Flat slabs in convergent regions Top-down tectonics Broad passive upwellings (sometimes with an island on top)

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

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

Sheared, anisotropic boundary layer

MOVING PLATE For all practical purposes any magma source below about 150 km depth can be regarded as “fixed”. This is well within the surface boundary layer as defined by anisotropy & heterogeneity. On the other hand, sources near the core-mantle free-slip boundary are highly mobile. 220 km LID LVZ REGION B

Passive upwelling balances subduction flux and is captured by spreading ridge active passive

French et al. SCIENCE “ Plumes” are >1000 km broad, therefore PASSIVE

Nature

Intraplate volcanoes in China & Korea, inboard of the Japan Trench (JT) Yellowstone, Snake River Plains, Karoo, Deccan… occur in similar terranes.

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 No hotspots LVA STAGNANT SLABS–A FIXED REFERENCE FRAME SLIP-FREE BOUNDARY

Free-slip Slip-free 60 Myr later start Non-fixed non- vertical upwellings

200 km Non fixed boundary track

Tomographic View on Western Mediterranean Geodynamics W. Spakman and M.J.R. Wortel

RIDGETRENCH OIB BAB

EXTENSIONAL STRESS EXTENSIONAL STRESS

~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

MORGAN PHIPPS MORGAN (MPM) MODIFIED PLUME MODEL (Plumes feed hotspots & ridges) …but passive upwellings may feed both ridges & hotspots. e.g.

RIDGE FEEDING UPWELLINGS (RFUs) RFU LLAMA TZ

Modified Plume Model (Morgan Phipps Morgan) PGZ

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

CLUSTER ANALYSIS (Lekic et al.) Most hotspots & backtracked LIPs are in ridge-like upper mantle structures (maroon). Note age contours.

There are remarkable correlations of present LVAs in the upper mantle and the original sites of breakup of Pangea and the locations of triple junctions and new plate boundaries in the Pacific hemisphere. Ridges, hotspots, LIPs, kimberlites….all backtrack to plate tectonic features associated with plate boundaries and spreading. These features correlate with upper mantle tomography with much higher statistical significance than with D”.

The low velocity zones (LVAs) associated with present day ridges are in the same places as they were when Pangea broke up & the antipodal Pacific plates reorganized & oceanic plateaus erupted. The surface expressions of ridges migrate but only within the confines of the ~2000 km LVAs associated with ridges at km depth. Hotspots & LIPs backtrack to these same ridge-related regions. Plates & D” are highly mobile since they are next to inviscid boundaries. Why is there apparently a fixed reference frame in the “convecting mantle”?

Steep gradients are indications of subadiabatic gradients

Along-ridge profile Ridge-normal profile ridge R i d g e Ridge adiabat T RIDGE FEEDERS True intra-plate hotspots do not need deep feeders for their T & isotopes, but they may be near passive upwellings

Maggi et al., 2006, GJI Some ridge segments are underlain by “feeders” that can be traced to >400 km depth, particularly with anisotropic tomography (upwelling fabric) Ridges cannot represent ambient midplate or back-arc mantle WHERE DOES MORB COME FROM? RIDGES MAY HAVE DEEPER FEEDERS THAN OIB (still upper mantle) 6:1 vertical exaggeration Only ridge-related swells have such deep roots

Both the top and lower boundary layers are rapidly moving in any whole mantle convection scheme However, slabs at 650 lie on a viscid boundary and provide a fixed reference frame

200 Myr of oceanic crust accumulation TRANSITION ZONE (TZ) REGION B Super- adiabatic boundary layer Thermal max 600 km 300 km Tp decreases with depth 600 km Thus, the ‘new’* Paradigm (RIP) (* actually due to Birch, Tatsumoto, J. Tuzo Wilson) Shear strain “fixed” Hawaii source MORB source Shear-driven magma segregation Sources deeper than ~ km can be regarded as effectively fixed

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 Depth km CONDUCTION REGION Vs Dry lherzolite solidus 50 ppm H 2 O Tp Any point on a geotherm can be assigned a Tp (the surface projection of a hypothetical adiabat)

A gravitationally stable rock column These can only become buoyant if they partially melt These can eclogitize below 60 km Helium-rich carbonatites degas above 60 km, not deeper (e.g. SCLM)

Material displaced out of TZ by slabs is not the same as modern subducted material; it can be cold & ancient (e.g. high 3 He/ 4 He)

2% LVZ

Density is too low to be able to sink into lower mantle even when cold STABLE STRATIFICATION Transition Zone is source of midocean ridge basalts

Crust & upper mantle eclogite: garnet & pyroxene-rich fertile rock Seismic LID

All hotspots, ridges & backtracked LIPs occur in red & yellow regions LVZ Near base of thermal boundary layer (Region B)

PLATE Low-velocity zone Intra-plate magmas such as Hawaiian tholeiites are derived from the low-velocity zone (LVZ) part of the sheared surface boundary layer (LLAMA). They are shear-driven not buoyancy-driven. The upper 220 km of the mantle (REGION B) is a thermal, shear & lithologic boundary layer & the source of midplate magmas. 200 km FOZO 1600°C

Schuberth et al. (2009) Representative geotherms for whole mantle convection with hot core & constant CMB Temperature CMB superadiabatic subadiabatic adiabat Any point on these geotherms projected adiabatically to the surface yields its potential temperature A subadiabatic mantle is stable and will not convect unless sinking material displaces it.

Why is there a strong bias of volcanoes, both plate boundary and intraplate to equatorial & southern latitudes…& volcanism, tomography, gravity field…to degree 2 patterns (C20)…& alignment of most continents & geoid lows to a polar band? The non-hydostatic geoid, the ellipticity of the Earth, rotation, tidal breaking, tomography, at all levels distribution of volcanoes all are dominated by C 20 (Ray, DLA). Fluid dynamic & self compression effects force the mantle to convect at the longest possible wavelengths– the long wavelength parts of these features control the moments of inertia & the rotation axis of the Earth..regions of extension, including ridges, concentrate in low & southern latitudes…+30 to -45 degrees latitude…& hug the boundaries of large geoid highs. Plate reconstructions show that subduction repeatedly occurs along the same bands. Regions that are warmed from above by the insulating effects of large plates & not cooled from below by stagnant slabs tend the control the locations of divergence of plates; the colder regions control the locations of convergence & subduction. These effects also control the boundary conditions at the top of the lower mantle, topography & temperature. much of the long-wavelength geoid originates in the deep mantle, the dynamic topography appears to originate from density variations in the upper mantle… There are more than 1600 spherical harmonic coefficients used in modern global tomography but only one has any degree of correlation between the top & the bottom of the mantle (Anchor)…& most of the power is in degree 2 and 3.

The issue arises, why is there a strong concentration of volcanism in equatorial and southern latitudes…and of geoid lows, continents, island arc volcanism and subduction zones in a narrow polar band? [NewTOE FIG. 6.1]. Most of the power in the geoid and in tomographic models at all depths is in degree 2. This does not imply whole mantle convection since shorter waveslength features are not correlated but it does imply thermal, rotationall or gravitational control on long wavelength fetures in both the upper and lower mantles…volcanism, tomography, gravity field…to degree 2 patterns (C20)…and alignment of most continents and geoid lows to a polar band? The non-hydostatic geoid, the ellipticity of the Earth, rotation, tidal braking, tomography, at all levels distribution of volcanoes all are dominated by C20 (Ray, DLA). Fluid dynamic and self compression effects all force the mantle to convect at the longest possible wavelengths and the long wavelength parts of these features control the moments of inertia and the rotation axis of the Earth.. regions of extension, including ridges, concentrate in low and southern latitudes…+30 to -45 degrees latitude…and hug the boundaries of large geoid highs.

Plate reconstructions show that subduction repeatedly occurs along the same bands. Regions that are warmed from above by the insulating effects of large plates and not cooled from below by stagnant slabs tend the control the locations of divergence of plates; the colder regions control the locations of convergence and subduction. These effects also control the boundary conditions at the top of the lower mantle, topography and temperature. …much of the long-wavelength geoid originates in the deep mantle, the dynamic topography appears to originate from density variations in the upper mantle… More than 1600 spherical harmonic coefficients are used in modern global tomography but only one has any degree of correlation between the top and the bottom of the mantle (Anchor)…most of the power is in degree 2 and 3.

About 32 of the world’s active non-arc volcanoes or volcano clusters in the oceans occur in region O1 (19% of area) which roughly corresponds to the 20 Ma age contour. Only about 7 occur well away from the LVAs associated with spreading ridges and most of these are in oceanic region O2 (27% of area).

200 Myr of oceanic crust accumulation TRANSITION ZONE (TZ) REGION B Super- adiabatic boundary layer Thermal max 600 km 300 km Tp decreases with depth 600 km (RIP) (* actually due to Birch, Tatsumoto, J. Tuzo Wilson) “fixed” Hawaii source MORB source Shear-driven magma segregation Plate vs Plume

…you ought to have done a better job! -Ernest Rutherford “ The probability that 18…out of 24 randomly chosen points lie within the belts (23.5% of the CMB area) is about 1 in 7 million” ( p =1.47 ·10−7 ) …considered by authors to be “remarkable” CMB & backtracked LIPs 69

slabs Slabs Over- ridden oceanic plates, mantle fluxed by slab volatiles Pacific plateaux formed at boundaries & triple junctions of new plates

Region D” is the smallest & least accessible boundary layer It has received an inordinate amount of attention from seismologists & geochemists It has even been claimed to be the largest plausible source for Hawaii & Yellowstone It is next to the fluid core, meaning that it is a free-slip boundary

VS

Ultra-refractory harzburgite (stable) Eclogite (unstable) lherzolite SiO 2 -poor eclogite MORB-st eclogite (stable) LID TZ coldhot An oscillating rock group Oceanic crust Lower arc crust (metastable) 650 (cold) 410 (warm) subadiabatic LID

Cooled cooled + + Cold slab Maximum lateral density & thickness differences, thermal & lithological, at level of neutral buoyancy! Runaway ascent + (-) density excess (deficit) Large blobs of mafic low-melting materials, containing much of the mantle LIL (U, Th…) transiting between crust & TZ ECLOGITE ENGINE THE BLOB MODEL

Works with ambient mantle heat No TBL required Huge volume changes with little  T Cools off mantle Interior to plate tectonic cycle THE ECLOGITE ENGINE Working fluid:basalt/eclogite/magma Lubricant: carbonatites Paired volcanic provinces (heads:tails) CONTINENT or CONTINENTAL DRIP THEORY

temperature Vs Passive upwellings from below 200 km are cold! adiabats Vs

MORB LVZ LITHOSPHERE Ocean Island 220 km OIB UPDATE OF CLASSICAL PHYSICS-BASED PLATE MODELS (Birch, Hales, Elsasser, Uyeda, Hager…)* after Hirschmann 2010 Pepi *not Morgan, Schilling, Hart, DePaolo, Campbell… -200 C INSULATING LID

Actual anomalies are ~2.5 times larger

The pale yellow areas become slightly positive when a background reference model with slabs at 650 km is added back in and the blue regions above 650 become bluer. In contrast to global reference models, the actual Earth has a global LVZ between 600 and 650 km. Correcting to this new reference turns the red patches above 650 into pale blue or neutral patches & blue regions into flat slab like anomalies km

Relative wavespeeds

COLD WARM REGION B TZ Ridges & near-ridge hotspots COOL

Plate (conducting) Depth 1600 ToCToC GEOTHERMS illustrating the thermal bump and subadiabaticity (every point on these curves has a potential temperature= the adiabatically decompressed T at the surface) UPPER MANTLE -The highest potential temperature in the mantle is near 200 km -Tectonic processes (shear, delamination) are required to access this B LLAMA (shearing) Ridge adiabat D”D” CMB Depth LOWER MANTLE UPPER MANTLE TZ Midplate (& backarc) T ridge midplate Boundary layer (lithosphere) bump 200 km