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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

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does strong slab parallel flow exist in the mantle wedge

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

slide2

Kneller et al. (2005)

Lassak et al. (2006)

slide3

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)

slide4
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
slide5

Honshu

Nakajima and Hasegawa (2004)

Consistent with 2D corner flow

With B-fabric in wedge corner

slide6

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)

slide7

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

slide8

Kamchatka - Levin et al. (2004)

Arc-normal in wedge corner, arc-// beyond arc

Not consistent with melt-free 2D corner flow

slide9

Chile/Argentina

Anderson et al. (in prep.)

Arc-// beyond arc

Not consistent with simple 2D corner flow

slide13

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
slide14

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
slide15

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
slide19

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
slide20

2D corner flow

Cagnioncle et al. (2006)

slide21

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)

slide22

3D flow around slab edge

Kincaid et al. (2006)

slide23

No rollback

Trench parallel

Partial trench parallel

In cross section is corner flow

Kincaid et al. (2006)

slide24

Rollback:

No more

corner flow

Slab translates

Kincaid et al. (2006)

slide25

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)

slide26

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)

slide27

Upwellings or downwellings beneath arc

Behn and Hirth

(this meeting)

slide28

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)

slide29

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

slide30

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

slide32

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

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