1. Lecture #14

2. DefinitionFold is the shape or volume changes from one material(bedding), which show as an arc or group of arc on the lineor plane structure on that material.Fold is an important phenomena, that indicatedeformation characteristic especially the geometrydescription with distortion and rotation.An inhomogeneous deformation that occurred onmaterial/rock that has line and plane structure.Need to consider that an element which is before hascurve form can change become plane or straight line oran element can remain as plane structure or straight lineafter deformation. 3. Fold Geometry DescriptionBy geometry fold can be descript as single curve planeFold shape has varieties, from simple until complexFor simplification purpose, fold can be assume as ancylinders surface with fold axis as the surface frameworkand the element can be shown at the fold profile. 4. Elements of fold:i. Hinge point; the maximum point on the curve of the foldedbedii. Crest; the highest point of the curveiii. Trough; the lowest point of the curveiv. Inflection point; the boundary point between two differenttype of curvev. Fold axis (hinge line); the maximum curve line on the surfaceof folded planevi. Axial plane; plane that form through axis line on fold. Thisplane not always a planar plane, but can be curved also knownas axial surface.vii. Fold limb; generally is the side of the folded plane, which islocated between the hinge zone and inflection line. 5. Basic Fold ClassificationGenerally fold being classified based on the characteristicwhich can descript the element geometricallyClassification and fold naming, normally not directlyrepresent the occurrence characteristic.By indirect fold forming can represent the occurrencecharacteristic or fold forming and type or materialEx: tight fold represent major deformation, parallel foldrepresent on the component bedding.Based on interlimb angle; symmetry characteristic; foldposition; isogon. 6. Interlimb AngleIs the smallest angle that form between fold limband being measured on the profile of fold.This angle represent the tightness of foldClassification based on Fleuty (1964);Interlimb Angle Fold Description180 120 Gentle120 70 Open70 30 Close30 0 Tight0 Isoclinals 7. Symmetry CharacteristicSymmetry is one of the criteria that represent form of a cylindricalsurface.Symmetry characteristic being determined by the plane that through thehinge line and divided the interlimb angle equally, which known assymmetry characteristic, and this fold known as symmetry fold, and hasorthorhombic symmetric characteristic.Series of fold known as symmetric if each of the fold has symmetrycharacteristic and pattern periodically .In symmetrical fold, amplitude and wave length, the comparison of thiselement will be easy to descript.If the plane trace through the hinge line and not as symmetry plane, thefold known as asymmetry fold, which has monocline symmetrycharacteristic.Therefore it is necessary to add asymmetry characteristic such as thenature of slope plane direction (vergence) or relative direction of anti-formhill with sin-form hill such as wind direction, left right or clockwiserotation for fold with plunge axis. 8. Fold PositionFolding that the limb dip goes to different direction known as anticlineFolding that the limb dip goes to the same direction known as synclineFold position can get from the fold axis and axial plane, surface.Fleuty (1964) based on the dip tendency of axial plane and trend of foldaxis :Angle Term Axial Plane Dip Axis Line Trend 9. Rickard (1971) classification based on the combinationof trend and pitch magnitude, and the dipping of theaxial plane.To give more consistent position on the inclined foldExample:1. Upright fold: dip 85, trend 25: upright, gently,plungging fold.2. Inclined fold: dip 70, trend 45; steeply, inclines,moderately plungging fold.3. Reclined fold: dip 56, trend 55; moderately inclinedfold 10. IsogonLines that connected points from two surface foldwhich the dip or apparent dip are same.Isogon can be used as a basic fold classification basedon the parallel nature of the isogon and the directionof the convergent and divergent (the inside part of theisogon used to determined the direction of isogonconvergency- Ramsay 1967).Fold with heavy isogon convergence: Parallel foldFold with weak isogon convergence: Similarity foldor Divergent fold 11. Construction of parallel fold profileMethod that can be used to construct the parallel fold:Arc methodHiggins method (1962)Bust method (1929) 12. Arc-method 13. Higgins, 1962 14. Busk, 1929 15. Construction of the unparallel foldprofile use Boundary RayBedgley, 1965