Hi Mohamed,

subduction of oceanic lithosphere is driven by the slab pull force, which mainly arises from a density contrast between the upper plate formed by bouyant continental crust (volcanic arcs) or less dense young oceanic crust (island arcs). Another important factor for slab pull forces and rate of subduction is the length of the subduction zone, as pointed out by Forsith and Uyeda (in the 70's I think). The larger the subduction zone, the faster the rate of subduction. Older, cooler and hence, denser oceanic crust is more prone to sink than young, hot and less dense oceanic crust. This is why many obducted ophioltes were formed from relatively young, hot and not dense oceanic crust.

On the other hand, as the oceanic plate sinks, basalt and gabbros and ultramafic lower units undergo metamorphic reactions, first to anfibolites and then to eclogite. These reactions allow for the desity increase of the subducting slab, accommodating for higher P conditions and reaching mineral stability. Such density increase of the subducting slab positvely contributes to the slab pull force, further sinking the slab into the mantle.

These metamorphic reactions are aided, and actually take place, because the oceanic lithosphere is highly hydrated upon its formation (i.e., serpentinised at mid ocean ridges). Hydrous minerals are more prone to react upong chaning P,T conditions.

Regarding the 660km discontinuity, some slabs have been "imaged" by seismic tomography (see the works of Van der hilst, or Spakman) stagnant and thickenning at the 660 km discontinuity, other trespassing it, others becoming parallal to it. This is probably controlled by the rate of subduction and the rate of metamorphic reactions taking place to increase the density of the slab.

I hope I have answered your doubts. Still, there are many issues we do not fully understand on this planet.

Pablo

Hi Pablo,

Thank you for your answer, it is very informative. I have previouslt imaged the pacific slab in Japan in my Tomography studies. i see all the cases of subduction until the stagnation. now it is very clear.

Actually, I tried to solve the mystery about the existence of High velocity anomaly in the upper mantle (>150 km) I found in my study which is not a subduction zone area but a volcanic area, Some how an amount of High-V material is found there along with the low-V magma related materials (shallower than 150 km). It seems to me that the old volcanic eruptions leaves this High-V materials as the remanence of old solidified basaltic magma?? does it make sense ??

Hi Mohamed,

as you know, Japan is an island arc. This means it consits of an oceanic plate being subducted underneath another oceanic plate. Higher velocities should correspond to the downgoing dense and cold slab, whereas lower velocities could correspond to the hotter astenospheric wedge above, that is currently being hydrated from the fluids released from the downgoing slap (fluids are released during metamorphic reactions). The astenospheric wedge should undergoes melting due to the input of fluids, generating the volcanic arc above.

Wheter such high velocity zone corresponds to the "restitic", depleted astenosphere (the source of basaltic-andesitic magma) that underwent partial melting, I can't really tell. I am not that familiar with seismic tomography and its limitations.

Regards,

Pablo

Another factor to take into account is that the lithospheric mantle in the subducting slab is colder than the underlaying asthenospheric mantle.

The term 'floating' in your question implicitly assigns a lower density to the lithospheric slab and a positive (upwards) buoyant force. This term is often used, informally, but it is misleading. Only the crustal part of the oceanic plate is less dense than the asthenosphere. But most of the oceanic lithospheric slab is similar to the underlaying unperturbed asthenosphere, just cooler and therefore adiabatically denser, as shown by the classical model explaining the seafloor depth by Parsons&Sclater (1977) and others. That's why older oceanic plates present a deeper bathymetry.

In other words, the plates are not 'floating' on top of the asthenosphere, but they are resting on it in spite of their higher adiabatic density. Oceanic plates cannot sink vertically because they cannot displace the asthenosphere globally. They sink laterally in subduction zones in the same way that a sheet of paper left horizontally in the air will initially move laterally, rather than vertically. It's part of the large-scale mantle convection process.

The worlds most unpopular, and unloved theory of tectonics would

indicate that the energy required to initiate subduction is 20 times the

available energy, which would make subduction physically impossible.

This would leave two possibilities, one is obduction, and the other is

a theory where planet's grow larger over time, and material is redistributed

from under continents to feed the spreading ridges in the middle of the

oceans. Both ocean floors, and continents would rise in elevation as

the Earth and other planets grow larger, but ocean floors would rise faster

than continents because of the redistribution of materials laterally.

The result would be a bending of the layers at the interface between

the thinner ocean floor, and the thicker continents that gives the illusion of

subduction, but a reversal of direction of the flow of materials from deep

below the continents, to below the ocean floors that would be aided by

both "dynamic" isostasy, and the upward heat flow of heated and rising

materials of slightly lower density to higher and more stable elevations.

The change in thinking would be from isostatic equilibrium to

the planets seeking an Iso-dynamic equilibrium thru the lateral, and

vertical movement of materials to locations of lower densities, and

higher heat flows toward the planet's surface. Since this is an

ongoing process, the interface of bending , ( the margins ), would

be quite active in differential vertical movements, creating bending,

folding ?, and faulting of the edge of the continents where volcanoes

tend to form, and are subject to Earthquakes which release the

increasing stresses in the rock created by the differential vertical motions.

Plate Tectonics, a static radius Earth, and Subduction have a very

powerful influence on the thinking of Geologists, and Geophysics,

but, perhaps we should use our newly acquired tools of

tomography, in conjunction with thermodynamics, and fluid motions,

and gravity to take a hard look at what makes the Earth move

internally, and in which direction.

In addition to the above answer, we can also look at where we find Limestone, and fish fossils on the surface of the Earth.  Mostly we find them atop the continents, and we find coral islands elevated above the  current global sea level elevation.  In Bolivia we find the Salar de Uyuni  ( " unified salt " ), and in the middle we find a Limestone Reef at an elevation in excess of 10,000 feet above the current sea level.  The conclusion that I came to by looking at the mountain ranges in the area is that there were wave formed benches at elevations between 14,000 feeet and 16,000 feet above the current ocean levels.  The total Volume of the Water on the surface of the Earth keeps increasing as more and more water comes up out of the planet.  So why are the Continents elevated? Why are they not completely submerged under oceans if the total volume of the water keeps increasing.  Subduction of liquids, like water is physically impossible, and subduction of rock takes 20 times the available energy, so that is not occuring either.  The answer is that the surface area of the Earth, and thus the Volume of the ocean basins is increasing faster than the increase in the volume of the water moving up and out of the planet toward the surface.  Part of the materials to feed the oceanic spreading ridges comes from under continents, so continents get thinner at the edges, while remaining thicker in the middle where the major mountain ranges and high plateaus occur.  In addition the Ocean floor rises farther and faster than the Continents, so a bending moment occurs at the edges of continents.  This can be simulated by interleafing a magazine with a Phone book, and then holding them out at arms length.  Due to the difference in weight, the magazine will rise, and the phone book will sink, and the illusion of subduction will occur right in your hands.

The Oceanic water table peaked in elevation globally around 95 million years ago, and by 65 million yers ago, most of the water had poured off the continents into the ever enlarging ocean basins.  It reached a possible lowest elevation around 15,000 to 20,000 years ago, then the ice started melting and raised the oceans about 400 feet in elevation ( 120 meters ).  Now that more ice is melting, it is starting to rise again, a few millimeters per year.  This rise can only occur until all of the ice on Earth is melted, then the water table will very slowly lower in elevation as the planets surface area increase provides ever more ocean basin volume.

The Earth is a very complicated system.  Getting the right concept that can explain multiple changes in the system, over vast time periods is the real challange.