Fold Types

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Symmetrical Folds

formed by equal pressure from both sides

the angle of dip on each of the limbs is the same

the axial plane will be vertical

they can be either an antiform or synform fold

an anticline has the dip arrows pointing out from the axial plane trace and will have the oldest rocks in the centre

a syncline has the dip arrows pointing into the axial plane trace and will have the youngest rocks in the centre of the fold

Asymmetrical Folds

formed by more pressure on one limb than the other

they can be either anticline or syncline folds

one limb with a higher angle of dip than the other

the limb will a low angle of dip will have a wider outcrop than the limb with the higher angle of dip

the axial plane will be inclined

Overfolds

formed by a lot more pressure on one side than the other

both limbs dip in the same direction but at different angles

overfolds can be either anticlinal or synclinal

the axial plane is inclined from the vertical

One limb of the fold has been inverted to that the older rocks overlie the younger.

Recumbent Folds

are formed from very high pressure on one side

the axial plane must be sub-horizontal

both the limbs and the axial plane will be at very low angles

one limb of the recumbent fold is inverted

rocks are usually incompetent or plastic in order to absorb such high levels of stress.

Isocline

folds are formed by large amounts of pressure equally from both sides

they can be recognised by parallel limbs at angles of over 70 forming closed and tight folds

the axial plane is usually vertical

they can be recognised on maps by outcrops of repeated and parallel beds

Nappes

form some of the world's largest tectonic features

these are huge recumbent fold that have broken along thrust planes

movement is horizontal along thrust planes which are lines along which a recumbent fold breaks

many nappes travel tens of kilometres over the underlying rocks and form some of the largest fold structures in the world

Domes

an anticlinal fold

beds dip outwards in all directions

oldest rocks in the centre

roughly circular outcrop patterns

Basins

synclinal fold

beds dip inwards in all directions

youngest rocks found in the centre

roughly circular outcrop patterns

A fold is a flexure in rocks, where there is a change in angle of dip in a bed.

an upfold is anticlinal or an antiform with rock dips pointing outwards. It is an anticline is the ages of the rocks are known and an antiform is the shape is only

a downfold is synclinal or a synform with rock dips pointing inwards.

All folds are produced by compressive forces acting horizontally, so are usually formed at destructive plate margins.

Fold Parts:

fold limbs: the section of fold between one hinge and the next

hinge: the line along which there is a change is amount or direction of dip

axial plane: a plane that joins the hinges of all the beds. It bisects the fold.

axial plane trace: the intersection of the axial plane with the surface

plunge: angle of dip on the axial plane

Fold Attitude:

open: the interlimb angle is between 120 and 70

closed: the interlimb angle is between 70 and 30

tight: the interlimb angle is less than 30

isoclinal: the two limbs are parallel

symmetrical: the axial plane is vertical, the limbs have the same dips

asymmetrical: the axial plane dips, the limbs have different dips

overturned: both limbs dip in the same direction at different amounts and one limb is inverted

recumbent: the axial plane is horizontal or sub-horizontal

nappes: form when a recumbent fold has been broken and thrust

Fold Types

Folds