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slide1

Do hotspots correlate with– asthenospheric shear (Conrad et al., King), extensional stress (Favela, Lithgow-Bertelloni), plate architecture & anisotropy (LLAMA), the lower mantle (Burke & Torsvik), absence of subduction (Anderson); distance from cratons, distance from active tectonic regions, near ridge-like passive upwellings…plate divergence associated with upper mantle downwellings (Husson & Conrad 2014)?

Yes. Does this mean that all of the above correlate with each other? Of course not.

Don L. Anderson

2 May 2011 (March 2014)

hotspot correlations have been proposed with
“Hotspot” correlations have been proposed with:
  • Faults, FZs, edges
  • Plate Extension
  • Diverging flow lines
  • Lack of anisotropy
  • Absence of subduction, cratons, anisotropy
  • Asthenosphere shear
  • Low seismic velocity and average velocity
  • Gradients in one parameter or another
  • Transition zone thickness; depth of 650
  • Lower mantle
  • Far from tectonically active regions
  • Median value of wavespeed at CMB
continued
continued
  • Previous supercontinent positions
  • Duration of magmatism
  • Middle of LVZs
  • Edges of LVZs
  • No anomaly bands
  • Strong anisotropy
  • Absence of anisotropy
  • Gradients of some parameter
strong non correlations
Strong non-correlations
  • Transition zone (TZ) parameters (depths of 410 & 650; thickness)
  • Magma temperatures & TZ parameters
  • Wavespeeds at any depth
  • Helium content
  • Presence of any one of Courtillot’s criteria
  • Heatflow
slide6

Extensional regions (red) correlate with magmatism

(note that all volcanoes are in extensional regions, including on Nazca plate (JF,SF), Cameroon, Afar, Easter, Reunion, Atlantic etc.; also all ridges)

slide10

Ritsema & Allen

Edges of slabs?

slide11

All active hotspots & all reconstructed positions of LIPs lie above regions that did not experience Jurassic-Eocene subduction. LVsLs in the lower mantle fall below these regions

HOTSPOTS (CIRCLES) are where slabs are not

Predicted locations of slab material; correlations best @ 800km

slide12

The Degree 2 Paradox

THE TRANSITION REGION

Region B

“…the key to a number of geophysical problems…”Francis Birch 1952

Center

of LLAMA

350 km

Degree 2

Degree 2 pattern in TZ is due to past subduction

Region C

Upper & lower mantle regions cooled by subduction

Scale change

slide13

Long-lived slabs in the transition region cool off the top of the lower mantle. Stagnant long-lived blobs in lower mantle do the same to the upper mantle.

High velocity

Apparent continuity of tilted oS2 feature does not imply whole-mantle convection

slide15

IT IS CLEAR FROM BACKTRACKING OF EASTERN HEMISPHERE LIPS THAT ALL OF THEM FORMED FROM MANTLE THAT WAS UNDER SUPERCONTINENTS FOR LONG PERIODS OF TIME AND OVER REGIONS OF THE MANTLE THAT THEN BECOME SITES OF HOTPOT MAGMATISM AND OF SPREADING RIDGES. PACIFIC LIPS AND HOTSPOTS ALL STARTED ON RIDGES & TJs. Essentially all backtracked LIPs end up over today’s ridges.

slide16

110 km. The slowest [hottest?] regions follow the midoceanic ridges and subduction zones (backarc basins). Fig. 2 (middle). Shear velocity at a depth of 110 km. Total range in velocity is +-4.5%. Note the sinuous LVA following the Atlantic and Indian ocean ridges. Continental shields are very fast. Fig. 3 (bottom). Shear velocity at a depth of 210 km. Total range is +-4%. Note the sinuous LVAs in the Atlantic and Indian oceans, which are offset from the current ridges.

310 km

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