Rock Deformation Flashcards
Where do rocks get deformed?
Where tectonic forces exert pressure (stress) on rocks -> Mostly at or near plate boundaries
what is stress?
force applied per unit area
what does stress cause?
strain
what does uniform stress (=confining pressure) do to rocks
squeezes rock (no shape change)
what does differential stress (=directed pressure; unequal application of stress) do to rocks?
distorts rock
what is yield stress?
amount of stress needed to deform the rock (to make it “yield”)
what does yield stress depend on?
rock strength
what is strain?
change in shape of body by deformation
what are the two types of strain?
(1) Brittle deformation - breaks/fractures (faults, joints)
(2) Ductile deformation - bends/flows (folds)
Type depends on composition, P, T, strain rate.
when are rocks stronger?
at higher pressure (greater depths)
when are rocks weaker?
at higher temperatures
deformation at shallower depths…
brittle
deformation at greater depth…
more ductile
what are the three types of differential stress?
tensional stress -> EXTENSION
compressional stress -> COMPRESSION
shear stress -> Lateral shift
what are some example of geological structure formed by deformation?
Brittle deformation - Joints - Faults Ductile deformation - Folds
what are joints?
fractures with no movement
what do joints result from?
may result from cooling -> contraction (e.g. basalt columns)
OR
pressure unloading -> sheeting
what are faults?
fractures along which displacement has occurred (one side moves relative to the other)
what does a sudden, rapid slip on fault plane lead to?
earthquakes
slip of cm to 10s of m in one event
what are the types of faults?
Strike-slip faults
-Movement is horizontal (to left or right)
-Parallel to the strike of the fault
Dip-slip faults
-Movement up or down
-Parallel to the dip of the fault surface
-e.g. Normal faults and Reverse (thrust) faults
San Andreas is a transform plate boundary, strike-slip fault. is it left or right lateral?
right lateral
how is the type of dip-slip fault defined?
the relative motion (slip) of:
hanging wall vs. footwall
(i.e. does the hanging wall move up or down)
what happens at a normal fault?
Hanging wall (upper block) moves DOWN the dip of the fault surface
- Caused by extensional stress
- Leads to crustal thinning
what happens at a reverse (thrust) fault?
Hanging wall (upper block) moves UP the dip of the fault surface
- Caused by compressional stress
- Leads to crustal thickening, shortening
what is a thrust fault?
reverse fault with dip angle ≤ 45°
what are folds?
Warping of rock layers at all scales (micro to mega)
- Shortening – contraction
- Form over long time periods as rocks yield to compressional stress – ductile behaviour
- Mountain belts – regional folding of rocks, and many folds can be refolded, overturned etc
what is an example of ductile extensional deformation?
boudins
