Lecture 2- Joints, Cleavage, Veins, Faults, Earthquakes Flashcards
What are joints?
fractures with minimum opening (mm to cm) with no displacement
What do joints form in response to?
extensions and perpendicular to stretching direction
How do joints form from lavas?
By contraction vertically due to thermal gradient cool at tip and base
What are plumouse structures?
feather like on top of joints and develop when the crack is propagating laterally
What can plumouse structures allow you to distinguish?
Joints from bedding planes
Why are joints imporant?
Paleostress analysis
Oil exploration
Volcanic hazards
Seismic hazard
Landslide Hazard
How are joints arranged in simple shear?
‘En echelon’ in shear zone (will step down) and dip in shear direction
What are joints like for pure shear?
Parallel and overlap
What are veins?
joints filled with minerals that precipitate from fluids circulating through fractures
How do veins form from pure shear?
straight veins (lens shape)
How will simple shear veins form?
Sigmoidal veins (S shape) as rotation occurs the core and growth rotates
What can veins form?
large ore deposits (gold, copper, lead, zinc, silver)
What is a fault?
planar surface or zone which one side is displaced relative to the other
What is the range for displacement?
mm to Km
What are the 2 main forms of deformation?
Brittle
Ductile
What is brittle deformation?
rock break into rigid blocks each portion separated by discontinuities
What is ductile deformation?
apply stress, no discontinuity but will deform as single piece
What are the conditions of brittle deformation?
<350*c
or
High strain rates
What are the conditions of ductile deformation?
> 350*c
or
Low strain rates
What deformation leads to faults?
brittle deformation
What are the typical conditions for fault formation if by brittle deformation?
Shallow depth (less than approx ~15km)
where temp is below 350*c
How do faults form?
when the internal friction of rocks is overcome due to stress
What happens when stress approaches internal friction?
micro-cracks form a fault form when these microcracks merge together
What plays a key role in fault formation?
pre-existing weaknesses
What are the 2 blocks in a fault called?
Hanging wall (above the fault plane)
Footwall (below the fault plane)
What is a normal fault?
the hanging wall moves down relative to the footwall
What is a reverse (thrust) fault?
the hanging wall moves up relative to the footwall
What are strike slip faults?
the two blocks are displaced laterally (i.e.
in the horizontal plane). There is no hanging wall or footwall
What is Dextral and Sinistral with strike-slip faults?
Dextral- right lateral
Sinistral- Left lateral
What are thrust faults?
low angle <30* reverse faults
What are detachment faults?
low angle ~30* normal faults (normal fault typically 60*)
What does thrust play a key role in?
Orogenesis (mountain building)
What does detachment fault allow?
deep rocks to be exhumed at the surface
What is heave?
horizontal displacement between 2 points which were adjacent
What is throw?
vertical displacements between 2 points that were once adjacent
What is total slip?
displacement along the fault plane between two points that were once adjacent
How do faults terminte?
Faults may terminate laterally by decreasing progressively their displacement.
What is displacement at tip points?
zero
What is a transfer fault?
when faults terminate against another fault
What are relay ramps?
when a fault terminates by overlapping eachother
How do normal faults affect stratigraphy?
OMIT and thin the crust (erase)
How do reverse faults affect stratigraphy?
Repeat stratigraphy so thicken crust
What are the 2 divisions of fault rock?
Incoesive
Coesive
Where do incoesive rocks faults form?
shallow part of crust
Where do coesive fault rocks form?
deep part of crust
What are the 2 types of incoesive rock?
Fault breccia (>30% visible grains)
Fault gouge (<30% visible grains)
What is the reason for the difference between fault breccia and fault gouge?
Fault gouge has experienced more deformation and friction
What are the coesive fault rocks?
Crush breccia - Matrix <10%
Cataclasite - matrix predominate
What are mylonites?
Fine grained rocks formed by dynamic recrystallization of new minerals during strain at high temperature
What depth do mylonites form at?
> 15km
shear zones where deformation is ductile
What is the stick phase of the stick-slip mechanism?
stress increases and elastic strain builds up over years
What is the slip phase?
when the elastic limit of rocks is reached and a fault forms
What is the speed of elastic recovery?
recovered in seconds
What is generated when elastic strain is recovered?
seismic waves
What are the types of seismic waves?
Compressional P waves
Shear S waves
What are the characteristics of compressional P waves?
Faster, arrive first to seismograms (aka “primary”)
Travel as a caterpillar
Travel in both solids and liquids
What are the characteristics of Shear S waves?
Slower, arrive after P waves (aka “secondary”)
Travel as a snake
Can only travel in solids
What is the seismic cycle?
Interseismic (strain build up) and Coseismic (strain recovered)
What is the primary cause of earth quakes?
Faults
How do faults grow?
repeated slip; so, large-offset faults are the products of hundreds/thousands of earthquakes
Why is repeated slip likely to occur on faults?
more likely to be reactivated due to the presence of the weakness
What increases at a fault with continued slip?
displacement and length
What is the ratio of length to displacement?
Length 10,000 times displacement (10km length = 1m displacement)
What is aseismic slip?
If the frictional resistance on the fault is low elastic energy cannot be stored, and displacement is accumulated at a constant rate without causing any earthquake
Where does aseismic slip commonly occur?
uppermost part of the crust (3-4km) where normal stress is lower (not enough friction)
What is the depth range for stick-slip?
~4 to ~15 km
Why does aseismic slip occur after 15km?
as rocks become ductile due to high temperature (>350*c)
How can earthquakes occur below 15km?
at subduction zones in the subducting plate as the temperature at the plate interface is <350*c
What is the thermal gradient at a subduction margin compared to normal thermal gradient?
subduction - 10c per km
Normal - 30c per km
What are the depth of shallow, intermediate and deep earthquakes?
Shallow <99km
Intermediate 100-300km
Deep 300~700 km
