Fold Types

siobhan.quirk
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Undergraduate Geology - Part 1 (Geological Structures) Note on Fold Types, created by siobhan.quirk on 05/12/2013.

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Page 1

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