Structural Geology: Intro

Structural Geology: Intro

1. What is it?

   • Description (of geometry)
   • Distribution (of structures)
     • Observed in: rocks, field, lab and geophysical data
   • Interpretation (of formation)

2. Deals with: Secondary structures

   • Primary structures = deposition
     • Need to know what it looked like before deformation
     • Modified by:
       • Strain: Change shape
       • Rotation: Tilting of Primary structures
       • Translation: Movement with no shape change (faulting?)
   • Secondary structures = (may) modification of primary structures

3. Deformation structures

   • Brittle:
     • Distinguished by DISCONTINUITIES
       • EXAMPLE: Features or Faults
   • Ductile:
     • CONTINUOUS
       • EXAMPLE: Folds, Boudins, Shear Zones, and penetrative fabrics such as Foliations and Lineations.
         • FAULTS: Recognised in terrain (DEMs)
         • FRACTURES: Cracks with a competent/tensile opening
         • FOLDS: Shortening of pre-existing layering
         • BOUDINS: Form due to layer-parallel extension. Seen by thickness variation within competent layers that with sufficient strain will separate into boudins.
         • FOLIATIONS: Layering defined by either the preferential orientation of platy minerals or mineral segregations that develop during metamorphism and deformation.
         • STRETCHING LINEATIONS: Alignment of minerals during deformation, typically at elevated temperature. Long axis of prismatic materials.
         • SHEAR ZONES: Ductile equivalent to faults. Tabular zones, the rocks on either side are displaced relative to each other.
   • Non-tectonic Deformation
     • Not directly related to plate tectonics
     • BUT still falls in to "faults, folds and fabrics" categories
     • EXAMPLES:
       • MIGMATITE: Form when a rock partially melts, and you end up with layers of solidified melt (leucosome) and residual host rock (melanosome).
         • Are on the boundary between metamorphic and igneous rocks.

4. Strike and Dip of Geology

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