Does strong slab-parallel flow exist in the mantle wedge?

Thanks to: David Abt, Catherine Rychert, Mariela Salas, Laura Martin, Alexis Walker (Brown University)Geoff Abers, Laura Auger, Ellen Syracuse, Terry Plank (Boston University)J. Marino Protti, Victor Gonzalez(OVSICORI, Universidad Nacional)Wilfried Strauch, Pedro Perez, Allan Morales(INETER)MARGINS

Kneller et al. (2005)

Lassak et al. (2006)

Fischer et al. (2000)

Examine:• Local S splitting• Paths outside wedge corner

Where is strongly 3D flow required?

Terms:

2D = wedge corner flow coupled to surface plate motions

3D = strong slab-parallel flow

Local S splitting fast directions relative to arc strike

Region fore-arc beyond arc• Ryukyu (Long & van der Hilst, 2006) //• Cascadia (Currie et al., 2004) //• Honshu (Nakajima & Hasegawa, 2005) // normal• Aleutians (Yang et al., 1995) //• Izu Bonin (Anglin & Fouch, 2005) variable• N. New Zealand (Morley et al., 2006) // normal & N• Tonga (Smith et al., 2001) // rotation to normal• Marianas (Pozgay et al., in prep.) // rotation to normal• Alaska (Christensen & Abers, in prep.) normal //• Kamchatka (Levin et al., 2004) normal //• S. America (Polet et al., 2000) variable• S. America (Anderson et al., in prep.) // • Nicaragua/Costa Rica (Abt et al., in prep.) normal? // + complexity

HonshuNakajima and Hasegawa (2004)

Consistent with 2D corner flow With B-fabric in wedge corner

TongaSmith et al. (2001)

Arc-// in wedge corner, BUT gradual rotation to arc-normal in back-arcNot consistent with melt-free 2D corner flow

After Turner and Hawkesworth (1998)

Rose Diagrams - plotted at station

Spatial Averaging

Marianas - Pozgay et al. (in prep.)

Arc-// in wedge corner, but stays arc-// beyond arcNot consistent with melt-free 2D corner flow

Kamchatka - Levin et al. (2004)

Arc-normal in wedge corner, arc-// beyond arcNot consistent with melt-free 2D corner flow

Chile/ArgentinaAnderson et al. (in prep.)

Arc-// beyond arcNot consistent with simple 2D corner flow

Costa Rica & Nicaragua - TUCAN ExperimentAbt et al. (in prep.)

Costa Rica & Nicaragua - TUCAN ExperimentAbt et al. (in prep.)

Costa Rica & Nicaragua - TUCAN ExperimentAbt et al. (in prep.)

Inversion: • model: 70% single xtal olivine, 30% single xtal opx• parameters: olivine a-axis azimuth, plunge & strength • split waveform for each path in successive blocks• calculate synthetic splitting at surface• invert residuals (data - synthetic splitting) using iterative damped least-squares method

Inversion: • model: 70% single xtal olivine, 30% single xtal opx• parameters: olivine a-axis azimuth, plunge & strength • split waveform for each path in successive blocks• calculate synthetic splitting at surface• invert residuals (data - synthetic splitting) using iterative damped least-squares method

Inversion: • model: 70% single xtal olivine, 30% single xtal opx• parameters: olivine a-axis azimuth, plunge & strength • split waveform for each path in successive blocks• calculate synthetic splitting at surface• invert residuals (data - synthetic splitting) using iterative damped least-squares method

SKS splitting indicates additional arc-// fast anisotropy below and farther into back-arc

Hypotheses for anisotropy sampled by local S

Beyond arc:• 2D corner flow + melt fabric• 3D flow around slab edge (or tear)• Flow along slab driven by changes in slab dip• Upwelling/downwelling beneath arc (Behn & Hirth)

Fore-arc:• Direction controlled by flow +/- B-fabric• But watch for upper plate, slab contributions

2D corner flow

Cagnioncle et al. (2006)

2D corner flow + melt fabric

Cagnioncle et al. (2006)

Oriented melt with arc-// strike (melt LPO effects not required)

• Marianas, Tonga, C. America require broader melting zones

• C. America SKS?

3D flow around slab edge

Kincaid et al. (2006)

In cross section is corner flow

Trench parallel Partial trench parallel

Kincaid et al. (2006)

No rollback

Rollback:No more corner flow

Slab translates

Kincaid et al. (2006)

3D flow around slab edge

Challenge:

Need slab-// flow over 500 km from slab edge, close to slab- enhance with slab dip changes

- enhance with low viscosities in mantle wedge

Supported by:

Geochemical evidence for flow around corner

After Turner and Hawkesworth (1998)

Tonga Costa Rica/Nicaragua

Herrstrom et al. (1995), Abratis & Woerner (2001), Feigenson (2004) - signature of Galapagos hotspot

3D flow around slab edge

Mehl et al. (2003)

Challenge:

Need slab-// flow over 500 km from slab edge, close to slab- enhance with slab dip changes

- enhance with low viscosities in mantle wedge

Supported by:

Geochemical evidence for flow around corner

In situ LPO data from Talkeetna arc

Upwellings or downwellings beneath arc

Behn and Hirth(this meeting)

Upwellings or downwellings beneath arc

Behn and Hirth(this meeting)

• Hard to match width of arc-// fast zone

• May explain 3D variations in anisotropy resolved in C. America

Feedbacks

Broader melt zones required in flow, T, melting models

If anisotropy = 2D corner flow + melt

If anisotropy = flow parallel to slab

Need 3D flow, T, melting models

velocity & attenuation images

Feedbacks

Broader melt zones required in flow, T, melting models

If anisotropy = 2D corner flow + melt

If anisotropy = flow parallel to slab

Need 3D flow, T, melting models

velocity & attenuation images

Marianas

C. America

Tonga

Conder and Wiens (2006)

Feedbacks

Broader melt zones required in flow, T, melting models

If anisotropy = 2D corner flow + melt

If anisotropy = flow parallel to slab

Need 3D flow, T, melting models

velocity & attenuation images

V, Q (T, volatiles, melt, grain size, dislocations)

V, Q (T, volatiles, melt, grain size, dislocations)