The forces involved in orogenic processes are obviously substantial. From a mechanical engineering point of view it is difficult to imagine how the solid oceanic crust of the Nazca plate will sink below the length of South America under the influence of a small density variation, and in so doing pull the South American plate over it, thereby creating the Andean mountain range. It must be understood that the mantle with its high viscosity 5 is to all intents and purposes as solid as the oceanic crust.
The difficulties associated with explaining the unidirectional movement of the various plates on an omni-directional convection current system has been elaborated on in Section 3. In contradiction the unbalanced centripetal forces, appear to be of sufficient magnitude to push the South American plate over the unyielding Nazca plate. Under these conditions the orogenic process that is still uplifting the Andes is inevitable.
It is now suggested that the geological actions depicted in Fig 16 can be explained as being a function of ‘continental force-push’ as distinct from ‘slab-pull’ forces In the same manner the sub-duction of the Nazca plate can be described as an inevitable consequence of the forces generated at the Earth’s surface and as such has a passive rather than active role in tectonic movements.
As the direction of the ‘Force Push’ will force the plates away from the heavier side towards the lighter side of the planet, these plates will be forced over those plates that are already on the lighter side which are under being pushed down into the mantle by the generated compression forces (see Section 8 & Appendix 1) . This is what is presently occurring at the rim of the Pacific Basin. This action will also account for the absence of a sub-duction zone on the African plate side of the earth which to date has been a source of contention in the explanation of tectonic movement by convection current forces. In the case where the continent to continent collision occurs, uplift will be by continuous compression due to the inertial force pushing against the upstream side of the moving plate. The movement of the Indian subcontinent into the Asian plate at the Himalayan boundary is a case in point.
Fig. 16. Using the same diagram it is possible to interpret the subduction of the Nazca plate under the South American plate by the 'slab-pull' system as depicted in (a) or by the forcing of the South American plate over the Nazca plate by the 'continental force - push' system as depicted in (b). In the latter case (the Maurer Model) the convection currents are considered to have a passive role in tectonic movements.