Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

EOS410PltTec > Plate Driving Forces > Flashcards

Plate Driving Forces Flashcards

1
Q

Source of energy to drive plates

A
  • Heat, radioactive decay in core and mantle

- Surface by mantle convection

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Relative importance of driving and resistive forces

A
  • Plate characteristics vs plate velocity

- Clues from stress field w/in plates

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Edge-Force Mechanism

A
  • Plates move in response to forces applied to their edges

- Ridge-push, Slab-pull, Trench Suction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What are the possibilities for Trench suction force?

A
  1. Overriding plate collapses towards steepening plate
  2. Slab ‘rollback’
  3. Secondary convective flow induced by motion of lithosphere
  4. Active volcanism, in back-arc, forces lithosphere apart
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Absolute plate velocity

A
  • Slower if attached to large continent

- Faster if attached to large subduction zone

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

SHmax

A

Maximum horizontal stress

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Shmin

A

Minimum horizontal stress

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
  1. EQ focal mechanisms
  2. Borehole breakouts
  3. In situ stress measurements
  4. 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

EOS410PltTec (12 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions