2. Current Theory of Plate Tectonics

2.1 Pioneers of Plate Tectonics Theory

The remarkable fit of South America to Africa had been remarked on ever since coastal outlines had been mapped. In 1912 Dr Alfred Wegener (Germany) published his Theory of Continental Drift based on fossil and meteorological similarities in the Americas, Africa and parts of Northern Europe. He postulated that all the Continents had been joined together in one landmass, which he termed Pangea (Greek for one land – entire Earth). (Fig 1) Wegener was a meteorologist, not a geologist, and his observations were not given serious consideration by the professional geologists at the time.

In 1937, geologist Alexander du Toit (South Africa), published his book 'Our Wandering Continents' which broadly supported Wegener, and proposed two continents, Laurasia and Gondwana which later collided to form Pangea. His geological evidence included the comparison of tillites in and fossils in South America and South Africa. His detailed work also allowed him to demonstrate that the Caledonian Mountains, formed during Caledonian Orogeny and ran continuously from Scandinavia, via NW Scotland, Western Ireland, Eastern Greenland to the Appellation Mountains in New England (USA) until split by the break-up of Pangaea. Du Toit coined the terms Gondwanaland, Laurasia, and Tethys Sea to add to Pangaea. Despite the overwhelming evidence, this work was not fully accepted, as du Toit, like Wegener before him, did not offer a mechanism to explain how the continents drifted apart and together.

In 1944 Arthur Holmes published the proposal, first that heated convection currents in the mantle, due to the heat generated at the core, could provide the mechanism to account for orogenic activity. However further evidence was needed to sway the many sceptics.

During the 1950s and 1960s, new evidence, particularly with regards to palaeomagnetism, seafloor mapping using sonar and seismic measurements, showed that the observations could only be accounted for if the continents moved over the surface of the Earth if Pangea had existed and continental drifting had occurred. The ocean floors were found to have bands of reversed magnetic polarity in a striped and mirrored pattern about a raised mid-ocean ridge. (Fig 2) The ages of the basalts ranged from the Jurassic, most distant from the ridges to the present, such as in Iceland. Palaeomagnetism showed that the poles had apparently been in different places in different continents, only possible if the continents themselves had moved.

Prof. Harry Hess (Princeton USA) checked these new discoveries against his own meticulous mapping of the Pacific Basin during his tenure in the US Navy in WW2. He identified the topography of mid-ocean ridges and the abyssal plain, with deep trenches close to continental margins. In 1962 Hess published ‘The History of Ocean Basins' in which he linked the Pacific Basin’s deep trenches to destruction and recycling of oceanic crust through the generation of magma that issued from continental volcanoes, and the creation of new oceanic crust at the mid-Atlantic ridge (Fig 3). He concluded that the breakup of Pangea and subsequent Continental or Plate movement was due to the combined action of 'Ridge Push' forces at divergent boundaries (Fig 4) with the 'Slab-Pull' forces at convergent boundaries. In this manner, Hess was able to balance the creation of new oceanic crust by the equivalent sub-duction of older oceanic crust. He attributed the forces involved in the action of heated circulation currents within the Earth's mantle.

A paper published in 1963 describing “Magnetic anomalies over ocean ridges”, subsequently known as the Vine-Matthews-Morley hypothesis, described the patterns of magnetic reversal in ocean basalts. This demonstrated the symmetry of ocean spreading and allowed measurement of the rate of plate movement away from mid-ocean ridges.

In 1965, John Tuzo Wilson (Canada) proposed ‘transform faults’ which linked divergent plate boundaries at ocean ridges to convergent boundaries at subduction zones. They provide a mechanism whereby spreading of adjacent segments of oceanic crust can be accommodated by horizontal movement.

Dan McKenzie applied thermodynamics to the problem of convection and published “The viscosity of the Lower Mantle” in 1966, opening the way to subsequent models for plate tectonics.

The next major breakthrough came in 1979 when the experimental submersible Alvin mapped the mid-Atlantic ridge and observed the creation of new oceanic crust at 'black Smokers' along the ridge (Fig 3).

These discoveries and subsequent research all contributed to the current Theory of Plate Tectonics based on a cycle of creation and destruction of oceanic crust and continual plate movements, accompanied by mountain building and metamorphism.

Finally, 30 years after he died, Wegener was credited with being the father of the science of Plate Tectonics.

The model^1,19,53 shown diagrammatically in Fig 4 suggests that the subduction of the colder and denser oceanic crust into the mantle by ‘slab pull’ forces resulting from gravity and convection currents in the upper mantle is the major force responsible for driving continuing tectonic plate movements. This 'slab pull' force is also linked to the recycling of oceanic crust at deep ocean trenches at convergent plate boundaries and orogenic and volcanic activity within the overriding plate.

The force attributed to ‘Ridge Push’ at spreading ocean ridges is now considered ^{13,16,27,38,47,49} to be insufficient to generate significant plate movement. The lack of distortion, other than at the transform faults, of the stripes showing magnetic reversal on either side of the mid-ocean (Fig 2), demonstrates the absence of a lateral push force, which would be expected owing to the discontinuous nature of MOR spreading. It is surprising and puzzling, therefore, that with this high level of agreement^{12,14,25,34,40} regarding convection currents as being the major driving force for plate movement and subduction, there is an absence of a magnitude ‘action-reaction’ mechanical force diagram which unambiguously represents the ‘slab pull’ force vector.

There is still no generally agreed model of convection currents in the mantle and the role and origin of mantle plumes is hotly debated. Two proposed circulation systems ^4,14 are summarised in Fig 5.

Although this paper investigates the rotational velocity-derived circumferential stress forces as the primary cause of tectonic and orogenic activity, a brief discussion on some aspects of convection current-driven plate movements is considered relevant. Dewey^10,11, van Andel⁴⁶, and Davies⁸ discuss the geometrical aspects of tectonic movement using Euler’s Theorem, which states that the displacement of a plate over a spherical surface from one position to another can be regarded as a simple rotation about a suitable axis through the centre of the sphere. This basically implies that in the case of the South American plate, the angular velocity will vary along its length. It is extremely difficult to understand how a convection current will match this rotational mode from the equatorial to the much smaller diameter polar latitudes. If the west-east convection currents were or are localised along a south-north axis within the upper mantle then, taken in isolation, a case for the movement of the South American plate may be made. However, as the African plate has been relatively stationary, the north-south convection currents must have moved the present Indian plate in a north-north-east direction into the Eurasian plate. This implies that the opposing heated convection currents must have been, and still are, stable over the 140Ma period since the end of the Jurassic (Fig 6).

It is interesting to note that Davies ⁸ states that, as the plate near the pole of rotation may be rotating about a vertical axis relative to the mantle, it would be inaccurate to think of the mantle motions in terms of simple roll cells of convection. In a spherical shell, the flow may need to connect globally in a complex manner. Davies⁸ also summarises other contemporary work which suggests that the ‘return flow’ from subduction under the north-west Pacific back to the East Pacific Rise may pass under North America. This would approximate to a great circle path, with the flow under North America probably having a southerly component that would not be inferred from the local part of the plate system. A further difficulty arises when trying to understand how the convection based ‘slab–pull’ forces, which moved the components of Pangea northward from their original position in the Permian, changed direction in the Jurassic to cause the break-up of Pangea in mainly east and west directions alongside the simultaneously north- and north-eastward clockwise rotation of the Indian and Australian plates (often referred to as the Indo-Australian Plate). Nor can the existing current convection hypothesis reconcile the variation in the velocity of the different plates as illustrated by Park³⁸ and Hamblin¹⁷. Overall, it is difficult to reconcile the sustained unidirectional movements of the various continental plates from their positions as part of Pangea over 275Ma ago to their present positions, with the clearly omni directional convection current flow patterns.