John Woodhouse Symposium

OxfordMarch 2014

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

Top-Down Plate-Driven upwellings

Ridges capture upwellings (Marquart)

Broad passive upwellings

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

COLD

WARM

REGION B

TZ

Ridges & hotspots

COOL

410

650

No hotspots

LVA

STAGNANT SLABS–A FIXED REFERENCE FRAME

SLIP-FREE BOUNDARY

Broad depleted

Ridge-feeding upwellings

Fractionation & contamination

1000-2000 km 650 km

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)

200 km

Non fixed boundary

track

~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

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