Transcript LLVSPs vs. LVAs - MantlePlumes.org

“The probability that 18…out of 24 randomly chosen points lie within the belts (23.5%
of the CMB area) is about 1 in 7 million”
( p =1.47 ·10−7 )
…considered by
authors to be
“remarkable”
CMB &
backtracked
LIPs
…you ought to have
done a better job!
-Ernest Rutherford
69
About 32 of the world’s active non-arc volcanoes or volcano clusters in the
oceans occur in region O1 (19% of area) which roughly corresponds to the 20
Ma age contour. Only about 7 occur well away from the LVAs associated with
spreading ridges and most of these are in oceanic region O2 (27% of area).
(Leki´c et al., 2010).
Distance of hotspots from Plume Generation Zones at
CMB (-1% contour)
Most hotpots formed on or
near ridges
50% of hotspots &
25% of LIPs formed
>1000 km away from
CMB “plume
generation zone”
Evidence that ‘most’ (1/2) hotspots
are from plumes from the CMB
2000 km
1000 km
Most of these are over ridge-related or ridge-like LVAs, are on active or abandoned
ridges, or are underlain by slabs or are on tectonic shears or rifts
Although the correlations of hotspots with the edges of
LLSVPs (i.e. the “no-anomaly” contour and with the edges of
LVAs associated with ridges can both be considered
“remarkable”, a straightforward hypothesis test shows that
the upper mantle correlation is far superior (p of chance
occurrence is an an order of magnitude lower).
The so-called Plume Generation Zone in D” actually
corresponds almost exactly with the median value of D”
wavespeeds (The second quartile).
The above does not imply that the upper mantle LVZ
correlates with the lower mantle LLSVP; in fact, they are
uncorrelated (Ray etc.). They have completely different
shapes (see previous 2 slides).
About 32 of the world’s active non-arc volcanoes or volcano clusters in the
oceans occur in region O1 (19% of area) which roughly corresponds to the 20
Ma age contour. Only about 7 occur well away from the LVAs associated with
spreading ridges and most of these are in oceanic region O2 (27% of area).
(Leki´c et al., 2010)
Hotspots are where supercontinents and ridges were
Hotspot locations show almost perfect correlation with the
lithospheric stress field associated with upper mantle
downwellings but have much poorer correlations with stresses
inferred from upwellings and from lower mantle effects. For a
search radius of 100 km, only 3 minor volcanic provinces are in
regions of substantial inferred convergence rather than
extension…the chances that the correlation between positive
divergence and hotspot locations could be randomly obtained is
almost null (≪1%). On the other hand, half of all hotspot
locations are more than 1000 km away from the vertical
projections of so-called Plume Generation Zones at the CMB
(Torsvik, Burke) and this was considered to be a remarkably good
correlation.
Near base of thermal boundary layer (Region B)
Schaeffer 2013
LVZ
All hotspots, ridges & backtracked LIPs occur in red & yellow regions
Upper (B) & Lower (D”) Boundary Layers of the Mantle
Hypothesis test
Null Hypothesis: Region D” (the Core-Mantle Boundary)
correlates better with hotspots & backtracked LIPs than any
other region of the mantle. D” therefore contains Plume
Generation Zones, e.g. fixed points above the core (Burke,
Torsvik).
Result: the hypothesis fails
EXAM QUESTION
If A & B both correlate with C, does A correlate with B?
ANSWER: if A and B are positively correlated and B and C are positively correlated then
are A and C also positively correlated? Is the positive correlation transitive? For
example, if the price of stock B increases along with the price of stock A, and the price of
stock C increases with the price of the stock B. Then is it the case that the price of the
stock C increases with the price of the stock A always?
Yes, one may jump into conclusion that they do so. Not because of he/she may be
thinking about Pearson’s correlation coefficient, but it may due to thinking in line with
causation.
In fact, if one knows about Pearson’s correlation coefficient he/she may not conclude
so.
An article by Langford, Schwertman and Owens in The American Statistician gives rather
deep look at the problem.
LVZ
Excellent
correlations
with hotspots
& LIPs
CMB
D” boundary
layer (poor
correlations)
Region D” exhibits little correlation with upper mantle, surface
tectonics, hotspots or the transition zone
CMB (D”)
slabs
Overridden
oceanic
plates,
mantle
fluxed by
slab
volatiles
Slabs
Pacific
plateaux
formed at
boundaries
& triple
junctions
of new
plates
The low velocity zones (LVAs) associated with present day
ridges are in the same places as they were when Pangea
broke up & the antipodal Pacific plates reorganized &
oceanic plateaus erupted. The surface expressions of ridges
migrate but only within the confines of the ~2000-km wide
LVAs associated with ridges at 150-200 km depth.
Hotspots & LIPs backtrack to these same ridge-related
regions.
Plates & D” are highly mobile since they are next to inviscid
boundaries.
Why is there apparently a fixed reference frame in the
“convecting mantle”?
Plate reconstructions show that subduction repeatedly occurs along the same
bands. Regions that are warmed from above by the insulating effects of large
plates and not cooled from below by stagnant slabs tend the control the
locations of divergence of plates; the colder regions control the locations of
convergence and subduction. These effects also control the boundary conditions
at the top of the lower mantle, topography and temperature.
…much of the long-wavelength geoid originates in the deep mantle, the dynamic
topography appears to originate from density variations in the upper mantle…
More than 1600 spherical harmonic coefficients are
used in modern global tomography but only one
has any degree of correlation between the top and
the bottom of the mantle (Adam’s Anchor)…most of
the power is in degree 2 & 3.
Surface hotspots correlate
with ridges & ridge-like
mantle structure & with
extensionanal stress
EXTENSIONAL STRESS
STATISTICS ~100% of hotspots fall in LVAs of the upper mantle, mostly those
associated with ridges, & in regions of extension
Only ~3 hotspots are not near yellow/red. All LIPs backtrack to red.
If the upper and lower mantles
correlate, this is why…
European, African, Asian (Changbai), Yellowstone & most continental “hotspots” are
underlain by slabs
Cold slab
Cooled mantle
CAN BOTH UPPER MANTLE & LOWER MANTLE BE
COOLED BY LONG-LIVED FLAT (STAGNANT) SLABS?
start
Free-slip
Slip-free
Free-slip
60 Myr later
Non-fixed nonvertical
upwellings
Houser, Masters, Flanagan, Shearer (20008)
410
650
Cool 650, warm 410