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John Woodhouse Symposium Oxford March 2014

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

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  1. John Woodhouse Symposium Oxford March 2014

  2. 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

  3. Broad passive upwellings Ridges capture upwellings (Marquart) Top-Down Plate-Driven upwellings

  4. 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

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

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

  7. Vs 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

  8. Non fixed boundary track 200 km

  9. 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.

  10. 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

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