Re: Now, our famous geologist confuses continental rift and ocean.



Florian wrote:

Stuart <bigdakine@xxxxxxx> wrote:


Fact is, I haven't seen such a blatant manipulation of the written
word outside of creationist circles. I don't see where Doglioni refutes
slab pull; he thinks slab pull alone is too simplistic. Thats hardly a
refutation.

Fact is, *I* haven't seen such a blatant manipulation of the written
word outside of creationist circles.

Doglioni does clealry refutes the slab pull concept (see below)


This demonstrates perfectly the mindset of EEr's.

This demonstrates prefectly the mindset of Stuart Weinstein. I'm glad to
put your nose in your own bull***.

Doglioni wrote:

"The main reasons why the slab pull is not considered here a good
mechanism for moving plates and for triggering subduction are listed.

1) The dip of the slab is independent from the age of the oceanic
lithosphere (Cruciani et al., 2005), considered to control the increase
of the lithosphere's density and likely, therefore, to be the magnitude
of the slab's pull forces.

2) Subduction involves even continental lithosphere even deeper than
100-150 km (Ampferer, 1906; Dal Piaz et al., 1972; Tr�mpy, 1975; Ranalli
et al., 2000; van Hinsbergen et al., 2005), although the subducted
average continental crust is most probably buoyant with respect to
mantle rocks (Hermann, 2002).

3) The oceanic lithosphere is a frozen shallow (30-100 km deep)
asthenosphere, previously depleted below ridges. Therefore, the oceanic
lithosphere is the differentiated lighter upper part of the mantle: then
why should it be heavier a priori than the undepleted deeper (100- 300
km) asthenosphere lying beneath the old oceanic lithosphere where a
pyrolite density of 3400 kg/m3(Jordan, 1988; Kelly et al., 2003) is
inferred? Moreover, hydrothermal activity generates serpentinization of
the mantle along the ridge that decreases the density even more.

4) If the oceanic lithosphere is heavier than the underlying mantle, why
are there no blobs of LID falling in the upper mantle, below the
western, older side of the Pacific plate?

5) Down-dip compression (Fig. 9) affects most of the W-directed slabs,
all below 300 km (Isacks and Molnar, 1971), and most of them at an even
shallower depth (e.g., Frepoli et al., 1996).

6) We do not have hard constraints of the real composition of the
country's upper mantle: there could be more fayalite, making the upper
mantle more dense and the slab's negative buoyancy smaller, or none.

7) The 700 km long W-Pacific slab, where only the upper 300 km show some
potential down- dip extension seismicity [but it could be generated also
by a horizontal shear in the mantle, Giardini and Woodhouse (1986)]
should pull and carry the 10,000 km wide Pacific plate, 33 times bigger,
winning the strong shear resistance at the plate base, and the opposing
basal drag induced by the relative eastward mantle flow inferred from
the hotspots migration (Fig.10).

8) At the Earth's surface, the oceanic lithosphere has a low strength
under extension [e.g., 8x1012N m-1, Liu et al. (2004)] and is able to
resist a force smaller than that requested by the slab pull [3.3x1013N
m-1, Turcotte and Schubert (2002)]. If the slab pull is the cause of the
motion of the Pacific plate, this observation argues in favour of a
stretching of the Pacific lithosphere before the slab pull is able to
move the plate. In other words, the plate cannot sustain the tensional
stresses, that could eventually be due to slab pull (Fig. 11). The
problem of low lithospheric strength could be, however, partly
counterbalanced by the mantle flow and viscous tractions acting on the
plates and induced by slab sinking (e.g., Lithgow- Bertelloni and
Richards, 1998). Due to the low temperature and high pressure, the
strength of the subducted oceanic lithosphere rises to some
2x1013-6x1013 N m-1 (Wong a Ton and Wortel, 1997) and would make the
eventual pull, induced by density anomalies related to phase changes at
depth sustainable. In short, the subducted slab is probably able to
sustain the load induced by the slab pull but probably, this load cannot
be transmitted to the unsubducted portion of the plate without breaking
it apart.


9) Faster plates surrounded by long slabs and trenches? It is a circular
reasoning because long subduction zones might be a consequence of fast
movements of plates. Moreover, plates are considered fast in the NNR
reference frame (Conrad and Lithgow-Bertelloni, 2003). For example,
measuring plate motions in the hotspot reference frame, i.e., relative
to the mantle, Nazca is very slow relative to the mantle, so the
relation plate velocity-slab age and length of the subduction zone (Fig.
12) is not true.

10) Plate velocities in the hotspot reference frame seem to be inversely
proportional to the viscosity of the asthenosphere rather than to the
length of the subduction zones and the age of the downgoing lithosphere.
In fact the Pacific, which is the fastest westerly moving plate (Gripp
and Gordon, 2002), has the lowest viscosity values (Pollitz et al.,
1998).


11) The horizontal velocity of plates is 10-100 times faster than
vertical velocity (subduction related uplift or subsidence) suggesting
that vertical motions are rather passive movements. Moreover, the
kinematic analysis of section 2 shows that subduction rates appear
controlled rather than controlling horizontal plate motions (Fig. 8).


12) The energy for compressing an orogen is probably larger than the one
supposed for the slab pull.

13) Slab pull has been calculated to be potentially efficient only at a
certain depth [e.g. 180 km, McKenzie (1977)]; and if it is shallower
than that? How does subduction initiate?

14) Some plates in the hotspot reference frame move without any slab
pulling them, e.g., the westward movements of N-America, Africa and
South America (Gripp and Gordon, 2002).

15) There are rift zones formed between plates not surrounded by oceanic
subduction to which one can attribute the pull for moving the
lithosphere (e.g., the Red Sea).

16) Although the knowledge of the rheological behaviour of the subducted
lithosphere is very poor it can be conjectured that the downgoing slab,
being progressively heated, could potentially lose strength, diminishing
the possibility of mechanically transfering the pull (Mantovani et al.,
2002).


17) Kinematically, subduction rollback implies that the volumes left in
the hangingwall of the slab have to be replaced by a horizontal mantle
flow, whether this is a consequence or the cause of the retreat
(Doglioni et al., 1999). However, in order to allow the slab to move
back, the slab retreat needs the mantle in the footwall of the slab to
also move away in the direction of the slab retreat, regardless whether
this motion is generated by the slab pull or is an independent mantle
horizontal flow. But the energy required to push the mantle forward is
much greater than the slab pull can effort.

18) When describing the plate motions relative to the mantle, e.g. in
the hotspots reference frame, along E- or NE-directed subduction zones
the slab might move out of the mantle, e.g., in the opposite direction
of the subduction. It is sinking because it is overridden by the faster
upper plate (Fig. 13).

19) Within a slab, eclogitization is assumed to make the lithosphere
denser. However, eclogitization is concentrated in the 6-8 km thick
oceanic crust, whereas the remaining 60- 80 km thick lithospheric mantle
does not undergo the same transformation. Therefore only 1/10 of the
slab is apparently increasing density, but the main mass of the slab
(90%) does not change significantly.

20) The density increase due to eclogitization is in contrast with the
exhumation of the eclogitic prism that is usually detached with respect
to the "lighter" lithospheric mantle (G.V. Dal Piaz, pers. comm.).

21) Why should the lithosphere start to subduct? This crucial point
arises particularly when considering an oceanic hydrated and
serpentinized lithosphere that has not yet been metamorphosed by the
subduction process, and consequently it is still less dense (G.V. Dal
Piaz, pers. comm.).

Yes, ..but so what? He didn't actually say slab pull was a *BAD*
explanation did he? He only listed 21 reasons why it was not a *good*
one. My objection stands, I still don't see where he refutes slab-
pull. All of those 21 reasons are just reasons why slab pull is too
simplistic. Hardly refutation, is it?

Stuart.




This long list casts doubts on the possibility that the slab pull can
actually trigger subduction, slab rollback, and drive plate motions."

--
Florian
"Toute v�rit� franchit trois �tapes. D'abord elle est ridiculis�e.
Ensuite, elle subit une forte opposition. Puis, elle est consid�r�e
comme ayant toujours �t� une �vidence." - Arthur Schopenhauer
.