1. Folds (plicative dislocation)
When rocks suffer ductile deformation due to
compressional stress, they will bend, forming what we call
a fold.
These include the formation of folds, monoclines and
flexures.
2.
3. • Hinge:
• Where curvature of the fold is at a maximum
• Crest (arch, top) & Trough (core, bottom):
• Where fold surface reaches a minimum and maximum respectively
• Limb:
• Beds between two hinges
• Antiform & Synform:
• Convex upwards or convex downward folds respectively
• Anticline & Syncline:
• Older or younger beds at the core respectively. Can be used in
conjunction with antiform and synform, i.e. an antiformal syncline
• The ages of the strata forming a fold are described by syncline and
anticline, as described above.
4.
5. • The part of the fold where the layers bend is called a crest or arch.
• The lateral parts adjacent to the arch are called limbs of the fold.
• The imaginary plane passing through the inflection points of the layers is the
axial surface of the fold.
• The line of intersection of the axial surface with the relief surface represents
the centerline of the fold or axis.
• The line formed at the intersection of the axial surface with the inflection
surface of one of the layers that make up the fold is called a hinge.
• The distance along the center line between the points of unambiguous
bending of the hinge is the length of the fold;
• fold width - the distance between the center lines of two adjacent anticlines
or synclines;
• height – the vertical length between the crest of an anticline and the trough
of an adjacent syncline, measured along the same layer.
6. • Axial Surface (or Axial Plane if it’s not curved) of a stack of folds passes
through the hinge lines and most nearly divides the fold into two equal
parts. This concept is best seen by picture below.
• Limbs = sides or flanks of folds.
• Hinge line = where the limbs of the fold meet. It is also the line of
maximum curvature.
7. Fold types
• Folds will form either concave down “hat-like” or concave up “cup-like”
structures. The concave down structures with two limbs pointing down are
known as anticlines.
• Anticlines = folds pointing upwards.
• Anticlines are a common fold type found in the Earth’s crust. When upright,
the limbs (flat inclined surfaces) of these folds dip away from the axial surface
(the plane intersecting the maximum curvature of the fold) to form a concave-
downward structure. More generally, any type of fold that has this geometry
is called an antiform. Anticlines, however, are more specific in that rock layers
get younger away from the axial surface. An example of an anticline is shown
in the map below.
8.
9. • The concave up structures with two limbs pointing up are known as
synclines.
• Synclines = folds pointing downwards.
• Like anticlines, synclines are also a common fold type that often
forms together with anticlines during compression. When upright,
synclines are trough shaped with beds dipping toward the axial
surface. Like an antiform, the term synform is used to describe any
structure with this general geometry. The more specific term syncline,
however, indicates that layers get younger toward the axial surface. It
is important to specify antiform from anticline and synform from
syncline as folds can be rotated so that overturned synclines become
antiforms and overturned anticlines become synforms. Figure 1
shows an example of a syncline in the field.
10.
11. Hinge line-|hɪn(d)ʒ lain| şarnir xətti
Limbs- |lɪmz| qırışığın qanadları
Axial surface- | ˈ æksɪəl |ˈsɜːfɪs|
12. Upright (Open) Fold: vertical/near vertical axial surface
and limbs have similar dips (though in opposite
directions!).
13. • Inclined (asymmetrical) Fold: - the axial surface is inclined to the
horizon, and the limps fall on different sides under the different
angles.
14. Overturned Fold: beds on one limb are upside down. the
axial surface is steeply inclined, and the limbs fall (inclined)
on one side at different angles. In these folds are distinguished
normal and overturned limbs.
20. Dome
• In simple terms, a dome is a fold that looks like an upside-down bowl. Beds
dip away in all directions from the dome center to create a circular
geometry. Similar to an anticline, beds in the center of the dome are oldest
and become progressively younger toward the outside edges (part A in the
figure). If you were to construct a vertical cross-section in any orientation
through the dome, you would always get an anticline!
21. Basin
• A basin is similar to a dome in that it is a circular shaped fold. In this case,
however, all beds dip toward the center of the fold with the youngest strata in
the center and progressively older beds moving outward (part B in the photo on
the right). If you were to draw a vertical cross-section in any orientation through
a basin, you would get a syncline. A great example of a basin is the Michigan
Basin.
22. Classification of folds by position of the axial plane
(surface)
• Depending on the position of the axial surface in space, the following
different types of folds are distinguished.
• Symmetrical folds |sɪˈmetrɪk(ə)l| |fəʊld| - the axial surface is vertical,
and the limps fall on different sides under the same angles.
23. Classification folds based on interlimb angle
• Another classification for fold types is based on the angle between the two
limbs. An illustration of this classification is presented in Figure. The broadest
folds (180⁰-120⁰) are termed gentle folds. When looking at an outcrop, folds
that have distinctively obtuse angles would fall under this category. Folds with
slightly smaller angles (120⁰-70⁰) are termed open folds. In outcrop, these
folds look more or less to be at right angles. Folds with interlimb angles of
70⁰-30⁰ are termed close folds. Folds that have distinctively acute angles in
outcrop are generalized as close folds. Folds with angles of 30⁰- < 5⁰ are
termed tight folds. These folds look extremely acute in outcrop to the point
that both limbs are nearly parallel to each other. Lastly,folds with interlimb
angles of < 5⁰ -0 are termed isoclinal folds. Both limbs are parallel to each
other.
25. • The folds with fan-shaped limps are termed concentric |kənˈsentrɪk| folds,
the folds with a wide chest-shaped lock are termed box |bɒks| folds.
26. Diapir /ˈdīəˌpir/ fold –
Salt dome -
|sɔːlt| |dəʊm|- duz gümbəzi
Diapir folds and salt domes are different types of
anticline folds. Their formation is associated with
the presence in the core of these folds of plastic
rocks (clay, salt, gypsum), which, under the action
of enormous pressure overlying rocks, are
compressed and penetrated into these rocks,
forming gentle arch and steep side surfaces.
27. • Flexure is a tectonic structure, which is a knee-like bend of a layered strata,
knee-shaped curved monocline.
28. Indication of flexure Syndepositional Post-sedimentation
Thickness Maximum in the lowered limb, minimum – in closing limb Is not different
Facies composition Fine-grained clastic, clayey and carbonate rocks in lowered limb.
Coarse-grained clastic and reef facies in the closing limb.
Coarse-grained clastic facies in an uplifted limb.
Is not different
Perfection of section The most complete in the lowered limb, with breaks in the uplifted
limb
Similar
pressure distribution Concentrated exclusively in the closing limb Has a different nature in the top and
bottom of the deformable layers
30. • Morphological classification of flexure
• Concordant flexure, limbs dip unidirectional
Discordance flexure, limbs dip in all directions
Structural terrace, closing limb is horizontal
Horizontal, upper (uplifting) and lower (lowered) limbs are horizontal
Vertical, closing limb is vertical
31. Monocline
• Monoclines are folds consisting of two horizontal (or nearly so) limbs
connected by a shorter inclined limb. They can be compared to anticlines,
which consist of two inclined limbs dipping away from each other, and
synclines, which consist of two inclined limbs dipping towards each other.
