Transcript SGES 1302 Lecture8

SGES 1302
INTRODUCTION
TO EARTH SYSTEM
LECTURE 8: Geological Structures: Fold
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Folds
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Structures produced when a originally planar surface becomes bent
or curved due to deformation by compressional forces
Results in shortening and thickening of the crust
Results of ductile deformation because it develops without fracturing
and the deformation is distributed over the entire structure.
Folds in rocks vary in size from microscopic crinkles to mountainsized folds.
They occur singly as isolated folds and in extensive fold systems of
different sizes, on a variety of scales.
Folds commonly related to convergent plate margins
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Fold Geometry and Nomenclature
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Hinge: zone of maximum curvature and
separates the 2 limbs
Limbs: less curved areas between the
hinges
Hinge line: line connecting the hinges
(line of maximum curvature)
Inflection point: point on a limb where
there is a change in sense of curvature
Fold axis: imaginary parallel generator
lines that outline the folded surface
(parallel to hinge line)
Fold axial surface: a surface that passes
through the hinge lines of each
successive fold surfacesFold profile
plane: reference plane perpendicular to
the hinge line, used to describe fold
shape
Interlimb angle: angle between the limbs
in the profile plane
Plunge: angle between the fold axis and
the horizontal
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Classification of Folds
Based on
 1) direction of closing,
 2) orientation of hinge line & axial surface,
 3) interlimb angle, and
 4) dip-isogon & nature of profile
Based on closing direction:
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Anticline – upwards younging antiform (a)
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Syncline – upwards younging synform (b)
Fold symmetry:
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Symmetric fold – both limbs of equal length
 Asymmetric fold – limbs have unequal length
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monocline – one limb is relatively short
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Classification of Folds
Based on axial surface & hinge line
 Plunge of Hinge Line
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0° - 10° Horizontal
 10° - 80° Plunging
 80° - 90° Vertical
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Dip of Axial Surface
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0° - 10° Recumbent
 10° - 70° Inclined
 70° - 90° Upright
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Eg. Shallow south-plunging
inclined anticline
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Classification of Folds
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Based on inter-limb angle
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gentle – 120°-180°
open - 70°-120°
close - 30°-70°
tight - 0°-30°
isoclinal - 0°
Gives a quantitative
estimate of the intensity
of folding
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