2. Tectonic Plate Movements

Current Wisdom

2.1 Brief history of the birth of Tectonic Plate movements

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 were joined together in one landmass, which he termed Pangea (Greek for one land). (Fg 1i)

In 1937, Alexander du Toit (South Africa), published his book 'Our Wandering Continents'. In which he based his observations on the geological similarities of the coal deposits in Northern Europe and common glaciated markings in South America, Southern Africa and Australia. His detailed work also allowed him to demonstrate that the presently named Caledonian Mountain range, which starts in Cumbria in the United Kingdom, skirts through Ireland and continues as the Appellation Mountains in New England (USA) were joined together in Pangea. Du Toit also coined the terms Gondwanaland, Laurasia, Tethys Sea, and the acronym SAMFRAU syncline to describe a continuous fold line through South America, South Africa and Australia. 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 were caused to wander.

Despite the scepticism, other evidence being collected by various field geologists, particularly with regard to the orientation of the magnetic lines in iron rich magmas showed that these anomalies could only really be accounted for if Pangea existed. In 1944 Holmes published the idea that heated convection currents in the mantle due to the heated core could account for orogenic activity.

The next major breakthrough came in the late 1950's when the experimental submersible Alvin mapped the mid Atlantic ridge and noted the creation of new oceanic crust at 'black Smokers' along the ridge (Fig 2i)

Collected core samples from the Atlantic floor also showed mirror imaged reversed polarity paleo- magnetic lines either side of the Mid Atlantic ridge (Fig 3i), which varied in age from the present to the Jurassic era.

Prof. A Hess (USA) checked these new discoveries against his own meticulous mapping of the Pacific Basin during his tenure in the US Navy in WW2. He correlated the Pacific Basin with it's deep trenches and truncated mountain ranges, the recycling of oceanic crust with mantle magma (andesite) that issued from continental volcanoes, with the creation of new oceanic crust at the mid Atlantic ridge (Fig 4i). In 1962 Hess published his findings in which 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 5i) with the 'Slab-Pull' forces at convergent boundaries. He attributed the forces involved to the action of heated circulation currents within the Earth's mantle. In this manner Hess was also able to balance the creation of new oceanic crust by the equivalent sub-duction of older oceanic crust.

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

Fig 5i

Fig 1i

Fig 2i

Fig 3i

Fig 4i

2.2 The Limitations of the Hess Model of Plate Tectonics

This hypothesis by A. Hess1, 19,53shown diagrammatically in Fig 3 suggests that the downward movement (subduction) of the colder and denser oceanic crust into the mantle by ‘slab pull’ forces resulting from the heated circulatory currents in the upper mantle, is the major force responsible for tectonic plate movements. This ‘slab-pull ‘force is also credited with the creation of the trenches in addition to the orogenic and volcanic activity on the uplifted plate. Other credits include the recycling of the oceanic crust at convergent boundaries, and magma intrusion from the split mantle onto the ocean floor being responsible for forcing the continents apart at divergent boundaries. The Hess model is extensively described in the literature. Park38, Hamblin17 and Davies8 are given as typical references which are continually cited in later publications.

While there is wide acceptance of the Hess model of convection currents, a number of research engineers, typically13,16,27,38,47,49, find it difficult to accept that the out-flowing magma along the mid-ocean ridges can contribute to the forces needed to drive continents apart. The lack of distortion (other than at the transform faults) of the disconnected strips either side of the mid-ocean ridge of intruded magma which show reversals in the Earth’s magnetism (Fig 4) demonstrates the absence of a lateral push force. It is surprising therefore that with this high level of agreement12,14,25,34,40, regarding convection currents as being the major driving force for plate movement and subduction, the absence of a magnitude ‘action-reaction’ mechanical force diagram allowing the ‘slab pull’ force vector to be unambiguously represented, is puzzling. It is also surprising that there is still no consensus regarding the origin and direction of the heated currents in the mantle. Experimental data obtained from igneous petrology studies5, seismic wave propagation5,52,54, mathematical1 and thermal modelling13,20,21 as well as consideration of mantle plumes (hot spots)3,14 has resulted in several different heat convection current systems being proposed23,28. Two of these proposed circulation systems including plumes4,14 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. Dewey10,11, 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 Davies8 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.

Fig 3

Fig 5

Fig 6