(1) Mr. Waleed Al-Omar Questions and Answers:

Abstract

Two views attempting to explain the driving forces behind the motion of earth’s plate, plate tectonics. The one theory relate the motion of the plates to convection and the other view relate the motion of the plates to a combination of “ridge push” and “slab pull” by the subducting plates. Through studying the deformation, and the orientation of the deformation, of the deep continental roots, the paper endeavors to proof that mantle plays a major role in plate motion. The technique used was based on angular variations of P-wave delays to determine the orientation, azimuth and dip angle, of the deformation.

Q.1) Does the orientation of minerals supports the mantle role in plate motion?

Yes, Studies shoes that in North America the mantle movement is faster than the overlain plate. It also shows that arrivals of from southwestern direction are faster than arrivals from other direction. This supports the idea of driving force from below

Reply: I agree.

Q.2)   How is that related to the thickness of the lithosphere?

Variation in the thickness corresponds to the speed of the mantle movement.

Reply: I agree, and this is illustrated in Figure (2).

Q.3) How is the dip direction supports the claim?

The dip direction depends on the driving force of the plate motion. If the plate motion was caused by mantle convection than the dip is in the direction of the of the movement. However, if the plate movement was caused by slab pull and ridge push thean the dip orientation is the opposite.

Reply: I agree.

(2) Mr. Jubran Questions and Answers:

Q.1 Which are the forces you think can be responsible for driving plates?

Ans. 1. Buoyancy and viscosity
2. Trench suction
3. ridge push
4. slab pull
5. thermal convection

Reply: I agree.

Q.2 If the north american plate is driven from below then what should happen to the motion of north america?

Ans. It should slow down and finally come to rest.

Reply: I agree with slowing of motion and disagree with its stopping.

Q.3 How would lithospheric thickness effect degree of coupling with mantle.
Ans. The thicker the lithospher, the stronger the coupling.

Reply: I agree.

Q.4 How can we test the assumption that hold simple shear responsible for lithospheric deformation.

Ans. If this assumption is true, then fast axes must have a dip angle shallower than 45 degree from the horizontal.

Reply: I agree, as explained in Figure (3)

Q.5 If the plate is driven by side forces or by below then where do you think orogenesis to occur?

Orogenesis will occur on side toward which root is moving. (see fig.1)

Reply: I agree, while orogenesis will occur on the contracting side.

(3) Mr. Mohammed Bulaihed Questions and Answers:

Q#1: Why the author used a technique based on angular variations of P-wave delays?

A#1: To determines both the azimuth and the dip angles for a set of stations in North America.

Reply: I agree.

Q#2: What the anisotropic layers that the deep Canadian Shield consists of?

A#2: The shallower one has subvertical foliation plane, and the deeper one has subhorizontal foliation plane.

Reply: I agree, as mentioned by the author referring to his paper with Silver in 2000.

Q#3: Why North America has slowed dramatically throughout the past 100 m.y.?

A#3: Because the south western motion of North America currently places the western part of the stable continent over the downwelling, and if the motion continues and the stable continent centers itself over the downwelling, the lateral force acting on it will be zero, and the motion will stop.

Reply: I agree. However I disagree with its stopping, while plates all over the world will be moving and the driving forces like mantle convection will still be there.

(4) Mr. Hussam A. Busfar Questions and Answers:

Q:1)
Stations in western United States have northeast trending fast directions. What does this fact suggest?

A:1)
I would believe that the observed variation in fast direction would be more dependent on the decoupling factor. So I would suggest that western United States has thinner lithosphere and thus is less influence by the flowing mantle beneath it, or in other words, it is more weakly coupled with the mantle than other parts of the United States, and more influenced by gravity pull and drag.

Reply: I agree, and the coupling relation with lithosphere thickness is illustrated in Figure (2).

Q:2)
What came first gravity drag-pull, or mantle flow?

A:2)
I think this question supports the idea that mantle flow does indeed play a role in moving the lithosphere. Because I would assume that during the early stages of the birth of earth, the whole earth had fairly the same surface. So how did we get the drag-pull effect if nothing was being subducted (dragged by gravity) beneath the other? This might bring up the idea that probably the mantle flow caused the plates to break up and then the effect of drag-pull came in.

Reply: I think it’s a logical prediction. However, it is very hard to understand the whole phenomenon initiation.

Q:3) Will north America really stop?

A:3) I don’t think that it will completely stop, because simply you will still have the mantle flow, and buoyancy effects acting on everything on the face of the earth.

Reply: I agree, while plates all over the world will be moving and the driving forces like mantle convection will still be there.