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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
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
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
CTO Annual Meeting, Nov. 8, 2006
Carl Tape
Predicted
Observed
CTO Annual Meeting, Nov. 8, 2006
Study of the Japan slab structure
Min Chen
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
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
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
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
CTO Annual Meeting, Nov. 8, 2006
Laura Baker & Paula Smith
QuickTime™ and aBMP decompressor
are needed to see this picture.
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
CTO Annual Meeting, Nov. 8, 2006
5-20 km
200 km
Laura Baker & Paula Smith
CTO Annual Meeting, Nov. 8, 2006
Subduction zone evolution and low viscosity wedges and channels
Vlad Manea
CTO Annual Meeting, Nov. 8, 2006
Low Viscosity Wedges (LVW) versus No LVW
Vlad Manea
CTO Annual Meeting, Nov. 8, 2006
Vlad Manea
CTO Annual Meeting, Nov. 8, 2006
Low viscosity channels (LVC) have much the same effects as LVW
Vlad Manea
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.
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
CTO Annual Meeting, Nov. 8, 2006
Extra Slides
CTO Annual Meeting, Nov. 8, 2006
Carl Tape
CTO Annual Meeting, Nov. 8, 2006
Predicted
Observed
Carl Tape
CTO Annual Meeting, Nov. 8, 2006
Study of the Japan slab structure
From Min Chen
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
CTO Annual Meeting, Nov. 8, 2006
Vlad Manea