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

1. 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

2. Kneller et al. (2005) Lassak et al. (2006)

3. Where is strongly 3D flow required? Terms: 2D = wedge corner flow coupled to surface plate motions 3D = strong slab-parallel flow Examine: • Local S splitting • Paths outside wedge corner Fischer et al. (2000)

4. 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

5. Honshu Nakajima and Hasegawa (2004) Consistent with 2D corner flow With B-fabric in wedge corner

6. Tonga Smith et al. (2001) Arc-// in wedge corner, BUT gradual rotation to arc-normal in back-arc Not consistent with melt-free 2D corner flow After Turner and Hawkesworth (1998)

7. Marianas - Pozgay et al. (in prep.) Arc-// in wedge corner, but stays arc-// beyond arc Not consistent with melt-free 2D corner flow Spatial Averaging Rose Diagrams - plotted at station

8. Kamchatka - Levin et al. (2004) Arc-normal in wedge corner, arc-// beyond arc Not consistent with melt-free 2D corner flow

9. Chile/Argentina Anderson et al. (in prep.) Arc-// beyond arc Not consistent with simple 2D corner flow

10. Costa Rica & Nicaragua - TUCAN Experiment Abt et al. (in prep.)

11. Costa Rica & Nicaragua - TUCAN Experiment Abt et al. (in prep.)

12. Costa Rica & Nicaragua - TUCAN Experiment Abt et al. (in prep.)

13. 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

14. 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

15. 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

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

19. 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

20. 2D corner flow Cagnioncle et al. (2006)

21. 2D corner flow + melt fabric Oriented melt with arc-// strike (melt LPO effects not required) • Marianas, Tonga, C. America require broader melting zones • C. America SKS? Cagnioncle et al. (2006)

22. 3D flow around slab edge Kincaid et al. (2006)

23. No rollback Trench parallel Partial trench parallel In cross section is corner flow Kincaid et al. (2006)

24. Rollback: No more corner flow Slab translates Kincaid et al. (2006)

25. 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 Tonga Costa Rica/Nicaragua Herrstrom et al. (1995), Abratis & Woerner (2001), Feigenson (2004) - signature of Galapagos hotspot After Turner and Hawkesworth (1998)

26. 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 In situ LPO data from Talkeetna arc Mehl et al. (2003)

27. Upwellings or downwellings beneath arc Behn and Hirth (this meeting)

28. Upwellings or downwellings beneath arc • Hard to match width of arc-// fast zone • May explain 3D variations in anisotropy resolved in C. America Behn and Hirth (this meeting)

29. Feedbacks Broader melt zones required in flow, T, melting models If anisotropy = 2D corner flow + melt velocity & attenuation images Need 3D flow, T, melting models If anisotropy = flow parallel to slab

30. Feedbacks Broader melt zones required in flow, T, melting models If anisotropy = 2D corner flow + melt Marianas velocity & attenuation images C. America Tonga Need 3D flow, T, melting models If anisotropy = flow parallel to slab

31. Conder and Wiens (2006)

32. Feedbacks Broader melt zones required in flow, T, melting models If anisotropy = 2D corner flow + melt V, Q (T, volatiles, melt, grain size, dislocations) velocity & attenuation images V, Q (T, volatiles, melt, grain size, dislocations) Need 3D flow, T, melting models If anisotropy = flow parallel to slab