Rheology

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Rheology

• Structures of the deeper crust
• An intro-3/19.02

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Outline of todays

• Weak rocks and flow
• Diapirism of salt
• Stress-strain experiments at higher P, T
• The brittle-ductile transition
• Deformation and re-crystallization

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Salt diapirs - known for a long time Intrude
sedimentary sections Important oil traps
DIAPIRS
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Gulf of Mexico salt diapirs
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Some of the best of all salt diapirs_ Zagros, Iran
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Salt Valley Utah
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Cross-section through diapirs
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What do we learn from salt?

• Rocks can behave in a non-brittle fashion
• Flow of rocks!! (especially over geologic time
  scales)
• Need to investigate the rock mechanics at higher
  T and P could granites flow at 20 km?
• Remember that crustal earthquakes are confined
  to within the upper 10-15-20 km any mechanical
  significance?

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Limitation of experiments size of the system
matters, maybe, but most importantly TIME Strain
rate is the rate over which deformation is
applied
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The brittle and the ductile crust
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The implications

• Earthquakes no deeper than transition
• Lower crust can flow!!!
• Lower crust decoupled from upper crust
• No strength to the deeper crust.

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A more complete picture
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And so how does a ductile rock look like? Can
we see them at the surface of the Earth? Can we
simulate their textures? Can we study their
strain /stress regimes?
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Such rocks are commonly exposed throughout the
continents. Not as common as the shallow rocks,
of course. Rocks that were at as deep as 150 Km
are seen at the surface of the Earth - sometimes
they contain diamonds!!!!!! SOOO we can attempt
to study deformation in the deeper Earth.
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prograde
retrograde
P
T
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Relationships between deformation and metamorphism

• Connection between structural processes and
  metamorphism
• Tectonites are subject to grain-size reduction
  but because this process take place at high
  pressures-temperatures, tectonites are also
  subject to grain growth via recrystallization.

time
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Static recrystallization
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Dynamic recrystallization
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Rotation of grains
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Summary

• Continuous deformation in the deeper crust is
  mostly ductile
• Effectively rocks can flow over geologic time
  scales
• Forget faults, large-scale folds instead
  penetrative deformation at smaller scale
• Deformation is intimately associated with
  metamorphism