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CTO Annual Meeting, Nov. 8, 2006
An integrated view of subduction zones from
geochemistry, seismology, and
dynamics
Reported by Mike Gurnis
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CTO Annual Meeting, Nov. 8, 2006
Emerging threads from CTO studies of subduction
• Observational controls on slab dip, Carl Tape, M. Gurnis, H. Kanamori & M. Simons
• Evidence for a Low Velocity Layer above the Japan slab, Min Chen, J. Tromp, D. Helmberger & H. Kanamori
• Full mass and energy coupling in subduction modeling, Laura Baker, Paula Smith, P. Asimow & M. Gurnis
• Subduction zone evolution and low viscosity wedges and channels, Vlad Manea & M. Gurnis
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CTO Annual Meeting, Nov. 8, 2006
Controls on slab dip inferred from subduction zone parameters
• Simple view of plate forces predicts that slab dip should increase with plate age (A) and that dip should decrease with convergence velocity (Vcmp)
• Much of our understanding harks back to Jarrard (1986), a multiple linear regression analysis that has been missing in recent studies
• A new analysis has been needed:– more, revised data (plate velocities, age of slabs, age of
subduction zones)– Multiple linear regression
• The new multiple regression analysis suggests that:– For intermediate depths (<125km), dip is controlled by A, over-
riding plate type, distance to boundary edge, and the age of the subduction zone, but not Vcmp.
– For deep depths (>125km), dip is controlled by Vcmp and the age of the subduction zone, but not A.
Carl Tape
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CTO Annual Meeting, Nov. 8, 2006
Carl Tape
Predicted
Observed
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CTO Annual Meeting, Nov. 8, 2006
Study of the Japan slab structure
Min Chen
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CTO Annual Meeting, Nov. 8, 2006
Study of the Japan slab structure
• Japanese Hi-net array. High sensitivity of a wide & broad array with 600 3-component stations
• Compare waveforms to those calculated with many 2-D FDM and 3-D SEM
• Used 2 deep focus events with simple sources (589 km, Mw=6.4; 492 km, Mw=6.1)
• Waveforms from tomography models or simple tabular, high velocities slabs do not produce prominent secondary arrivals
Min Chen
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CTO Annual Meeting, Nov. 8, 2006
QuickTime™ and aBMP decompressor
are needed to see this picture.
Study of the Japan slab structure
FD model of SH-waves
Min Chen
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CTO Annual Meeting, Nov. 8, 2006
Study of the Japan slab structure• Low Velocity Layer (LVL) above the slab produces
secondary arrivals• Polarity reversal of later arriving phases indicates a low
velocity waveguide• Waveform change is dependent on LVL depth; only
depths ~300km fit for all distances • Preferred LVL has -14% shear velocity reduction with a
thickness of 20 km• Tradeoffs between LVL thickness and velocity contrast• Deeper than 150 km, layer may be serpentinized
peridotite
Min Chen
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CTO Annual Meeting, Nov. 8, 2006
I. 2-D ConManvariable viscosity
thermal flow model solves conservation of mass, energy,
and momentum
II. pHMELTS (adiabat_1ph)
a thermodynamically-based melting and solid equilibration model that takes into account water in both hydrous and
nominally-anhydrous minerals, and in melts
pHMELTS: Asimow et al. (2004); based on pMELTS (Ghiorso et al., 2002)
and with the adiabat_1ph front-end by Smith & Asimow (2005)
achieved through an iterative, particle-based feedback mechanism:
full coupling between two separate models:
initial distribution of 40,000Lagrangian particles captures
steady-state thermal and velocityconditions
Laura Baker & Paula Smith
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CTO Annual Meeting, Nov. 8, 2006
Laura Baker & Paula Smith
QuickTime™ and aBMP decompressor
are needed to see this picture.
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CTO Annual Meeting, Nov. 8, 2006
Southeastern Costa Rica Subduction Zone: 90.0 mm/yr convergence rate, 30 degreeslab dip, 15 Ma slab thermal age, 30 km over-riding lithosphere
Laura Baker & Paula Smith
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CTO Annual Meeting, Nov. 8, 2006
5-20 km
200 km
Laura Baker & Paula Smith
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CTO Annual Meeting, Nov. 8, 2006
Subduction zone evolution and low viscosity wedges and channels
Vlad Manea
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CTO Annual Meeting, Nov. 8, 2006
Low Viscosity Wedges (LVW) versus No LVW
Vlad Manea
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CTO Annual Meeting, Nov. 8, 2006
Vlad Manea
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CTO Annual Meeting, Nov. 8, 2006
Low viscosity channels (LVC) have much the same effects as LVW
Vlad Manea
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CTO Annual Meeting, Nov. 8, 2006
If the weakening is confined to shallow depths, then slab dip decreases and leads to
flat lying subduction
Vlad Manea
QuickTime™ and aMicrosoft Video 1 decompressorare needed to see this picture.
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CTO Annual Meeting, Nov. 8, 2006
Some important points & ideas to pursue
• Slab dip is not simply controlled by plate age & convergence; subduction zone age may be even more important
• Low Velocity Layers may be a pervasive feature above slabs and the Japan LVL extends to ~300 km depth
• The coupling of thermodynamics and fluid dynamics suggest that Low Viscosity Channels (LVCs) can form above slabs through slab dehydration
• The strength of LVCs can change slab dip• The depth of mantle wedge weakening can lead to
divergent evolutionary pathways: Shallow LVCs can lead to flat slabs while deep LVW/LVCs to steep slabs
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CTO Annual Meeting, Nov. 8, 2006
Extra Slides
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CTO Annual Meeting, Nov. 8, 2006
Carl Tape
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CTO Annual Meeting, Nov. 8, 2006
Predicted
Observed
Carl Tape
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CTO Annual Meeting, Nov. 8, 2006
Study of the Japan slab structure
From Min Chen
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CTO Annual Meeting, Nov. 8, 2006
Central Costa Rica Subduction Zone: 87.0 mm/yr convergence rate, 45 degreeslab dip, 18 Ma slab thermal age, 30 km over-riding lithosphere
Laura Baker & Paula Smith
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CTO Annual Meeting, Nov. 8, 2006
Vlad Manea