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Folds

Folds. Chapter 7. Strike. Strike. Dip direction. Dip direction. Antiform. Antiform. Synform. +. +. Mechanical layering . Compression. Ductile deformation. =. FOLDS. Younger layer. Older layer. Strike. Strike. Dip direction. Dip direction. Antiform. Anticline. Antiform.

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Folds

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  1. Folds Chapter 7

  2. Strike Strike Dip direction Dip direction Antiform Antiform Synform + + Mechanical layering Compression Ductile deformation = FOLDS

  3. Younger layer Older layer Strike Strike Dip direction Dip direction Antiform Anticline Antiform Anticline Syncline Synform Antiformal synclines and synformal anticlines can form due to refolding

  4. Hinge line: Join points of maximum curvature along the same layer (pages 383-384) http://earth.leeds.ac.uk/folds/describing/3d/index.htm

  5. To construct the AXIAL SURFACE of a fold (page 384) • Find the hinge lines of different layers http://earth.leeds.ac.uk/folds/describing/3d/axsurf1.htm • Construct the IMAGINARY surface containing all the hinge lines for that fold http://earth.leeds.ac.uk/folds/describing/3d/axsurf2.htm

  6. The hinge lines could be: • Horizontal • Plunging • Vertical The section perpendicular to the hinge line is called the PROFILE section or profile plane of the fold http://earth.leeds.ac.uk/folds/describing/3d/profile.htm

  7. The trace of the axial surface on the profile section is called AXIAL TRACE (page 384) Axial trace The hinge lines can be straight or curved. Straight segments of hinge lines are considered for descriptive purposes

  8. Fleuty diagram (page 387, fig. 7.25) Classifies folds on the basis of: • The DIP of the axial plane, and, • The PLUNGE of the hinge line Dip of axial plane CANNOT be lower than the plunge of the hinge line (why?)

  9. RECLINED (plunge of hinge line ≈ dip of axial plane) Dip of axial plane Upright Steeply inclined Recumbent Moderately inclined Gently inclined 90 80 60 30 10 0 0 Sub-horizontal 10 Gently plunging 30 Plunge of hinge line Moderately plunging 60 Steeply plunging 80 Sub-vertical 90

  10. Fold axis vs. hinge line (page 387) • Fold axis is a mathematical concept (IMAGINARY) • ALWAYS a straight line • Generates CYLINDRICAL (or near-cylindrical) folds (page 388) • Useful for detailed structural analyses, identifying multiple generations of folds etc.

  11. Fold axis vs. hinge line contd. • Hinge line is a measurable line (REAL) that joins points of maximum curvature on a folded surface • May or may not be a straight line • All types of folds have hinge lines • When the hinge line (or a segment of hinge line) is straight, it is considered parallel to the fold axis

  12. Cylindrical folds (page 388) Mathematical definition: • Folded surface generated by a straight line (fold axis) moving parallel to itself in space Real-life characteristic: • Folds with straight hinge lines Importance: • Cylindrical folds indicate a SINGLE folding event

  13. Cylindrical folds on stereograms (page 389) • On a stereogram, the great circles representing the limbs (or the planar segments of the folded surface) intersect along the FOLD AXIS (β- diagram) • The POLES of the limbs (or the planar segments of the folded surface) lie on one great circle (π-diagram) • The pole to this great circle (called π-circle) represents the fold axis • The π-circle represents the PROFILE SECTION of the fold

  14. Ode to the profile section • The profile section is the plane perpendicular to the hinge line (or fold axis) • If the limbs of a fold are planar, the angle between them (INTERLIMB ANGLE, page 390) is measured on the profile section • The fold axial plane is usually the plane bisecting the interlimb angle (the best way to find its orientation)

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