Deformation (Structural Geology) Flashcards
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
What is the strike line?
line representing the intersection of a geological feature (a plane) with the horizontal
WHat is the dip?
inclination of a plane (of a geological feature) to the horizontal
What is the relationship between the strike and dip?
dip is always at 90 degrees to the strike
What types of brittle deformation exist?
joints and faults
What are joints?
fractures with no displacement
What are faults?
fractures along which movement has occurred
What tyeps of faults are there?
dip-slip (normal, reverse, thrust), strike-slip, oblique-slip
What are strike-slip faults?
shear horizontal movement - one block slides horizontallly past another (parallel to strike line), so there is no relative vertical motion
What are dip-slip faults?
sldiding occurs up or down the slope (dip) of the fault
What are oblique-slip faults?
anything not strike-slip or dip-slip - sliding occurs diagonally along fault plane
What are normal dip-slip faults?
hanging rock slides down relative to other rock due to pulling tensile forces (must dig down to seam of coal)
What are reverse dip-slip faults?
hanging rock slides up relative to other rock due to compressive forces (must dig up to seam of coal)
What are thrust faults?
reverse fault with low angle
How does the abundancy of joints relate to that of faults?
more abundant to faults
When do joints form?
as soon as a rock can deform brittly
In what type of rock are joints most regular?
homogeneous rock
What is a normal result of uplift - rasing a rock to the surface?
decrease in pressure causes expansion and brittle deformation, creating joints
What can joints vary in?
orientation, spacing, length
What is a slickenside and how is it formed?
polished surface of fault (lineated ones have groooves indicating direction of fault movement);
caused by slip on fault
What is a fault gauge?
ground up rock of a fault zone; can be impermeable
What is fault scarp?
small step on the ground surface where one side of a fault has moved vertically wrt the other
What are vein?
mineral-filled fractures
HOw are veins formed?
water gets into fracture (joint, etc.) and precipitates minerals from walls
What two types of folds can be made?
anticlines and synclines
Whate are folds cuased by?
compression (stress)
What are anticlines?
up-fold - has arch-like shape; limbs dip away from the hinge
What are synclines?
down-fold - trough-like shape; limbs dip toward the hinge
What is the usual symmetry of folds?
symmetrical
What does an asymmetrical fold look like?
one side is shorter than the other - wave is dipping to one side
What does an overturened fold look like?
peak of fault bent so much it overshoots one of the “limbs”
What do recumbant folds look like?
sideways
What do isoclinal folds look like?
perfect “u” - limbs don’t dip toward or away from hinge - perfectly straight
Why is erosion required to bring folds to the surface?
folds form at depth
What is a plunging fold?
dips below axis
On what scales can deformations be found?
microscopic, mesoscopic (cm - 100 m), regional (km), tectonic (1000s km)
What is the scale of amountain belt?
1000s of kms
