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


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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?

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Does strong slab parallel flow exist in the mantle wedge l.jpg

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 l.jpg

Kneller et al. (2005)

Lassak et al. (2006)


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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)


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Local S splitting fast directions relative to arc strike

Region fore-arcbeyond 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


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Honshu

Nakajima and Hasegawa (2004)

Consistent with 2D corner flow

With B-fabric in wedge corner


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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)


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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


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Kamchatka - Levin et al. (2004)

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

Not consistent with melt-free 2D corner flow


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Chile/Argentina

Anderson et al. (in prep.)

Arc-// beyond arc

Not consistent with simple 2D corner flow


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Costa Rica & Nicaragua - TUCAN Experiment

Abt et al. (in prep.)


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Costa Rica & Nicaragua - TUCAN Experiment

Abt et al. (in prep.)


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Costa Rica & Nicaragua - TUCAN Experiment

Abt et al. (in prep.)


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  • 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


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  • 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 l.jpg

  • 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


Slide18 l.jpg

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


Slide19 l.jpg

  • 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 l.jpg

2D corner flow

Cagnioncle et al. (2006)


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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)


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3D flow around slab edge

Kincaid et al. (2006)


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No rollback

Trench parallel

Partial trench parallel

In cross section is corner flow

Kincaid et al. (2006)


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Rollback:

No more

corner flow

Slab translates

Kincaid et al. (2006)


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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 l.jpg

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 l.jpg

Upwellings or downwellings beneath arc

Behn and Hirth

(this meeting)


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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 l.jpg

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 l.jpg

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


Slide31 l.jpg

Conder and Wiens (2006)


Slide32 l.jpg

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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