inter-limb angleGeometric elements of folds
Review of geometrical elements of folds
Classification of folds (continue)
Based on four main features or properties:
1 direction of closing and facing
2 attitude of axial surface
3 size of inter-limb angle,and
4 shape of profile
5 dip isogons
Classification of fold based on direction of
closing and facing
Downwards-facing
upwards-facing
Facing towards the left
Classification of folds based on the attitude of axial surface
Trap for oil or gas
Classification of folds based on
the inter-limb angle
Gentle
Open
Close
Tight
Isoclinal
A, Constant Bed
Thickness
perpendicular to
Fold surface
B,Thickness constant
parallel to axial
surface
C,A common
center of curvature
D,Planar limbs
and sharp angular
hinge zone
Classification of folds based on the fold profile
Parallel folds
Detachment zone
Kink band 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
Classification of folds based on
shape of profile
? Variation in layer thickness
the perpendicular to
the axial surface
Method of measuring variation in layer thickness t? in a
fold.Measurement is made at the point where the tangent
to the outer curve makes an angle ? with the
perpendicular to the axial surface
FOLD CLASSIFICATION
BASED ON DIP
ISOGONES
Construction of dip isogons
The isogons join points on successive fold surfaces with the same
inclination ?,The isogons converge to the fold core here.
Fold classification based on dip isogons Class 1,fold with
convergent isogons; Class 2,fold with parallel isogons; Class 3,fold
with divergent isogons,Class1A,strongly convergent isogons.
Class 1B,parallel folds with isogons perpendicular to fold surface,
Class 1C,weakly convergent isogons.
Class 2:
Similar folds
Class 1A:
Thickened
limbs
Class 1B:
Parallel
folds
Class 1C,
Class 2,3:
Thinned
limbs
DESCRIPTION OF FOLD
SYSTEMS
? FOLD SYMMETRY
? FOLD VERGENCE
? PARASITIC FOLDS
? HARMONIC AND DISHARMONIC
SYSTEMS
? CONJUGATE AND POLYCLINAL
SYSTEM
Symmetric and asymmetric folds
Symmetric folds with limbs of equal length,Asymmetric folds with
Limbs of unequal length
Fold vergence,easterly vergence,or verging east according to the
Shorter limbs of the antiforms of the set of asymmetric folds.
W
E
PARASITIC FOLDS
Z – SHAPED
FOLDS S –SHAPED
FOLDS
M – SHAPED
FOLDS
Enveloping surface drawing through the hinge lines of
all folds can show the overall shape of the main
structure.
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
Conjugate folds(A,B) with opposite sense of
asymmetry and polyclinal folds(C) with
variable axial surfaces
Folds In Three Dimensions
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)
Cylindroidal Folds
NON – CYLINDROIDAL FOLDS
Fig.B and Fig,D, conic folds
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)
Problems
1,Distinguish between two dimensional folds and three
dimensional folds.
2,Distinguish between basin and dome.
3,What is the significance of the enveloping surface of
folds.
4,Could you explain the formation of symmetric and
asymmetric folds?
5,How to use the parasitic folds in field work?
Exercises
1,Construct dip isogons with 0,15,30,45,60 degrees of
the fold in Figure 3.11.
Review of geometrical elements of folds
Classification of folds (continue)
Based on four main features or properties:
1 direction of closing and facing
2 attitude of axial surface
3 size of inter-limb angle,and
4 shape of profile
5 dip isogons
Classification of fold based on direction of
closing and facing
Downwards-facing
upwards-facing
Facing towards the left
Classification of folds based on the attitude of axial surface
Trap for oil or gas
Classification of folds based on
the inter-limb angle
Gentle
Open
Close
Tight
Isoclinal
A, Constant Bed
Thickness
perpendicular to
Fold surface
B,Thickness constant
parallel to axial
surface
C,A common
center of curvature
D,Planar limbs
and sharp angular
hinge zone
Classification of folds based on the fold profile
Parallel folds
Detachment zone
Kink band 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
Classification of folds based on
shape of profile
? Variation in layer thickness
the perpendicular to
the axial surface
Method of measuring variation in layer thickness t? in a
fold.Measurement is made at the point where the tangent
to the outer curve makes an angle ? with the
perpendicular to the axial surface
FOLD CLASSIFICATION
BASED ON DIP
ISOGONES
Construction of dip isogons
The isogons join points on successive fold surfaces with the same
inclination ?,The isogons converge to the fold core here.
Fold classification based on dip isogons Class 1,fold with
convergent isogons; Class 2,fold with parallel isogons; Class 3,fold
with divergent isogons,Class1A,strongly convergent isogons.
Class 1B,parallel folds with isogons perpendicular to fold surface,
Class 1C,weakly convergent isogons.
Class 2:
Similar folds
Class 1A:
Thickened
limbs
Class 1B:
Parallel
folds
Class 1C,
Class 2,3:
Thinned
limbs
DESCRIPTION OF FOLD
SYSTEMS
? FOLD SYMMETRY
? FOLD VERGENCE
? PARASITIC FOLDS
? HARMONIC AND DISHARMONIC
SYSTEMS
? CONJUGATE AND POLYCLINAL
SYSTEM
Symmetric and asymmetric folds
Symmetric folds with limbs of equal length,Asymmetric folds with
Limbs of unequal length
Fold vergence,easterly vergence,or verging east according to the
Shorter limbs of the antiforms of the set of asymmetric folds.
W
E
PARASITIC FOLDS
Z – SHAPED
FOLDS S –SHAPED
FOLDS
M – SHAPED
FOLDS
Enveloping surface drawing through the hinge lines of
all folds can show the overall shape of the main
structure.
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
Conjugate folds(A,B) with opposite sense of
asymmetry and polyclinal folds(C) with
variable axial surfaces
Folds In Three Dimensions
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)
Cylindroidal Folds
NON – CYLINDROIDAL FOLDS
Fig.B and Fig,D, conic folds
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)
Problems
1,Distinguish between two dimensional folds and three
dimensional folds.
2,Distinguish between basin and dome.
3,What is the significance of the enveloping surface of
folds.
4,Could you explain the formation of symmetric and
asymmetric folds?
5,How to use the parasitic folds in field work?
Exercises
1,Construct dip isogons with 0,15,30,45,60 degrees of
the fold in Figure 3.11.
