alternative models for the origin of time-progressive
23rd May, 2008, Carlo Doglioni
think this is a helpful piece of work. However, the
model by myself and Cuffaro (Doglioni
et al. (2005)) has been misinterpreted.
None of the world's plate boundaries are fixed relative
to the mantle, and this can be easily demonstrated
kinematically. The so-called plume trails argued to
be moving against the mantle flow (implicit
in the net rotation of the lithosphere required by
any reference frame and supported by the asymmetry
of orogens and rift zones) are volcanic trails that
are stationary and close to a ridge or transtensional
area. These have been interpreted as shallow, unfixed
wet spots. Those geometries are in fact on both sides
of the ridge (therefore they do not represent any preferred
kinematics), and they neither represent nor contradict
the evidence for mantle flow. Otherwise they are along
the 90°E ridge
and the Laccadive transtensional boundary.
for this type of volcanism is proposed by Doglioni
et al. (2005), Figure 9. In this model the volcanism
does not represent the mantle motion relative to the
deep mantle. In the paper by Gripp
& Gordon (2002)
there is a description of the net rotation in HS3 where,
for example, Africa is moving "W-ward",
contrary to the volcanic trails in the eastern Atlantic.
25th May, 2008, Victor Puchkov
I did not try to interpret the model of Cuffaro & Doglioni (2007) and therefore I could not misinterpet it. My intention was to see what are the
consequences of this model because they are the best test for any theory. The
direction of the mantle flow in this model changes between NE and SE, and
directions of relative lithosperic plate motions in most of cases are opposite
to the mantle flow, changing between NW and SW. The melt anomalies are entrained
by the mantle flow and move in the same direction as the mantle flow, but more
slowly. If this is correct, the time progression of the volcanic trails must
always be directed in the NW-SW direction. Unfortunately such a conclusion conflicts
with my map. Thus, something is wrong either with the
model of Cuffaro & Doglioni (2007) or with the map I compiled from
many independent sources.
The way in which of Cuffaro & Doglioni (2007) explain
the cases which contradict there model is merely inventing
new, additional models. For example, the model suggested
ad hoc for the Southern Atlantic (Doglioni
2005; Figure 9) implies that the hot (or wet) spots
sit under the MAR and produce symmetric volcanic ridges.
But again, this is not the case according to my information
and map: neither Tristan, nor Helena sit on MAR
and they did not produce symmetrical patterns.
as I understand, yet another ad hoc model is suggested
by Cuffaro & Doglioni (2007) for
the Reunion and Kerguelen hotspots, implying a transtensional
nature for the 90°E and Laccadive tectonic lines.
But there are still the Easter, Galapagos, Madeira
and Canaries to explain, with E- and NE-ward time-progressions
On the other hand, I did not need
any additional models, easily correlating my map with
the ITRF-2005 scheme, and exactly following the classical
Wilson and Morgan explanation.
26th May, 2008, Carlo Doglioni
I have been aware of these points since I first saw
a global asymmetry along an
undulated flow pattern of plate motion. I was puzzled
as to why there was so much evidence for
global tuning, and about the contrasting kinematics
suggested by some volcanic trails. The asymmetries
across rifts, the opposing characters of orogens,
such as the dip of the foreland monocline, the subsidence
in the foredeep, the elevation of orogens regardless
of the nature and age of the subducting lithosphere,
their metamorphic evolution, the types of rocks involved,
the depth of the basal decollement, the gravimetric
signature, the heat flow signature, the kinematics
of the subduction hinge, and a number of other different
markers are all
verifiable data in favor of a polarized flow (Doglioni
1999; 2007). That is
why I studied the origin of plumes and I concluded
that volcanic tracks
may have different origins (wet spots, shear heating,
rifts, transtensional shears, etc.) but have in common
a shallow source (e.g., Smith & Lewis,
1999). So our model is not ad hoc, but based
on verifiable data. This evidence has to be disproved
before our model of global tuning can be rejected.
of a flow of plates was proposed by myself in 1990
based on the first-order tectonic features, but it
has been independently confirmed by space geodesy
(e.g., Crespi et al., 2007). The westward
drift is added to this flow, but it is an independent
observation and also not an artifact. The net
rotation of the lithosphere is based on a number
of independent types of evidence. We can question
what causes it, and its real velocity, but not its
existence. In Scoppola et al. (2006) you
may find a review and a new model on the subject.
The westward drift has been envisaged and/or measured,
and discussed by many researchers, including Wegener,
Rittmann, Le Pichon, Bostrom, Jordan, Knopoff, Moore,
Wang, Gordon, Ricard, Sabadini and myself. The ITRF
is artificially anchored to the Earth's center of
mass, and it is fixed a priori without any net rotation
(the sum of all plate motion vectors is constrained
to be zero), disregarding what the lithosphere is
doing relative to the mantle (see papers by Argus
on that subject). Therefore it is not representative
of any "absolute" plate
Regarding the concern about volcanoes
which Prof. Puchkov infers to be fed by the deep mantle,
the paper by Bonatti (1990) on wet spots
is particularly relevant. These do not need to be exactly
on the ridge. Rifting – "fissures" –
have also been invoked, e.g., by Natland
Figure 1 shows water variations along
the mid-Atlantic ridge. There is a good correlation
with the so-called hot spots. As you know, the asthenosphere
is richer in water, which
lowers the melting temperature. Therefore the so-called
hot spots, when they are in truth wet spots, are
actually cool spots! I am not saying that all plume
trails are wet spots. I do not think Hawaii is a wet
spot. In my opinion, the Hawaiian track is really an
intraplate plume. But nevertheless thermo-barometry
and geochemistry support an asthenospheric source
I reiterate that:
- the lithosphere
is decoupled from the asthenosphere and the deep mantle
(see shear wave splitting for example), and
- if the
source of plume trails comes from within or even
above the decoupling surface, the trails of age-progressive
volcanism do not represent the plate motion relative
to the mantle, especially when they are on or close
to plate boundaries (including transform zones) which
all move relative to the mantle.
the motion of Africa in HS3 with respect to the
eastern Atlantic trails calculated by Prof. Puchkov.
arguments proposed to disprove the net rotation of
the lithosphere with "westerly" moving
plume tracks are, from my point of view, indirect
but strong evidence for the shallow nature of plumes.
Figure 1. Variations
in water and sodium along the mid-Atlantic ridge, from
Ligi et al. (2005).
27th May, 2008, Don Anderson
I agree with the comments
of Carlo Doglioni except for a few semantic issues.
It is not helpful to refer to all melting anomalies
as "plumes" since
this term has a fluid dynamic and conventional usage
meaning. The alternatives to mantle plumes include
shallow features such as wetspots, coldspots or fertile-spots.
Mantle metasomatism even makes CO2- or carbonatitic-spots. "Mantle
plumes" must be hot and anchored in a deep thermal
Fertile blobs on the other hand can
melt at ambient mantle temperature and can originate
as delaminated arc crust, from above. Even Hawaii need
not be hotter than ambient mantle under a large long-lived
plate. Fertile blobs can also originate as subducted
seamount chains or temporarily subducted young buoyant
oceanic crust; there is a lot of such crust and many
seamount chains are currently entering subduction zones.
The mass flux of these exceeds the hotspot flux. The
central Pacific, under Hawaii, is more likely to be
fueled by these kinds of fertile blobs than by delaminated
supercontinent crust that seems to fuel the Indian
and south Atlantic ocean fertility spots.
In the asthenospheric
counterflow models, the hotspot motions on a given
plate will be approximately parallel. There is no agreement
on what to call these various alternatives to mantle
plumes but it is confusing to call them "plumes" since
they have none of the attributes of the Wilson-Morgan-Campbell
plumes. They may, in fact reflect an entirely different
form of convection, more akin to salt fingers, dikes,
Galileo thermometers, and diapirs than to the boundary
layer convection, heated from below, that motivated
the plume hypothesis. The actual form of convection
involved in melting anomalies is driven by internal
heating and phase changes, not heating from below combined
with thermal expansion. "Hotspots" are actually
due to high homologous temperature rather than to high
absolute temperature. The results are essentially the
same for seismology and petrology except that predicted
temperatures and heatflows are much lower.
Bonatti E., 1990, Not So Hot "Hot
Spots" in the Oceanic Mantle, Science, 250,
Crespi, M., Cuffaro, M., Doglioni,
C., Giannone, F., & Riguzzi, F., 2007, Space
geodesy validation of the global lithospheric flow. Geophys.
J. Int., 168, 491-506, doi: 10.1111/j.1365-246X.2006.03226.x.
C., D. H. Green & F.
Mongelli, 2005, On the shallow origin of hotspots
and the westward drift of the lithosphere. In:
G. R. Foulger, J.H. Natland, D.C. Presnall and
D.L. Anderson (ed.) Plates,
Plumes, and Paradigms.
Geological Society of America, Boulder, Colorado,
Doglioni C., 1990, The global
tectonic pattern. J. Geodynamics, 12,
Doglioni C., Harabaglia P.,
Merlini S., Mongelli F., Peccerillo A. & Piromallo
C., 1999, Orogens and slabs vs their direction
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Doglioni C., Carminati E., Cuffaro
M. and Scrocca D., 2007, Subduction kinematics
and dynamic constraints. Earth-Science
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Gripp A.E. and Gordon R.G.,
2002, Young tracks of hotspots and current plate
Geophys. J. Int., 150,
Ligi M., Bonatti E., Cipriani A., Ottolini L.,
2005, Water-rich basalts at mid-ocean-ridge cold
spots, Nature, 434, 66-69.
Scoppola B., Boccaletti D.,
Bevis M., Carminati E. & Doglioni C., 2006,
The westward drift of the lithosphere: a rotational
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Smith, A. D. & C. Lewis,
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last updated 26th May, 2008