7. The Forces associated with Subduction

This overview summarizes the present understanding of subduction zones, from the perspective of the higher density oceanic crust sliding under the lower density continental crust at convergent margins. The petrological activity associated with the subducting lithosphere, the formation of the accretionary wedge, the trench, and continental volcanic arcs, have and are still the subject of intense research. In fact, the understanding of 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. This treatise examines the forces responsible for the implementation of the subduction process and in no way contradicts the veracity of petrological activities involved.

Arguments will be put forward to demonstrate that subduction is an inevitable consequence of tectonic plate movements as distinct from being the driving force it is nevertheless important to examine the forces involved in order that its true role may be examined. Fig 7 shows general illustrations of the conventional three major subduction systems. Consideration will first be given to the oceanic-to-oceanic plate system.

Ophiolites obducted/thrust onto the top of the continental plate edge

Fig 7. © 2020 www.tectonic-forces.org

7.1 Oceanic – Oceanic Plate convergence. Subduction or Obduction

Subduction starts the moment two plates collide at a convergent margin. Fig 6iia illustrates this point. The inference from the illustration is that the uppermost plate has a lower density than the bottom plate. Having said that no relative movement will take place unless a compressive force is applied. Compressive and tensile forces by convection currents have been challenged and discounted in section 2. At present, there is still no consensus regarding the origin and flow patterns of the convection currents.

Taking this argument forward, it is important to consider the process of obduction at this juncture. Obduction refers to the situation where a higher density oceanic lithosphere is thrust over a lower density opposing plate instead of being subducted. Typical examples are seen in Oman, Himalayas, Cyprus, and in the movement of the Arabian plate towards the Eurasian plate. This action is noted by the exposure of ophiolite rocks which are fragments of the uppermost oceanic lithosphere located above sea level, and in many cases on top of the continental crust Moores, 1970; Dewey, 1976; Casey et al., 1981; Dilek et al., 1990; Cluzel et al., 2001; Cloos et al., 2005; Spandler et al., 2005] check it out.

Fig 8 demonstrates the collision between two oceanic plates. From an engineering viewpoint, the relative thickness and composition between the colliding plates will have a greater bearing than the density difference in determining which of the colliding oceanic plates subducts or obducts.

Ultimately the collision between the two oceanic plates will result in the continental plates either side coming together to form a continental plate to continental plate convergence. The oceanic plate to oceanic plate is basically a system in which the two plates are subjected to a pincer movement. This situation is noted at the closing of the Tethys ocean between the Arabian Plate and Eurasia.

7.2 Conventional Force diagram associated with Subduction

The diagrams showing the forces associated with the subduction process generally take the form as shown in Fig 9. The calculations generally follow a similar path in that the resultant forces causing tectonic plate movement are stated as:

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

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

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

The above-mentioned forces are generally stated as the following equation

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.

In solving for the numerical values of the forces involved in the above general equation many factors need to be considered. These include upward buoyancy forces, pressure, and temperature gradients to describe the density and hence the slab weight in the asthenosphere, isostatic considerations, and the variable water content and its effect on viscous drag coefficients. As both the approaches and the actual values vary between different authors, the simplified format of the equations will be considered in this section of this treatise.

Fig 9. Copyright R. Maurer, www.tectonic-forces.org, 2020

7.3 Consideration of Forces associated with Ridge Push

Examination of fig 9 clearly shows that the mid -oceanic ridge is split with magma extruding onto the seafloor. Fig 11 shows a low-pressure metamorphism diagram at a ridge. As the paleomagnetic lines either side of the ridge are reasonably parallel to the ridge (Fig 4) 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 the conventional N/m2. However, the lack of high-pressure distortion and metamorphism suggests that forces associated with magma extrusion at a ridge to be negligible with respect to the forces associated with the subduction process.

As F (ridge -push) approximates to zero the above equation is simplified to:

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

Fig 10

Fig 11

7.4 Consideration of the forces associated with Slab Pull

This section will demonstrate that both the origin and the slab break off into the mantle do not influence the tectonic movements of the continental plates. Subduction can be clearly shown to be direct consequence of tectonic plate movement as distinct from being the driver. This section thus challenges current wisdom and in doing so enables some of the puzzling anomalies associated with the Hess model to be clarified.

7.4.1 Upward Buoyancy Forces

In order that subduction takes place several major variables need to be considered. The first major variable resistance to be encountered at a convergent boundary are the upward buoyancy forces due to the higher density asthenosphere as shown in the Fig 11 table. Consideration of Figs 7&9 show the higher density (2.9 g/cm3) oceanic crust subducting under the lower density (2.7g/cm3) continental crust. Both layers however sit comfortably on the higher density (3.3g/cm3) upper mantle. An external force must therefore be applied to push the lithosphere (3.1g/cm3) downwards into the upper mantle (c.3.3 g/cm3). From a purely mechanical point of view, the extra force needed to overcome the upward buoyancy forces would be derived by the heavier bulk weight (albeit it of lower density) of the Continental plate moving over the oceanic lithosphere. This point is shown diagrammatic form in Fig 12.


Fig 12

7.4.2 Viscous Drag forces

The second major variable that the subducting lithosphere must overcome is the viscous drag force between the surface areas of the moving lithosphere over and then through into the asthenosphere. As there will be a substantial temperature gradient between the interfaces described above, the viscous drag force and lithospheric tensile force will vary along the length of the subducting lithosphere and with it the petrological processes. This in turn will have a bearing on the pattern of the stress induced faults This complicated stress pattern in the descending lithosphere will also be substantially influenced by the water content and thus the serpentisation processes. Ultimately the slab pull force is addition to the weight of the overriding continental crust will overcome both the viscous drag and buoyancy forces.

The faulting of the lithosphere overlying the asthenosphere by the induced tensile stresses can if close enough to the bend result in seaward volcanic island arcs.

The formation of mid ocean ridges may have more to do with the separation of the continental plates in the form of sea floor stretching rather than as a direct function of forces associated with slab pull. The basic equation will thus be modified to:

Total subduction force = F (slab pull) + M (weight of continental crust)