Review of folds
inter-limb angleGeometric elements of folds
Axis,axial plane and axial surface
D,Anticline,fold faces upwards E.antiformal syncline,fold faces
downwards,F,refolded recumbent anticline
Classification of folds based on the inter-limb angle
Detachment zone
Markedly asymmetric chevron with a very long
limb and a very short limb
The band is the zone in which the common short straight limb
bounded by the two adjacent fold surfaces
Symmetric and asymmetric folds
Symmetric folds with limbs of equal length,Asymmetric folds with
Limbs of unequal length
N
S
56
63
52
N
5 km
PARASITIC FOLDS,USE IN MAPPING
West Lake
East Lake
disharmonic folds (B) and harmonic folds (C)
Harmonic folds,correspond with each other in wavelength,
Symmetry and general shape,Disharmonic folds,wavelength
And shape of folds in adjacent layer are quite different.
Disharmonic folds
The wavelength of the inner,thinner layers is
much shorter than that of the outer layer
Cylindroidal Folds
（ After G.H.Davis,1984）
Folds that maintain a constant
profile are termed
Cylindroidal folds.
Such folds may be generated
by a line moving parallel to
itself,Many geologists call this
line as fold axis.
A.B.a part of a cylinder；
C,Surface tangential to cylinders
with different curvature radii
IDEAL CYLINDROIDAL FOLD
（ After Wilson,1961)
Periclines,domes and basins
A pericline is a fold
whose amplitude
decreases regularly
to zero in both
directions,so the fold
has precise limits in
space.
Brachyanticlines:
Anticlinal periclines
Branchsynclines:
Synclinal periclines
Dome,special brachy-
anticlines,the dip is
radial
Structural Dome (A) and Circular Basin (B)
Special type of anticline and special type of syncline
No hinge lines and no axial surfaces
Heavy dashed line represents the hinge line
of the fold in three dimensions
Doubly
plunging
Culminations,points of maximum elevation along curved hinge line
Depression,points of minimum elevation along curved hinge line
(After B.E.Hobbs et al,1976)
LECTURE 14 SUPERIMPOSED
FOLDS AND FOLD
MECHANISM
? SUPERIMPOSED FOLDS
? FOLD MECHANISM
? SHEAR ZONE
Superimposed Folds
Superimposed folds
A,Refolding of folds
F1 By a second fold F2.
B,Three main types of
outcrop pattern produced
by superimposition,Fold
axial traces shown in
color.
1 dome and basin
2 crescent and mushroom
3 double zigzag
The earlier axial surfaces are refolded by the second folding
Superimposed folds in Gangkou area,Northwest Jiangxi
FOLD MECHANISM
? BENDING
? BUCKLING
? Flexural flow (A) and flexural slip folds
Bending Flattening Buckling
Buckling,
force parallel to
the layer
Bending:
Force perpendicular
to the layer
Flexural flow (A) and flexural slip folds(B)
（ After J.G.Ramsay et al,1987）
Spaces in the hinge zones formed by flexural slip
(Filled with ore bodies)
Ideally similar fold formed by
slip oblique to the fold layer
Profile of the ideally similar fold
Formed by the mechanism illustrated
in the left figure,The thickness
parallel to the shear planes is constant.
The volume of folded
material in the thin lamella
ABCD on the limb of a fold
is the same as that in the thin
lamella EFGH on the hinge
of the fold.
(After Bruce E,Hobbs et al,
1976)
A mechanism for the
Formation of
similar folds
FOLDING PROCESSES AND LAYER GEOMETRY
? Flexural – Slip,Low temperature,high deformation rate
competent (mechanically strong) layers
[ Parallel Folds ]
? Flexural – Flow,Metamorphic temperature,moderate
deformation rate &/or alternating competent & incompetent
layers
[ Mixed layers of roughly parallel and similar geometry ]
? Passive – Flow,Higher metamorphic temperatures,low
deformation rate
[ Similar Folds ]
Folding of neutral surface
（ After J.G.Ramsay et al,1987）
A.strain distribution pattern； B.cleavage； C.tension fractures； D.shear fractures
Neutral surface
Initial element
Hinge
Hinge Hinge Sheath fold
SHEAR ZONE
Shear zone is a zone of brittle,brittle-
ductile,or ductile deformation between
two blocks that have moved relative to
each other.
A small shear
zone in a sample
from Jianou,
fujian Province
(Zeng Zuoxun
And Liu Lilin,
1992)
Different shear zone
（ After J.G.Ramsay,1980）
A.brittle,B.brittle-ductile,C.ductile-brittle,D.ductile
Transition from brittle shear zone to
ductile shear zone
Problems
? Distinguish flexural slip from flexural flow
? Why do we call some folds passive folds?
? What is the difference between buckling and
bending?
Exercise
Observe and draw two different folds of sample from the
structural scope (next door)
Preview the APPENDIX (STEREOGRAPHIC
PROJECTION)
inter-limb angleGeometric elements of folds
Axis,axial plane and axial surface
D,Anticline,fold faces upwards E.antiformal syncline,fold faces
downwards,F,refolded recumbent anticline
Classification of folds based on the inter-limb angle
Detachment zone
Markedly asymmetric chevron with a very long
limb and a very short limb
The band is the zone in which the common short straight limb
bounded by the two adjacent fold surfaces
Symmetric and asymmetric folds
Symmetric folds with limbs of equal length,Asymmetric folds with
Limbs of unequal length
N
S
56
63
52
N
5 km
PARASITIC FOLDS,USE IN MAPPING
West Lake
East Lake
disharmonic folds (B) and harmonic folds (C)
Harmonic folds,correspond with each other in wavelength,
Symmetry and general shape,Disharmonic folds,wavelength
And shape of folds in adjacent layer are quite different.
Disharmonic folds
The wavelength of the inner,thinner layers is
much shorter than that of the outer layer
Cylindroidal Folds
（ After G.H.Davis,1984）
Folds that maintain a constant
profile are termed
Cylindroidal folds.
Such folds may be generated
by a line moving parallel to
itself,Many geologists call this
line as fold axis.
A.B.a part of a cylinder；
C,Surface tangential to cylinders
with different curvature radii
IDEAL CYLINDROIDAL FOLD
（ After Wilson,1961)
Periclines,domes and basins
A pericline is a fold
whose amplitude
decreases regularly
to zero in both
directions,so the fold
has precise limits in
space.
Brachyanticlines:
Anticlinal periclines
Branchsynclines:
Synclinal periclines
Dome,special brachy-
anticlines,the dip is
radial
Structural Dome (A) and Circular Basin (B)
Special type of anticline and special type of syncline
No hinge lines and no axial surfaces
Heavy dashed line represents the hinge line
of the fold in three dimensions
Doubly
plunging
Culminations,points of maximum elevation along curved hinge line
Depression,points of minimum elevation along curved hinge line
(After B.E.Hobbs et al,1976)
LECTURE 14 SUPERIMPOSED
FOLDS AND FOLD
MECHANISM
? SUPERIMPOSED FOLDS
? FOLD MECHANISM
? SHEAR ZONE
Superimposed Folds
Superimposed folds
A,Refolding of folds
F1 By a second fold F2.
B,Three main types of
outcrop pattern produced
by superimposition,Fold
axial traces shown in
color.
1 dome and basin
2 crescent and mushroom
3 double zigzag
The earlier axial surfaces are refolded by the second folding
Superimposed folds in Gangkou area,Northwest Jiangxi
FOLD MECHANISM
? BENDING
? BUCKLING
? Flexural flow (A) and flexural slip folds
Bending Flattening Buckling
Buckling,
force parallel to
the layer
Bending:
Force perpendicular
to the layer
Flexural flow (A) and flexural slip folds(B)
（ After J.G.Ramsay et al,1987）
Spaces in the hinge zones formed by flexural slip
(Filled with ore bodies)
Ideally similar fold formed by
slip oblique to the fold layer
Profile of the ideally similar fold
Formed by the mechanism illustrated
in the left figure,The thickness
parallel to the shear planes is constant.
The volume of folded
material in the thin lamella
ABCD on the limb of a fold
is the same as that in the thin
lamella EFGH on the hinge
of the fold.
(After Bruce E,Hobbs et al,
1976)
A mechanism for the
Formation of
similar folds
FOLDING PROCESSES AND LAYER GEOMETRY
? Flexural – Slip,Low temperature,high deformation rate
competent (mechanically strong) layers
[ Parallel Folds ]
? Flexural – Flow,Metamorphic temperature,moderate
deformation rate &/or alternating competent & incompetent
layers
[ Mixed layers of roughly parallel and similar geometry ]
? Passive – Flow,Higher metamorphic temperatures,low
deformation rate
[ Similar Folds ]
Folding of neutral surface
（ After J.G.Ramsay et al,1987）
A.strain distribution pattern； B.cleavage； C.tension fractures； D.shear fractures
Neutral surface
Initial element
Hinge
Hinge Hinge Sheath fold
SHEAR ZONE
Shear zone is a zone of brittle,brittle-
ductile,or ductile deformation between
two blocks that have moved relative to
each other.
A small shear
zone in a sample
from Jianou,
fujian Province
(Zeng Zuoxun
And Liu Lilin,
1992)
Different shear zone
（ After J.G.Ramsay,1980）
A.brittle,B.brittle-ductile,C.ductile-brittle,D.ductile
Transition from brittle shear zone to
ductile shear zone
Problems
? Distinguish flexural slip from flexural flow
? Why do we call some folds passive folds?
? What is the difference between buckling and
bending?
Exercise
Observe and draw two different folds of sample from the
structural scope (next door)
Preview the APPENDIX (STEREOGRAPHIC
PROJECTION)
