Plate Driving Forces Flashcards
1
Q
Source of energy to drive plates
A
- Heat, radioactive decay in core and mantle
- Surface by mantle convection
2
Q
Possible mechanisms for plate motion
A
- Plates dragged along by the mantle: Mantle Drag
- Plates driven by forces applied to their margins: Edge-force model
3
Q
Relative importance of driving and resistive forces
A
- Plate characteristics vs plate velocity
- Clues from stress field w/in plates
4
Q
Mantle drag mechanism
A
- Plates move in response to viscous drag exerted on base of lithosphere by lateral motion of asthenosphere at top of convection cells
- Cannot be main mechanism
5
Q
Edge-Force Mechanism
A
- Plates move in response to forces applied to their edges
- Ridge-push, Slab-pull, Trench Suction
6
Q
Why can’t mantle drag be the main mechanism
A
- Poor coupling: driving lithosphere at 40mm/yr requires 200 mm/yr asthenosphere motion which is unreasonably fast
- Large cells of simple regular geometry cannot explain motion of small plates or plates w/ irregular margins
- However, it was likely important for supercontinent break-up
7
Q
Forces at ridges
A
- Ridge push: gravitational sliding away from elevated, hot, buoyant, ridge
- Ridge resistance: Resistance due to internal strength of elastic lithosphere (minor effect)
8
Q
Forces beneath plate interiors
A
Mantle drag:
- Force and resistance
- Viscous shear stress btwn lithosphere and asthenosphere
- Force if Velocity Asthenosphere > Velocity of plate
- Resistance if Velocity asthenosphere < Velocity of plate
- 8 times greater beneath continents than oceans
9
Q
Forces at subduction zones
A
- Slab Pull: Force, due to negative buoyancy (force of negative buoyancy) of cold dense slab
- Trench Suction: Force, extensional force landward of subduction zone
- Slab Resistance: Resistance, mainly at tip of descending plate (where it is 5-8 times greater than viscous drag on upper and lower slab surfaces)
- Bending Resistance: Resistance to elastic flexure of plate
- Overriding Plate Resistance: Friction btwn plates at subduction zone
10
Q
Driving vs resistive forces at subduction zones
A
- Slab pull = Bending Resistance plus Overriding plate resistance: Downgoing slab achieves terminal velocity
- SP > RB plus RO: Slab descends faster than terminal velocity, tension in slab
- SP < RB plus RO: slab descends slower than terminal velocity, Compression in slab
11
Q
What are the possibilities for Trench suction force?
A
- Overriding plate collapses towards steepening plate
- Slab ‘rollback’
- Secondary convective flow induced by motion of lithosphere
- Active volcanism, in back-arc, forces lithosphere apart
12
Q
Relative importance of driving forces
A
- Absolute plate velocity NNR vs. plate area: Velocity is independent of plate area (inconsistent w/ mantle drag mechanism)
- Plate velocity vs. percent plate circumference connected to subducting slab: Plate velocity is larger for plates attached to big downing slabs (favours FSP, and FSU)
- Plate velocity vs. continental area of plate: Plate velocity is slower if attached to large continents (mantle drag inhibits plate motion rather than speeding it up)
13
Q
Absolute plate velocity
A
- Slower if attached to large continent
- Faster if attached to large subduction zone
14
Q
SHmax
A
Maximum horizontal stress
15
Q
Shmin
A
Minimum horizontal stress
16
Q
Strike-Slip Fault
A
- Vertical fault plane
- Max compressive stress sigma 1 is in horizontal plane
- Direction of sigma 1 = SHmax = Pressure axis P
- Minimum compressive stress sigma 3 is at right angle stop P and is also horizontal
- Direction of sigma 3 = Shmin = Tensional axis T
- P and T are 45 degrees to fault plane, 90 degrees to each other
- SHmax>Sv>Shmin
17
Q
Thrust or Reverse Fault
A
- Fault planes dip at 45 degrees
- P axis tends to be in horizontal plane = SHmax
- T axis has large component in Vertical plane = Sv
- SHmax>Shmin>Sv
18
Q
Sv
A
- Vertical stress
- Density x gravity x z
19
Q
Principle stresses lie in approx. horizontal and vertical planes defined as:
A
- Sv
- SHmax
- Shmin
20
Q
Normal Fault
A
- P is mostly vertical = Sv
- T is mostly horizontal = Shmin
- Sv>SHmax>Shmin
21
Q
Indicators of stress orientations
A
- EQ focal mechanisms
- Borehole breakouts
- In situ stress measurements
- Geologic data
22
Q
Borehole breakouts
A
- 300-4km depth
- 16 percent of all stress orientation indicators
- Theory: circular hole in large plate under uniaxial compressive stress, SHmax
- Min. stress at ends of diameter parallel to SHmax, stress is tensile, fractures open against Shmin
- Max. ‘hoop stress’ at ends of diameter perpendicular to SHmax, brittle shear failure may occur, typical rock shear failure occurs at 22 degrees to SHmax
- Intersection of shear planes in rock, a portion of the rock falls away
- Measure max widening to infer SHmax
23
Q
In situ stress measurements
A
- Upper 1km
- 3 percent of stress orientation indicators
- Hydrofracturing, Overcoring
24
Q
Geologic data
A
- Upper 50m depth range
- 4 percent of stress orientation indicators
25
Q
Hydrofracturing
A
- In situ stress measurement
- Portion of borehole isolated by fluid-inflatable packers
- Fluid is pumped into isolated section and its pressure is monitored
- Induced fractures extend in direction of SHmax, open against Shmin
- Interpretation of pressurization and pumping curves: Estimate horizontal stress orientation and magnitudes
26
Q
Overcoring
A
- In situ stress measurement
- Stress in drillhole relieved by drilling a second annular hole around the first
- Measure displacements: determine original state of stress, opposite to measured strain
- Disadvantages: near surface only or in mine, subject to local topography or fracturing (e.g. near dam sites), may not represent regional stress field
27
Q
Geologic Data for stress measurements
A
- Fault slip
- Volcanic vent alignment
28
Q
Volcanic vent alignment
A
- for stress measurements
- Linear zones of cinder cones, trends of feeder dikes
- Trend of the fracture zone is parallel to SHmax
29
Q
Fault slip, for stress measurements
A
- For young, quaternary faults
- Striations on a number of faults, w/ varying trends
- Slip vector, fault plane strike/dip (historical/prehistoric EQ’s)
- Strike of young faults and primary sense of offset (slip for v. young grabens is perpendicular to their trend w/in 25 degrees)
30
Q
Applications of world stress map
A
- Stability aspects in mining, tunnels etc.
- Stress data in the oil patch (borehole stability, seal breach by fault reactivation, reservoir drainage and flooding patterns)
- Seismic hazard estimation (fault re-activation/slip depents on ratio of shear/normal stress, stress propagation from previous EQ’s, induced seismicity implications)
31
Q
Plate interiors are dominated by what type of stress?
A
- Compression
- Max stress is horizontal, thrust or strike-slip
- Ridge push, and collisional resistance (compression) favoured as plate driving forces
- Slab pull and trench suction = extensional stresses
32
Q
Implication of strong correlation btwn absolute plate motion and SHmax direction
A
- Forces driving plates are important, cannot distinguish relative importance
- Australia is exception to motion and stress direction
- Plate boundary forces: 1st order control on stress field
33
Q
Extension regime (normal faulting) in areas of high topography
A
- West U.S., N. Andes, East Africa, Himalayas
- Extension due to gravitational collapse, buoyancy forces 2nd order control on stress field
34
Q
E. Africa
A
- Lithosphere thinned in N-S along rift,
- Implies Extension in E-W, SHmax in N-S
- But mid plate regional SHmax is E-W
- Results in SHmax NE-SW, consistent w/ plate motion
