Why study geologic structures?

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Why study geologic structures?

Primary Geologic Structures

  • Geometric features developed during formation of a rock body

  • Microscopic to mountain scale

  • Formed in sedimentary or igneous environments

  • Provide clues about environment of rock formation

Sedimentary Structures

  • Bedding

  • Uncomformities

  • Cross bedding

  • Graded bedding

  • Ripple marks

  • Mud cracks

  • Trace fossils

  • Etc.

Igneous Structures

  • Flows

  • Veins

  • Dikes and Sills

  • Chilled or baked margins

  • Magmatic flow foliation

  • Flow tops marked by vesicles

  • Aa-aa vs. pahoehoe structure

  • Stocks and batholiths

  • Xenoliths or inclusions

  • Magmatic segregations

Secondary Geologic Structures develop after formation of the rock body

  • Folds

  • Fractures and joints

  • Faults and breccias

  • Foliation and lineation

  • Metamorphic fabrics like contact metamorphic aureoles and mylonitic S-C structures

σ1 horizontal, σ3 vertical — reverse faults

  • σ1 horizontal, σ3 vertical — reverse faults

  • σ1 vertical, σ3 horizontal — normal faults

  • σ1 horizontal, σ3 horizontal — strike-slip faults

Reverse faults: should form at ~30° dip

  • Reverse faults: should form at ~30° dip

  • Normal faults: should form at ~60° dip

  • Strike-slip faults: should form at ~90° dip

Thrust faultsmechanically unfavorable

  • Thrust faults — mechanically unfavorable

  • Low-angle normal faults — mechanically very unfavorable


  • Planar fabric formed by ductile flow of minerals under directed stress

  • Generally manifested as compositional layering

  • Foliation plane generally oriented at high angle to maximum stress direction

Just how deformed is this rock?

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