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John Woodhouse Symposium Oxford March 2014. Ridge crests occur above ~2000 km broad 3D passive upwellings… ’ hotspots ’ are secondary or satellite shear-driven upwellings. Near-ridge ‘ hotspots ’ sample deep & are coolish compared to midplate volcanoes. OIB. 1000-2000 km. MORB.

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John Woodhouse Symposium

Oxford

March 2014


Ridge crests occur above ~2000 km broad 3D passive upwellings…’hotspots’ are secondary or satellite shear-driven upwellings

Near-ridge ‘hotspots’ sample deep & are coolish compared to midplate volcanoes

OIB

1000-2000 km

MORB

Passive upwellings are broad & sluggish, to compensate for narrow fast downwellings


Broad passive upwellings upwellings…

Ridges capture upwellings (Marquart)

Top-Down Plate-Driven upwellings


In whole mantle convection simulations, both the surface & the core-mantle boundary move rapidly. Neither provides a stable reference system

FREE-SLIP BOUNDARY

http://mcnamara.asu.edu/content/educational/main/piles/2Dpiles.jpg


STAGNANT SLABS–A FIXED REFERENCE FRAME the core-mantle boundary move rapidly. Neither provides a stable reference system

Ridges & hotspots

No hotspots

REGION B

LVA

410

WARM

TZ

COLD

650

COOL

SLIP-FREE BOUNDARY


Fractionation & contamination the core-mantle boundary move rapidly. Neither provides a stable reference system

Broad depleted Ridge-feeding upwellings

650 km

1000-2000 km


Vs the core-mantle boundary move rapidly. Neither provides a stable reference system

Tp

seismic gradients imply

subadiabaticity over most

of the mantle

CONDUCTION REGION

Dry lherzolite

solidus

100

Depth km

50 ppm

H2O

Any point on a geotherm can be assigned a Tp

(the surface projection of a hypothetical adiabat)

Canonical 1600 K adiabat

Thermal bump region (OIB source)

Geotherm from seismic gradients

SUBADIABATIC REGION

300

T

1600

1800

2000 K

Modified after Xu et al. 2011, GJI


Non fixed boundary the core-mantle boundary move rapidly. Neither provides a stable reference system

track

200 km


STATISTICS ~100% of hotspots fall in LVAs of the upper mantle, mostly those associated with ridges, & in regions of extension

~3 hotspots are not near yellow/red. All LIPs backtrack to red.


50% of hotspots & 25% of LIPs formed >1000 km away from CMB mantle, mostly those associated with ridges, & in regions of extension“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


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