Week 13

Question 1

Mechanical thrusting paradox

Answer 1

Force applied must exceed rock strength

Question 2

Do Andersonian conjugates apply in nature?

Answer 2

Not necessarily

Question 3

What is the role of pore fluid pressure in compressional regimes?

Answer 3

• deform
• PFP builds (can’t escape)
  = hovercraft principle = lubricant
  = moves long distances

σc = σo + tanФ(σn - PFP)

σc = critical shear stress
σo = cohesive strength
tanФ = coefficient of internal friction
σn = normal stress

Question 4

Value of σo (cohesive strength) if rock fractures

Answer 4

0

Question 5

Critical taper model

Answer 5

= idea that constant alpha + beta angle maintained between topographic surface and basal thrust fault

= extension in orogenic wedge by NORMAL faults

1. Tectonic underplating
2. Uplift
   - brings HP rocks to surface e.g. eclogite/coesite in Alps and Norwegian Caledonides
3. Normal faults = extension

N.B. less successful for thick-skinned

Forces (stresses) act upon each part of wedge

Question 6

Forces in the critical taper model

Answer 6

τb = ρghα + 2Kθ

τb = basal shear stress

ρghα = ‘gravitational’ stress

2Kθ = ‘longitudinal push’

K = yield strength within wedge

Question 7

Pedicted characteristics of critical taper model

Answer 7

1. ‘Toe’ maintains critical angle with simultaneous compression and extension
   = f(rock properties, fluid P and décollement horizon strength)
2. τb and ρghα = important parameters for governing alpha and beta
   N.B. alpha may vary across wedge and over time

Question 8

Other models of compressional regimes

Answer 8

GLIDING

BULLDOZER

EXTRUSION

SPREADING

Question 9

How do you determine the slip vector from a focal mechanism?

Answer 9

The pole to the auxiliary plane

Gives kinetic information

Question 10

Escape/extrusion tectonics

Answer 10

Moves lithospheric blocks laterally out of collision zone

Question 11

Alternative to escape/extrusion tectonics

Answer 11

Rotating blocks that rotate on vertical axes (S-S component)

Question 12

Delamination model

Answer 12

Cold, dense roots “fall off” i.e. delaminated

Upward collapse of root

= mountain collapse gravitationally

e.g. TIBETAN PLATEAU

Question 13

Example of active collisional tectonics; facts

Answer 13

Arabia-Eurasia and India-Eurasia

• Indian N 40mm/yr w.r.t stable Asia
• marginal S-S zones
• long term plate convergence ~2000km shortening
• thrust, normal and S-S earthquakes
• earthquake depths –> basal thrust within basement

Question 14

Where is the seismogenic layer?

Answer 14

Up to 15km depth

Question 15

Example of active collisional tectonics; processes

Answer 15

1. Extensional faults in high-Himalayan
   - mid crust extruded out onto surface ~20Ma
   - gneisses to centre
   - monsoon = rapid erosion = encourages uplift = decreases normal stress
2. Channel flow
   - mid crustal processes = weak layer
   - extensional faults = wedge thinning
   - erosion
   = explains 1.

Question 16

Tibet; facts

Answer 16

“Roof of the world”

Biggest area of planet ~5km above SL

70km thick crust

N-S normal faults within plateau

S-S (N-S/E-W) within plateau

Thrust faults at margins (=crustal thickening/imbrication)
- where altitude 3.5km or

Question 17

Tapponier model for Tibet (and alternatives)

Answer 17

Crustal thickening propagated N in Tibet, S in Himalayas

OR

• lots of accreted deforming crustal blocks
• escape/extrusion tectonics
• rotating blocks (England and Molnar 1990)

Question 18

Accounting for extension, magmatism and very high elevation within Tibet

Answer 18

Elevated crust = buoyancy force

• excess GPE
• why plateau doesn’t shorten/uplift beyond a limit

Delamination model