23.vck_geometric classification of folds

8/10/2019 23.VCK_Geometric Classification of Folds

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Fold-Folds may be defined as wavy structure formed when originally flat and

planar surfaces such as bedding, foliation or other originally planar surface in rocksare bent or curved as a result of deformation.

Folds range in size from mm to km.

Are a manifestation of ductile deformation.i.e., form at depth where T, P are high and fracturing does not occur

Fault related folds as well,

Differential compaction, intrusion etc

Can form in varieties of deformational environments ofEarthss crust from near surface brittle condition tolower crust ductile conditions


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Limb

Limb

Hinge points

Anatomy of Fold

Axial trace

Inflectionpoint

Fold Axis

Interlimb

angle

Amplitude


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Based on the orientation of various anatomical features of

the fold, geometrically it can be classified into the following

1)Based on the sense of curvature

2) Based on the plunge of Fold Axis

3) Based on the orientation of axial plane

4) Based on direction of younging relative to sense of fold

closure:

5) Based on symmetry of folds

6) Based on the nature of Axial plane and Hinge line.

7) Based on interlimb angle

8) Based on the shape of the hinge

9) Based on number of hinge

10) Based on the geometrical relations among neighbouring

structures:

11) Morphological Classification


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3) Based on the orientation of axial plane

Upright fold

Recumbent fold

Inclined fold

Reclined fold

Overturned fold

Fleuty (1964) further made the following

classification based on the amount of dip of

axial plane-

Upright fold: dip 800-900

Steeply inclined fold: dip 600-800

Moderately inclined fold: dip 300-600

Gently inclined fold: dip 100-300

Recumbent fold: dip 00-100


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Based on the dip of the Axial Plane vs Plunge of the Fold Axis (Fleuty 1964)


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4) Based on direction of younging relative to sense of fold closure:

Syncline

Anticline

AntiformalSyncline

SynformalAnticline

Anticlinoriuum andSynclinorium


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5) Based on symmetry of folds

Symmetric fold axial plane is the plane ofsymmetry .Symmetric folds are sometime describe as

M-Type folds (Ramsay 1967)

Asymmetric fold axial plane is not theplane of symmetry

Example- S and Z-Type folds (Ramsay 1967)


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6) Based on the nature of Axial plane and Hinge line.

Plane cylindrical- axial planeand hinge line both straight

(Type 0)

Plane non cylindrical- axial plane

straight but hinge line curved (Type I)

Non plane non cylindrical- axial

plane and hinge line both are

curved (Type II)

Non plane cylindrical- axial plane

curved but hinge line straight (Type III)


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7) Based on interlimb angle

Gentle fold- between 180 and 120

Open fold- between 120 and 70

Close fold- between 70 and 30

Tight fold- between 30 and > 0

Isoclinal fold-00

Elastica- Negative interlimb angle

Interlimb angles are measured on profile plane of folds


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Round-hinged fold

broad hinge zonecompared to limb

Chevron fold straight limbs and sharp

hinge

Arrow-head fold sharp hinge anddistinctly curved limbs

Cuspate fold A train of folds with sharphinges on one set of closure and withrounded hinges on the oppositedirectedclosure

8) Based on the shape of the hinge


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9) Based on number of hinge

Single-hinged foldwith singlehinge between two pointsof inflection

Conjugate fold A double hingedfold with sharp hinges

Box fold A double hinged foldwith more or less rounded hinges,

flat top and steeply dipping limbs


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10) Based on the geometrical relations among neighbouring structures:

Periodic folds -A train of folds with more or less

the same geometry between alternate points ofinflection.

Non-periodic folds - Not periodic in nature.

Polyclinal folds A group of folds with non-parallel axial planes but with sub-parallelhinge lines

Disharmonic folds A group of folds in

which the folds of one layer differ in size orstyle from folds of an overlying or underlyinglayer.

Decollement A train of folds in a layerwhich becomes detached from the adjacent

layer.


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Morphological Classification of Ramsay-1967:

Dip Isogon:line of equal dip

Based on

Thickness

a. Orthogonal thickness (t)-measuredbetween tangents on limb

b. Thickness parallel to the axial trace ofthe fold (T)

t= T

cos


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Ramsay classification scheme for folds

Class

Comparison of

Curvature of

inner arc &

outer arc

Dip isogon geometry

(towards inner arc)

1 Cinner> Couter Dip isogons converge

1A Strongly Convergent

1BConvergent

(Parallel folds)

1C Slightly Convergent

2 Cinner= CouterDip isogons are parallel:

(similar folds)

3 Cinner< Couter Dip isogons diverge


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Thank You

23.vck_geometric classification of folds

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