UNIVERSITY OF SOUTH AUSTRALIA

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UNIVERSITY OF SOUTH AUSTRALIA

• INTRODUCTORY AND ENVIRONMENTAL GEOLOGY
• Structural Geology
• FOLDS and FAULTS

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Reference

• Press, F. and Siever, R., 2001 Understanding
  Earth, Freeman Co., N.Y., Chapter 10
• Clark, I.F., and Cook, B.J., 1983 Perspectives
  of the Earth. pages 377 - 399
• Hamblin, W.K., 1991 Earths Dynamic Systems.
  Macmillan, N.Y. (6th Edition) pages 147 - 164

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Structural Geology

• Structural geology is concerned with the
  determination of the 3D geometry of rock bodies.
  
• The shape of rock bodies can be controlled by the
  processes by which the body was formed or the
  shape can be modified by later processes, such as
  deformation.

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Original shapes of rock bodies

• Sedimentary rock bodies are generally formed in
  near horizontal sheets.
• Volcanic rock bodies are generally formed in
  sheets, intercalated (alternating layers) with
  sedimentary rocks.
• Intrusive rock bodies show a variety of forms
  (see igneous lectures).

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Horizontal Beds
Sedimentary rocks are deposited in horizontal
layers.
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Uniformly Dipping Beds
Horizontal beds can be tilted to form uniformly
dipping sequences. Beds face (get younger) in
the direction of dip.
Press Siever (2001)
Press Siever (2001)
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Sedimentary layers
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Deformational structures

• There are two main types of deformational
  structures
• ductile
• brittle.
• Ductile structures are formed by plastic
  deformation during which the rock remains
  cohesive
• Brittle structures are formed by the breaking of
  rocks

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Ductile features

• Ductile deformational features include
• folds and
• foliations.
• Folds are ductile structures formed by the
  bending of rock layers due to compression
• Foliation is a planar structure, commonly
  developed by compression in association with
  folding.

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Folds
Folds are curved bedding surfaces
Folded sequences are commonly composed of
alternate arches (ANTICLINES) and troughs
(SYNCLINES)
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Fold Nomenclature
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Fold Nomenclature

• Horizontal fold with vertical axial plane

Plunging fold with vertical axial plane
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Folds at Hallett Cove
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Doubly plunging folds
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Wilpena Pound

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Foliation
Foliation is a planar structure. Types of
foliation include Slaty cleavage Schistosity Fo
liation is commonly developed parallel to the
axial planes of folds
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Fold Belts

• Distribution of Folds
• Folded rocks occur in major crustal regions
  called FOLD BELTS or OROGENIC BELTS
• Fold Belts are elongate features in the crust

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Fold Belts
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Adelaide Fold Belt
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Formation of Folds and Foliation

• Folds and foliation are formed by compression of
  rock

Crustal compression takes place at or near plate
boundaries
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Brittle features

• Brittle deformational features include
• faults
• joints and
• veins
• Faults are brittle structures formed by the
  fracturing and displacement of rock
• Joints are fractures with no displacement
• Veins are fractures that have been filled with
  minerals.

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FAULTS
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FAULTS
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JOINTS
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VEINS

• Veins are fractures that have been infilled by
  minerals.
• Minerals commonly formed in veins are
• quartz
• calcite