L8 - Plate Driving Forces Flashcards
What are the main forces involved in a subducting slab? (4)
Ridge push
Slab pull
Basal drag
Mountain range resistance
What causes the driving forces of subduction? (1)
A consequence of gravity acting on lateral density contrasts
What causes ridge push? (2)
Magnitude? (1)
At any depth above the level of isostatic compensation
Pressure beneath the MOR > beneath old seafloor
2-3TN/m along strike
What is slab pull? (2)
Magnitude? (1)
Force is exerted on the plates by the sinking of cold material into the mantle
Material is colder than the surrounding mantle so buoyancy acts to drag plates towards subduction zones
Over 10TN/m
What are the resistive forces on the subducting slab that mean net force transmitted < slab pull? (3)
Drag from the surrounding mantle as slab moves through
Friction on the subduction zone megathrust
Work done in bending the plate in the outer rise
What does it mean that the outer rise of a subducting slab breaks in EQs? (1)
Force transmitted from a subducting slab to the horizontal part of the plate is limited by the strength of faults
What is known about basal drag? (3)
Under fast-moving plates, the seismic anisotropy is aligned with plate motion direction
Relative motion between the plate and underlying mantle is in plate motion direction
Doesn’t say if force is driving or resistive
What is the argument for basal drag being resistive? (1)
Where is it less clear? (1)
For fast-moving plates (India and Pacific), if basal drag was a driving force, not enough resistive force for the forces to balance
Under slow-moving plates (Africa) as there are few driving forces
What does the size of basal drag depend on? (2)
Plate velocity relative to underlying mantle
Thickness and viscosity of the asthenosphere
Where and why do mountain ranges exert a force? (2)
What does the force depend on? (1)
Magnitude? (1)
Exert it on surrounding lowlands
As P at a given depth beneath mountains > beneath lowlands (above isostatic compensation depth)
Thickness and density structure of the lithosphere
Up to 6TN/m
What does the relationship between plate velocity and the percentage of the circumference by plate boundary type imply? (1)
What is the bi-modal nature seen? (1)
What is the problem with these conclusions? (1)
Slab pull may be the dominant force
Slow plates have no subduction, fast plates have subduction
Too few tectonic plates for meaningful statistics