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Subduction or Overriding ?
(...expression of mantle convection or gravitational collapse of Pangaean lithosphere..)
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Subduction or Overriding ?
(...expression of mantle convection or gravitational collapse of Pangaean
lithosphere..)
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Abstract:- The concept of subduction, the active 'carrying-down' of oceanic lithosphere at the Benioff zone driving convection cells is central to Plate Tectonics, yet it is only one of two interpretations of what may be happening at these active continental margins. The other is "overriding', where the decoupled lithosphere on the continental side of the zone is being carried over the lithosphere on the oceanic side. In using the word 'subduction' Plate Tectonics does not discriminate between the two yet the difference is crucial: 'overriding' (related to spin) is not 'subduction' (related to convection). The consensus choice of subduction follows from a belief that this is a zone of mantle destruction, paired with the complementary creation of ocean floor at spreading ridges. The alternative notion of 'overriding' derives from a demonstrable architectural symmetry of geological structure of the ocean floors and continental margins with the planet's spin. On grounds that Plate Tectonics does not take into account this first-order spin-symmetry of the structure of the planet, and is self-contradictory in many of its inferences, notions of convection are held to be incomplete and in need of revision. In short, convection predicated on subduction as the corollary of ridge-spreading is considered to be invalid. |
Fig.1. Seismic section through the Tonga Trench. Lithospheric collapse ('overriding') of the upper-over-lower mantle across the Benioff ('subduction') zone. Overriding from left to right on elements of flat-lying structure at the 660km boundary layer (black lines) is indicated by offsetting of the 'subduction' zone (red highlight); earthquakes circled; structural domains right and left of the zone are highlighted. Asymmetry of movement indicated by the flexure confirms overriding against subduction as the simplest movement picture. (After Mussett and Kahn, 2000)
Historically, the term 'subduction' carries the connotation of convective return of the shell of oceanic 'crust' (lithosphere; 60-80km thick) to the deeper mantle, and is typically supported by figures like the inset above. However more recently seismic tomography has led to the increasing recognition of the pronounced stratification in the upper mantle, giving rise to the concept of layered or 'stacked' convection (where convection cells in the upper mantle piggie-back on convection cells in the lower mantle) as well as the notion of 'flat subduction' (googles 1,000 + entries at the present time: jan 2006).
Layered convection? Flat subduction? Layered convection is a challenge indeed to see through the fog of irregular seismic reality, ..and flat subduction is simply a contradiction in terms. How can subduction, returning oceanic lithosphere to the deep mantle be 'flat' and remain within 600km of the surface? The answer is it can't. Not unless it's not returning plates to the deep mantle at all, but is just cycling them in the upper mantle. Which is then hardly convective return to the deeper mantle touted by Plate Tectonics. And how does 'layered convection' explain the offset on the subduction zone in the figure which is supposedly a cooling slab falling into the mantle, other than an as ad hoc assertion.
This is precisely where Plate Tectonics is at the present day: "All scientists believe in convection, the only debate is whether layered convection exists or not". That is the view according to some. According to others:- "There are text books, monographs and hundreds of papers on the subject of thermal convection but few are applicable to the mantle .... understanding of the “exterior” problem (the surface boundary condition) is in its infancy."
So much for convection and subduction. The alternative, simpler, and as may be noted already adopted 'overriding' option, is graphically illustrated in the figure. The lithosphere on the left is simply 'leaning against' that on the right, slipping along those black-line seismic discontinuitites to offset the zone of earthquakes and overrride the right-hand lithosphere. On the larger scale the displacement on the zone is seen to be due to the collapsing, gravitationally correcting Pangaean lithosphere of the circumglobal 'mountain belt'/ 'elevation' overriding the mantle.
In comparing the two it should be remembered exactly what the benioff zone actually is, when temperature tends to lower the speed of seismic waves and pressure tends to increase the speed. The Benioff is a zone of increased seismic travel time (about 6% above average) which coincides with a zone of earthquakes. That's all. Plate Tectonics says that seismic waves travel faster in this zone because it is colder than the surrounding mantle and therefore more brittle. And being brittle it tends to fracture more easily to give earthquakes, the stress being seen as extensional due to the descending slab. (Colder? To a depth of 1600km? Going down really slow too. What's more, the zone gets wider, not narrower. Well, ..thermal modelling says yes, if you believe it)
Here, where the zone is simply regarded as a zone of increased stress due to gravitational collapse of the Pangaean crust (which remains today as the most elevated regions of the planet we call 'mountain belt') the stresses on the benioff are compression and shear due to the gravitational collapse of the overriding slab. So, to sum up, Plate Tectonics says the stresses in the Benioff zone are extensional due to the sinking slab. Overriding (here) says the stresses are compression and shear due to collapsing lithosphere beneath elevated crustal regions. The difference is important when estimating moment tensor solutions for the zone. The alternative case put here is entirely ignored by Plate Tectonics, which is wedded to the notion of convection and the subduction that supports it, and refuses to consider the signatures of spin.
The overriding option should be aggressively explored if only from the viewpoint of precaution.