How do you start subduction?
"There are two principal views on the physical mechanism leading to the initiation of subduction. The first and most common is that as the oceanic lithosphere ages and cools, its density increases so that an instability arises and the plate sinks spontaneously in the mantle under its own weight."2 However, numerical modelling has shown "that it is highly unlikely that the entire lithosphere at a fracture zone will spontaneously founder."3 "A self-sustaining subduction zone does not form from a homogeneous plate."2 Researchers found that "no combination of fault rheology or geometry produced self-sustaining subduction without applied convergence."3 "The Wilson cycle predicts that the Atlantic will close again to form a second Pangaea through widespread initiation of subduction in the Atlantic basin. Yet, with the exceptions of the narrow Caribbean and Scotia arcs, there is no evidence for subduction initiation (either intra-oceanic or at passive margins) within this ocean basin, despite 100- to 200-million-year-old passive margin sequences... Thus, the passage of time does not appear to increase the probability of subduction initiation in the Atlantic basin."5
"According to the other view, externally applied compressive stresses and moderate convergence are necessary to form a new subduction zone."2 "The most likely mechanism would be through a transfer of stress induced by a collision, leading to 'forced' subduction initiation elsewhere. Yet the response to recent collisions suggests otherwise. The formation of the Alpine-Himalayan chain represents the collision of India and Africa with Eurasia at about 35 to 50 million years ago in the closure of the Tethys Ocean. If large-scale collisional stress transfer occurred, we would expect subduction to have initiated elsewhere within the Indian and African plates. However, no new subduction zones have initiated south of either India or Africa... More than 50 million years have elapsed without the initiation of subduction."5
"In fact, the only subduction initiation that has occurred in the past 80 million years (with the notable exception of the 600-km-long Scotia Arc) has been intra-oceanic and entirely within the Pacific basin."5 "Although it is commonly assumed that subduction has operated continously on Earth without interruption, subduction zones are routinely terminated by ocean closure and supercontinent assembly."5 Researchers "hypothesize that dramatic reductions or temporary cessations of subduction have occurred in Earth's history."5
That would require subduction on Earth to begin repeatedly virtually from scratch! The truth is, "subduction initiation [is] probably the least well-understood aspect of plate tectonic theory."5 "While events such as the opening and closure of ocean basins suggest that subduction initiation is common, theoretical models suggest is should be quite difficult."3
"There would be no plate tectonics if there were no subduction zones. Yet how a subduction zone begins remains poorly understood."4 "Most theoretical studies have concluded that it is difficult to initiate a new subduction zone. Although several studies have examined the initiation of subduction, the dynamics of this process remain obscure. There remains substantial disagreement and uncertainty about the significance of different processes influencing subduction initiation, the material properties of tectonic plates, and even whether it is possible to initiate a totally new subduction zone in isolation from an existing one."2
"Subduction zones are really only crudely predicted by simple convection theory, and there is much to them that is highly atypical of convection. First, if one were to only consider the strength of cold super-viscous lithosphere, one would not expect to see subduction zones at all. Convection with purely temperature-dependent viscosity typical of the Earth's tends to form a cold, hard, and immobile layer on the top, and all convective motion occurs beneath it, as if it were a rigid lid." "Thus, subduction initiation presents a formidable problem to convection models. Second, in most forms of thermal convection, both surface boundary layers converging on a sheet-like downwelling will descend, while this occurs nowhere at any terrestrial subduction zone; i.e. all subduction is one-sided, with only one plate descending into the mantle. This asymmetric downwelling is yet another major enigma not yet well explained in convection theory."1
"Two forces must initially be overcome to make a subduction zone: fault friction (or growth of a lithosphere-cutting shear zone) and plate bending. Plate bending becomes the principal source of resistance."2 "Materials on both sides of a ridge or transform [fault] should be broadly similar as they are produced by similar processes in similar environments. Hence, it is unlikely that compositional buoyancy contrast across these weak zones would develop throughout their evolutionary histories." "Therefore, it is physically implausible why one side of an old ridge, or transform [fault] or fracture zone prefers to sink while the other side chooses to rise under any deviatoric stresses."4
For plate tectonics theory, "the factor which most strongly dictates where subduction initiation will occur is the initial tectonic state of the system. [That] include[s] former spreading centers, fracture zones, transform faults, passive continental margins, and subduction zones undergoing polarity reversal."2 So how did the very first subduction zones form, before there were plates and spreading ridges to provide compression? Nobody wants to touch that one.
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References
1. Bercovici, David. 2003. The generation of plate tectonics from mantle convection. Earth and Planetary Science Letters, Vol. 205, pp. 107-121.
2. Gurnis, Michael, Chad Hall, Luc Lavier. 10 July 2004. Evolving force balance during incipient subduction. Geochemistry Geophysics Geosystems, Volume 5, Number 7, pp. 1-31.
3. Hall, Chad E., Michael Gurnis, Maria Sdrolias, Luc L. Lavier, R. Dietmar Müller. 2003. Catastrophic initiation of subduction following forced convergence across fracture zones. Earth and Planetary Science Letters, Vol. 212, pp. 15-30.
4. Niu, Yaoling, Michael J. O'Hara, Julian A. Pearce. May 2003. Initiation of Subduction Zones as a Consequence of Lateral Compositional Buoyancy Contrast within the Lithosphere: a Petrological Perspective. Journal of Petrology, Volume 44, Number 5, pp. 851-866.
5. Silver, Paul G., Mark D. Behn. 4 January 2008. Intermittent Plate Tectonics? Science, Vol. 319, pp. 85-88.
John Michael Fischer, 2004-2009
www.newgeology.us
