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 time Intrude sedimentary sections Important oil traps DIAPIRS

Gulf of Mexico salt diapirs

Some of the best of all salt diapirs_ Zagros, Iran

Salt Valley Utah

Cross-section through diapirs

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 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