8. The Subduction Cycle Sequence

Subduction will start the moment two plates collide against each other. In section 7 above it was demonstrated that oceanic plate-to-oceanic plate will ultimately result in continental-to-continental plate collision. Thus, by inference continental plate-to- oceanic plate collision in an intermediate stage. This section will examine in depth the actual mechanics that result in subduction of the oceanic crust intermediate stage (fig 6b). Current wisdom displays the oceanic lithosphere as preferentially subducting under the continental plate and in doing so is shown to be responsible for the orogenic and volcanic arc activity on the uppermost continental plate. The calculations (sections 5&6) relating to the circumferential forces demonstrate the continental crust moving over the oceanic crust.

This poses a problem as to which of the two opposing moving crusts move at a faster rate and has the greater push. Consideration of forces and relative movement by either the oceanic crust bending downwards or by the outwards and upwards movement of the continental crust moving over the incoming oceanic crust the following system is created. Taking an everyday example of a person walking up a downwards moving escalator (Fig 13).

In order for the person to get on just stay on the escalator he must move against the downwards velocity (Voc) of the escalator. In moving upwards against the downwards velocity of the escalator, the man must move at Voc + delta V =Vcc

Taking the westward movement of the South American plate away from Pangea, its movement can be likened to that of figs 13&14&15 showing the velocities involved in moving against the oncoming and incoming subducting plate. As such subduction must be considered as a two- speed system in which the oceanic crust is bent and moves downwards towards the mantle.

Furthermore, the continental plate in moving over the oceanic lithosphere must push it downwards into the asthenosphere. It is this action that must be considered as the primary force for bending the oceanic lithosphere downwards into the asthenosphere. Thus, slab pull by inference will be a secondary and consequential action of the moving mass of the continental plate above it.

Fig 13

Fig 14

Diagram Illustrating the start of the Subduction Cycle