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 17. Passive upwellings are broad & sluggish, to compensate for rapid downwellings. They are best sampled at ridges, where there is no pre-existing boundary layer. MORB Passive upwellings are broad & sluggish, to compensate for narrow fast downwellings

Top-Down Plate-Driven upwellings Broad passive upwellings Ridges capture upwellings (Marquart) Imposed surface velocity larger than max hor. Vel. In free convection case…ridge parallel walls rise below and off the ridge and develop instabilities in the 3d dimension 500-800 km spacing…high Ra instabilities of the lower BL in between walls develop somewhat later * shorter wavelength…but locations are controlled by locations of ridges…for large plate drive only the chain of ridge plumes survive… 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

LVA TZ STAGNANT SLABS–A FIXED REFERENCE FRAME REGION B WARM 410 650 Ridges & hotspots No hotspots REGION B LVA 410 WARM TZ 650 COLD COOL SLIP-FREE BOUNDARY

Broad depleted Ridge-feeding upwellings Fractionation & contamination Broad depleted Ridge-feeding upwellings 650 km 1000-2000 km

T seismic gradients imply subadiabaticity over most of the mantle 1600 1800 2000 K Vs Tp seismic gradients imply subadiabaticity over most of the mantle CONDUCTION REGION Dry lherzolite solidus 100 50 ppm H2O Depth km 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) 9. Wavespeed gradients imply subadiabaticty below ~200 km. The average temperature increase inferred from seismic gradients across the upper 200 km of the mantle is ~1530 K. This causes the negative seismic wavespeed gradient and the Gutenberg low velocity zone, as has long been known but often ignored. Thermodynamic inversion of seismological data show that there is at least a 200 C decrease in temperature between 200 km and the base of the transition zone…gradients of wavespeeds exceed those predicted for adiabatic self-compression of a homogeneous mantle for depths between 200 and > 350 km depth. (seismic gradients imply subadiabaticity over most of the mantle Bullen, Birch) Geotherm from seismic gradients SUBADIABATIC REGION 300 T Modified after Xu et al. 2011, GJI

Non fixed boundary track 200 km

~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 Range 15%; CMB range is 5%; -3% corresponds to CMB range; 25 out of 37 are in salmon (68%) 5 are outside (32 in)….26 in by recount; 7 max are out; 30 in(81%) ~3 hotspots are not near yellow/red. All LIPs backtrack to red.

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