Deformation (Structural Geology)
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What are the two general types of deformation processes, with examples?
brittle (faults, joints…); ductile (folds)
What is stress?
force applied
What is strain?
deformation - size and shape change as a result of applied stress/force
What are the controlling factors of deformation, and how do they relate to those of metamorphism?
tempreature, confining pressure, directed pressure, time, fluids, original character of the rock; exact same!
What is often the only evidence of stress?
strain
How does temperautre affect deformation?
rock is more ductile as temperature increases; recrystallization (whose rates change with temp) changes rock behaviour
How does recrystallization change rock behaviour?
makes rock behave more ductile and in a more homogeneous fashion
How does tempearature change with depth, and how does this relate to deformation processes?
increases with increasing depth; brittle deformation more likely to happen near surface of crust
How does confining pressure affect deformation?
rock is more ductile as pressure increases
How does confining pressure change with depth?
increases with depth
How does directed/tectonic pressure affect deformation?
determines the orientation of deformation
What are the 3 types of tectonic pressure?
compressive (squeezing); tension (pulling apart); shear (tearing)
What is used to deduce stress?
strain - its geometry
What limits the possible orientations of deformation?
free surface of lithosphere/atmosphere
What is the major controlling stress in the crust, and what is its orientation?
gravity; vertical
What orientations can plate tectonic forces be in?
vertical or horizontal
How does time affect deformation and why?
fast deformation more likely to be brittle; slow deformation more likely to be ductile; since silicate minerals take time to change
How do solutions affect deformation?
affect friction
What does pressure solution do to deformation? (?)
slow ductile failure (?)
How does rock type affect deformation?
minerals have different physical properties - strength, inhomogenetities…
What is needed for the brittle/ductile transition in the crust?
10-15 km depth, 350-450 deg C
What happens to a rock when it the stress on it is below the elastic limit?
rebounds from deformation
What happens to a rock when the stress is in its ductile behaviour range?
when stress is released, rock stays bent; bending takes very little stress to increase
What happens when a rock reaches its rupture strength?
breaks