John Woodhouse Symposium Oxford March 2014
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Transcript of John Woodhouse Symposium Oxford March 2014
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John Woodhouse Symposium
OxfordMarch 2014
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Passive upwellings are broad & sluggish, to compensate for narrow fast downwellings
Ridge crests occur above ~2000 km broad 3D passive upwellings…’hotspots’ are secondary or satellite shear-driven upwellings
1000-2000 km
Near-ridge ‘hotspots’ sample deep & are coolish compared to midplate volcanoes
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Top-Down Plate-Driven upwellings
Ridges capture upwellings (Marquart)
Broad passive upwellings
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http://mcnamara.asu.edu/content/educational/main/piles/2Dpiles.jpg
In whole mantle convection simulations, both the surface & the core-mantle boundary move rapidly. Neither provides a
stable reference system
FREE-SLIP BOUNDARY
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COLD
WARM
REGION B
TZ
Ridges & hotspots
COOL
410
650
No hotspots
LVA
STAGNANT SLABS–A FIXED REFERENCE FRAME
SLIP-FREE BOUNDARY
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Broad depleted
Ridge-feeding upwellings
Fractionation & contamination
1000-2000 km 650 km
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1600 1800 2000 K
Canonical 1600 K adiabat
Geotherm from seismic gradients
SUBADIABATIC REGION
Thermal bump region (OIB source)
seismic gradients imply subadiabaticity over most of the mantle
Modified after Xu et al. 2011, GJI
T
Dep
th k
m
CONDUCTION REGION
Vs
100
300
Dry lherzolitesolidus
50 ppmH2O
Tp
Any point on a geotherm can be assigned a Tp(the surface projection of a hypothetical adiabat)
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200 km
Non fixed boundary
track
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~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
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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