Plate Driving Forces

Question 1

Source of energy to drive plates

Answer 1

• Heat, radioactive decay in core and mantle

- Surface by mantle convection

Question 2

Possible mechanisms for plate motion

Answer 2

• Plates dragged along by the mantle: Mantle Drag

- Plates driven by forces applied to their margins: Edge-force model

Question 3

Relative importance of driving and resistive forces

Answer 3

• Plate characteristics vs plate velocity

- Clues from stress field w/in plates

Question 4

Mantle drag mechanism

Answer 4

• 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

Question 5

Edge-Force Mechanism

Answer 5

• Plates move in response to forces applied to their edges

- Ridge-push, Slab-pull, Trench Suction

Question 6

Why can’t mantle drag be the main mechanism

Answer 6

• 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

Question 7

Forces at ridges

Answer 7

• Ridge push: gravitational sliding away from elevated, hot, buoyant, ridge
• Ridge resistance: Resistance due to internal strength of elastic lithosphere (minor effect)

Question 8

Forces beneath plate interiors

Answer 8

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

Question 9

Forces at subduction zones

Answer 9

• 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

Question 10

Driving vs resistive forces at subduction zones

Answer 10

• 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

Question 11

What are the possibilities for Trench suction force?

Answer 11

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

Question 12

Relative importance of driving forces

Answer 12

• 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)

Question 13

Absolute plate velocity

Answer 13

• Slower if attached to large continent

- Faster if attached to large subduction zone

Question 14

SHmax

Answer 14

Maximum horizontal stress

Question 15

Shmin

Answer 15

Minimum horizontal stress

Question 16

Strike-Slip Fault

Answer 16

• 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

Question 17

Thrust or Reverse Fault

Answer 17

• 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

Question 18

Sv

Answer 18

• Vertical stress

- Density x gravity x z

Question 19

Principle stresses lie in approx. horizontal and vertical planes defined as:

Answer 19

• Sv
• SHmax
• Shmin

Question 20

Normal Fault

Answer 20

• P is mostly vertical = Sv
• T is mostly horizontal = Shmin
• Sv>SHmax>Shmin

Question 21

Indicators of stress orientations

Answer 21

1. EQ focal mechanisms
2. Borehole breakouts
3. In situ stress measurements
4. Geologic data

Question 22

Borehole breakouts

Answer 22

• 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

Question 23

In situ stress measurements

Answer 23

• Upper 1km
• 3 percent of stress orientation indicators
• Hydrofracturing, Overcoring

Question 24

Geologic data

Answer 24

• Upper 50m depth range

- 4 percent of stress orientation indicators

Question 25

Hydrofracturing

Answer 25

- 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

Question 26

Overcoring

Answer 26

- 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

Question 27

Geologic Data for stress measurements

Answer 27

- Fault slip | - Volcanic vent alignment

Question 28

Volcanic vent alignment

Answer 28

- for stress measurements - Linear zones of cinder cones, trends of feeder dikes - Trend of the fracture zone is parallel to SHmax

Question 29

Fault slip, for stress measurements

Answer 29

- 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)

Question 30

Applications of world stress map

Answer 30

- 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)

Question 31

Plate interiors are dominated by what type of stress?

Answer 31

- 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

Question 32

Implication of strong correlation btwn absolute plate motion and SHmax direction

Answer 32

- 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

Question 33

Extension regime (normal faulting) in areas of high topography

Answer 33

- West U.S., N. Andes, East Africa, Himalayas | - Extension due to gravitational collapse, buoyancy forces 2nd order control on stress field

Question 34

E. Africa

Answer 34

- 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