An integrated view of subduction zones from geochemistry, seismology, and dynamics

CTO Annual Meeting, Nov. 8, 2006

An integrated view of subduction zones from

geochemistry, seismology, and

dynamics

Reported by Mike Gurnis


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


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


Carl Tape

Predicted

Observed


Study of the Japan slab structure

Min Chen



• 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


QuickTime™ and aBMP decompressor

are needed to see this picture.


FD model of SH-waves

Min Chen


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


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



QuickTime™ and aBMP decompressor

are needed to see this picture.


Southeastern Costa Rica Subduction Zone: 90.0 mm/yr convergence rate, 30 degreeslab dip, 15 Ma slab thermal age, 30 km over-riding lithosphere



5-20 km

200 km



Subduction zone evolution and low viscosity wedges and channels

Vlad Manea


Low Viscosity Wedges (LVW) versus No LVW

Vlad Manea


Vlad Manea


Low viscosity channels (LVC) have much the same effects as LVW

Vlad Manea


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.


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


Extra Slides


Carl Tape


Predicted

Observed

Carl Tape



From Min Chen


Central Costa Rica Subduction Zone: 87.0 mm/yr convergence rate, 45 degreeslab dip, 18 Ma slab thermal age, 30 km over-riding lithosphere



Vlad Manea

An integrated view of subduction zones from geochemistry, seismology, and dynamics

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