Rheology

Structures of the deeper crust

An intro-3/19.02

Outline of today’s

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

Salt diapirs - known for a long timeIntrude sedimentary sectionsImportant oil traps

DIAPIRS

Gulf of Mexicosalt diapirs

Some of the best of all salt diapirs_ Zagros, Iran

Salt Valley Utah

Cross-section through diapirs

QuickTime™ and aCinepak decompressor

are needed to see this picture.

QuickTime™ and aCinepak decompressor

are needed to see this picture.

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?

Limitation of experiments: size of the system matters, maybe, but most importantly TIME

Strain rate is the rate over which % deformation is applied

The brittle and the ductile crust

The implications

• Earthquakes no deeper than transition

• Lower crust can flow!!!

• Lower crust decoupled from upper crust

• No strength to the deeper crust.

A more complete picture

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?

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.

T

P

prograde

retrograde

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

Static recrystallization

Dynamic recrystallization

Rotation of grains

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