Presentation on theme: "…you ought to have done a better job! -Ernest Rutherford “The probability that 18…out of 24 randomly chosen points lie within the belts (23.5% of the."— Presentation transcript:
…you ought to have done a better job! -Ernest Rutherford “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 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).
50% of hotspots & 25% of LIPs formed >1000 km away from CMB “plume generation zone” 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 Most hotpots formed on or near ridges Distance of hotspots from Plume Generation Zones at CMB (-1% contour) Evidence that ‘most’ (1/2) hotspots are from plumes from the CMB 1000 km 2000 km
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.
All hotspots, ridges & backtracked LIPs occur in red & yellow regions LVZ Near base of thermal boundary layer (Region B) Schaeffer 2013
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.
CMB (D”) Region D” exhibits little correlation with upper mantle, surface tectonics, hotspots or the transition zone
slabs Slabs Over- ridden oceanic plates, mantle fluxed by slab volatiles 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.
EXTENSIONAL STRESS EXTENSIONAL STRESS Surface hotspots correlate with ridges & ridge-like mantle structure & with extensionanal stress
Only ~3 hotspots are not near yellow/red. All LIPs backtrack to red. STATISTICS ~100% of hotspots fall in LVAs of the upper mantle, mostly those associated with ridges, & in regions of extension
If the upper and lower mantles correlate, this is why…
CAN BOTH UPPER MANTLE & LOWER MANTLE BE COOLED BY LONG-LIVED FLAT (STAGNANT) SLABS? Cold slab European, African, Asian (Changbai), Yellowstone & most continental “hotspots” are underlain by slabs Cooled mantle