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 Hess Model & Convection Anomalies

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.

Convection currents and plate movements

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

2.3 Consideration of the Forces associated with Subduction

This overview summarizes our present understanding of subduction zones, using perspectives of the incoming plate, down going plate, mantle wedge, and arc-trench complex. Understanding the operation of subduction zones stands as one of the great challenges facing the Earth sciences in the 21st century and will require the efforts of global interdisciplinary teams (R.J. Stern).60

The multitude of teaching diagrams for subduction as seen in Fig 6ii all show a simplified situation in which the oceanic side of the lithosphere is depicted as a ‘slab’ descending into the mantle where it is ultimately melted into the magma . Although there are numerous papers describing the forces responsible for driving plate tectonic there are relatively few that give the magnitude of the forces involved.

The diagrams showing the forces associated with the subduction process generally take the form as shown in figs 6ii & 7ii. The calculations generally follow a similar path in that the resultant force causing tectonic plate movement are stated as:

F (Slab-Pull): the downward force of the heavy cold slab as it moves towards the mantle

F (Ridge-Push): the force created by the hot magma either via a plume or convection as it breaks through to the seafloor

F (Viscous-drag): the force opposing the downward motion of the slab. This force is calculated as a function of the velocity of descent of the slab

F (Buoyancy): The lithosphere is buoyant at the beginning of any subduction process due to the density difference between the lithosphere and upper asthenosphere

Where F (ridge-push) + Force (slab-pull) - Force (viscous-drag)- F (Buoyancy) =0.

This notation is used as all the forces acting on a body must balance out to zero.

Fig 6ii

Fig 7ii

2.4 Consideration of Forces associated with Ridge Push

The above format considers many factors such a temperature, thermal equilibrium, and pressure variations to describe density and thus the weight of the slab, isostatic considerations, variable viscous drag coefficients, and buoyancy forces are also considered. As the actual values do vary between different authors the simplified format of the equations will be considered for this section of the treatise.

Examination of fig 8ii clearly shows that the mid -oceanic ridge is split with magma extruding onto the seafloor. Fig 9ii shows a low-pressure metamorphism diagram at ridge. As the paleomagnetic lines either side of the ridge are reasonably parallel to the it and to each other and display little distortion, the opposing forces splitting the oceanic crust must be a tensile as distinct from a compressive force. In contradiction Billen(ref) calculates the ridge force = F (ridge force) to be of the magnitude of F= 1.59 x1011N/linear meter. The distance between the ridge and subduction zone is quoted as 5000km. The use of N/m to describe the force makes it difficult to compare with forces as calculated in this treatise. However, the lack of high pressure distortion and metamorphism suggests that forces associated with magma extrusion at ridge to be negligible with respect to the forces associated with the subduction process . Thus, the above equation is simplified to:

Force (slab-pull) - Force (viscous-drag)- F (buoyancy) =0.

As F(ridge-push) approximates to 0.

Fig 8ii: Sea Floor Spreading of Seafloor stretching.

Fig 9ii: Low-pressure metamorphism.

Consideration of the forces associated with Slab Pull

In looking at the forces associated with slab-pull many factors need to be considered:

  1. How much water is carried by the descending lithosphere into the serpentinized mantle.

  2. Does faulting of the outer trench high result in significant serpentinization of the lithospheric mantle?

  3. What is the thermal structure of subduction zones?

As stated above, examination of the illustrations all show the lithosphere ‘slab’ descending into the mantle. In fact, it was not possible to find an illustration in Google showing the start of the subduction process. This treatise proposes the following sequence:

Fig 10ii

‘Subduction’ Process Sequence - Momentum considerations

Subduction will start immediately when two moving plates collide. The probability of one of the plates being stationary is low. Current wisdom states that the Oceanic crust moves under the Continental crust. The calculations relating to the circumferential forces favour the Continental Crust moving over the Oceanic crust . This poses a problem as to which of the two opposing moving crusts has the greater push.

At the very moment subduction occurs 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 11ii).

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

Subduction may well be a function of a two- speed system in which the oceanic crust is bent and move downwards towards the mantle. The larger and heavier of the plates coming together will be the overriding plate due to its greater momentum (Mass x Velocity). Ultimately the momentum associated with the movement of the large Continental Crust albeit with a very small velocity (12 mm/year) will override the less massive and possible opposite slower moving Oceanic crust.

Fig 11ii

With reference to Fig 10ii (see above), the following sequence is offered as a viable explanation of the subduction process:

  1. Subduction starts the moment the Continental plate starts pushing on the Oceanic Plate.

  2. The Accretion process and Trench formation starts at the same time.

  3. Continental plate movement continues to deform the oceanic lithosphere to the point of forcing it downwards towards the mantle.

  4. Slab Pull puts the Oceanic Crust in tension and the Ridge (i.e. East Pacific Rise) opens by the cracking the lithosphere down to the mantle. Minor ridges may also open.

  5. Sea- floor spreading starts.

  6. Continued movement of the Continental Crust over the Oceanic crust (e.g. westward movement of S. America) will push the lithosphere on the oceanic crust downwards towards the mantle.

  7. The creation of the Slab -Pull force will only come into effect when the bent lithosphere penetrates the asthenosphere. Prior to that point there is just a compressional crunch between the two plates.

  8. As the bent Oceanic lithosphere enters the lithosphere the gravitational downward forces come into play in addition to the weight of the overlying Continental lithosphere. This downward force must overcome both the frictional resistance and the buoyancy effects due to higher density of the asthenosphere.

  9. Slab finally breaks off or melts in the Asthenosphere/Mantle.

  10. At this junction, the SLAB-PULL Force=0 as the slab falls away

  11. Continental mass continues its unrelentless movement driven by the circumferential stress forces without loss of momentum.

  12. Subduction starts again as the Continental plate starts pushing down on the slab free.

Fig 12ii

Does subduction drive Tectonic Plate Movement?

Subduction as presently understood is a process whereby the higher density oceanic crust will on a convergent margin sink below the lower density continental crust and in doing so causes the continental plate to move over it. This scenario needs to be re-examined as the absence of a slab and thus the absence of a slab-pull force does not impede the continental plate. It therefore follows that subduction cannot be considered as the major driving force of plate tectonics. As such, subduction of the oceanic plate must be considered as the consequence of continental plate movements rather than as the driving force of plate tectonics. Using the above hypothesis in which subduction and with or without Slab pull is not the driving force for tectonic processes it is not unrealistic to relate them to the unbalanced rotation of the Earth.

The four major basins making up Alaska appear to suggest the four cycles of slab formation followed by slab breakaway and the re-start of subduction.

With reference to the subduction process as described in (fig 10ii) it is feasible to explain the South Alaskan sequence (fig 12ii) as being due to the continuous movement of the overriding Continental plate . This movement was responsible for the successive separate orogenic cycles resulting from oceanic lithosphere bending followed by slab detachment. This cyclical activity led to the formation of the St. Elias Range which in turn led to the formation of the Alexander and Yukon Terranes and the present-day Accretionary Wedge as the Continental Plate moved south.

Orogenic Activity

It is obvious that the forces involved in pushing up the Andes Mountains to as high as 6,000 m above sea level has been, and still is, continuously sustained in one direction. The direction of the forces will be perpendicular to the alignment of the mountain chain. In this case, where the collision is between continental and oceanic crust, the uplift of the Andes is attributed to the noted subduction of Nazca oceanic crust by the 'slab pull’ mechanism18,19.

In contrast, the continuing uplift of the Himalayas (8,000m above sea level) along an east-west axis is attributed to the collision between two continental blocks. It is interesting to note that the subduction forces that were credited with moving India into central Asia are now totally credited with the continuing formation of the Himalayas. The continuously compressive and possibly isostatic forces now associated with the formation of the Himalayas appear to be far more complex than it would be if an obvious subduction zone were present at the India/Asia interface. Van Andel48 and Davies8 discuss this matter in some detail. From the purposes of this article point of view, the major significant similarity between the different orogenic processes (Andean, Himalayan, and Alpine) is the sustained manner of the unidirectional movements and the forces involved.

The creation of the Himalayan mountain range has the hallmarks of a head on collision whereby the momentum absorbed by the fast moving Indian plate is converted into effort to raise the Himalayan mountain chain. It can be likened to a motor car crashing to a halt on hitting a large tree

Summary of the forces associated with subduction on Tectonic Plate Movements

The forces associated with the subduction that influence Plate Movements may be further simplified as follows:

F (Net downward) =0 =F (weight=0)-F (viscous drag=0) -F (buoyancy=0)