wide variety of orientations
within polyphase structures as early lineations are
folded and new lineations are formed. Remember that
there are two classes of lineations related to folding:1) Lineations parallel to the fold axes (You can call this
the F direction).
2) Lineations parallel to the stretching direction or X
direction which is generally at right angles to the fold
axes (You can call this the X direction).
In areas which have been affected by numerous
deformation phases, early foliations (S1) may become
folded and new foliations (S2) may develop. The second
foliation is usually a crenulation cleavage which intersects
the first foliation to produce a crenulation lineation (L2 in
S1). The crenulation foliation may also intersect bedding
(S0) to produce a crenulation lineation (L2 in So).
Both F and X lineations can be found related to F1 and
F2 folds.
- Lineations formed during the first folding (F1 and/or X1
directions) phase may show a curvilinear form as they
are folded by F2 folds.
- Lineations formed by the second phase of folding are
orientated parallel to the F2 fold axes (F2 direction) or
parallel to the stretching direction of the the D2 strain
ellipsoid (X2 direction).
In areas of only two phases of folding, the crenulation
foliation S2, (approximately axial planar to the second
phase of folds) will be fairly constant in orientation, whereas
the orientation of L2 in S1 will vary according to the
orientation of S1 within the first phase fold structures (i.e it
will be different within the different parts of the same
structure and between types of interference patterns).
3D view of the F1 axial planar
cleavage (S1) intersecting the
F2 axial planar cleavage (S2)
F1 axial planar
cleavage (S1)
B
2
F axial planar
2
cleavage (S )
in this orientation
F2 fold
axial plane
A
The L2 intersection lineation in S2 changes in orientation
spatially. It is almost horizontal at A and steeply dipping at B.
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In this Type 3 re-folded fold, L1 linear structures at right angles to the
fold axis have been folded, whilst those parallel to the fold axis have not.
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Start of this Lecture
7) SHEATH FOLDS
Where strains vary across a shear zone, layers can become
stretched into tube-like shapes known as sheath folds. They
occur in high strain rocks within shear zones. Strictly, they are
not polyphase folds because they usually form during a single
phase of deformation with spatially varying strain. The fold
axes are curved, having been rotated almost parallel to the
sides of the zone of high strain during progressive deformation.
Sheath folds can be recognised from their sub-circular outcrop
patterns when viewed down the transport direction
8) MAPPING OF SUB-AREAS
B) Cross-section
A) 3D view of a sheath folds
Plane of
cross-section
A
distance
B
A
distance
C) Graph showing how strain
varies across the shear zone
B
strain
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Formation of a sheath fold
Time 2
Time 1
In areas of superimposed folding the division of structurally
homogeneous subareas is essential in order to analyse the
overall structure.
Time 4
fold axis
Time 3
The division should be made whilst in the field so that you can
find exposures with which to test your sub-divisions.
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Sheath folds can be recognised from their sub-circular
outcrop patterns when viewed down the transport direction
Start of this Lecture
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Division into sub-areas is based on:-
Examples:-
1) Areas where there is a constant orientation of a particular
generation of lineation.
1) Lineations deformed by flexural slip folds retain a
constant angle between L1 and F1. This occurs
because flexural slip folding does not strain the folded
layers (the layers do not change shape and retain
internal angles).
2) Areas where there is a constant orientation of a particular
foliation.
3) Areas where there is a constant fold axis orientation.
4) Areas distinguished by the axial surfaces of the various fold
structures, i.e. vergence boundaries.
The sizes of the sub-areas will be constrained by the spacing
of fold axial planes.
2) Lineations deformed by similar folds produced by
tangential longitudinal slip (TLS Folds) with high
strains, do not retain a constant angle between L1 and
F1. This occurs because the folded layer becomes
strained (i.e. it changes its shape so that internal
angles are altered.
The effects of folding mechanisms on the deformation of
lineations (A cautionary tale concerning stereographic rotation
of lineations to their pre-folding orientation)
- The style of folding can influence the orientation of lineations
within folds. If the beds are significantly strained then the
angular relationships between between lines on the originally
un-folded surface are changed. This is important when
considering palaeo-current data from folded strata. If the layers
are highly strained (e.g. isoclinal similar folds produced by the
TLS folding mechanism) then simple stereographic rotation of
lineations on bedding surfaces (e.g. flute casts) will produce
erroneous palaeocurrent directions. Therefore only use
simple stereographic rotation of palaeo-current data in
rocks which have low strains.
Start of this Lecture
FURTHER READING AVAILABLE
FROM THE ELECTRONIC LIBRARY
S. K. Ghosh, S. Hazra and S. Sengupta 1999.
Planar, non-planar and refolded sheath
folds in the Phulad Shear Zone, Rajasthan,
India, Journal of Structural Geology, 21,
1715-1729
G. I. Alsop and R. E. Holdsworth, 1999.
Vergence and facing patterns in large-scale
sheath folds, Journal of Structural Geology,
21, 1335-1349
F2 fold axis
At Badcall, N.W. Scotland, Lewisian Gneisses have a foliation that has a
vertical dip, but the strike changes from WNW-ESE to NW-SE. This large
fold therefore has a vertical fold axis. The gneissic layers are composed of
an earlier set of isoclinal folds. The overall structure is a Type 3 re-folded
fold geometry. The vertical fold axis (green dashed line) is the F2 axis. One
of our students, Ron Million in the red coat and flat cap, has spent many
years mapping this structure and the surrounding area.
F1 Folds in second photo
View towards the WNW
View looking ESE
These are F1 folds at
Badcall. Ron is standing
close to where he was in
the first photo
F1 folds curving around from
WNW to the NW
due to the larger F2 fold.
View towards
the WNW
Can you spot the
F1 fold?
View looking NW. Can
you spot the F1 folds at
the feet of another
student, Ioannis
Papanikolaou?
View to the NW, with more F1 folds
My
Gn loni
eis tic
se
s
Next
photo
Thrust-sense shear zone containing
mylonitic gneisses within less-deformed
Lewisian Gneisses, Scotland. Steve Hirons for
scale. The sheared foliations are again
isoclinal folds of light (quartz and feldspar
rich) and dark (amphibole-rich) gneiss. The
shear zone folds the F1 folds into a Type 3 refold.
A mafic layer in the core of an F1 fold is
re-folded by a shear zone that formed in
D2. A smaller D2 shear zone occurs
above the lens cap. The sense of refolding is Type 3.
Mylo
nites
A mafic layer in the core of an F1 fold is
re-folded by a shear zone that formed in
D2. The sense of re-folding is Type 3.
Mylo
nites
M
ylo
ni
te
s
A mafic layer in the core of an F1 fold is
re-folded by a shear zone that formed in
D2. A smaller D2 shear zone occurs
within the larger shear zone. The sense
of re-folding is Type 3.
View of a sheath fold within
Lewisian gneissic layering, N.
W. Scotland
Sketch
Note closed or “Bulls eye”
pattern indicating a
sheath fold
Another view
of the sheath
fold.
A view of two closed outcrop patterns of gneissic foliation defining sheath
folds (an antiform and a synform). The stretching lineation (green lines) in
these metamorphic rocks is parallel to the elongation of the sheaths.
Synform
Antiform
= dip of the foliation in this map view photograph