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

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

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

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  1. An integrated view of subduction zones from geochemistry, seismology, and dynamics Reported by Mike Gurnis

  2. 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

  3. 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

  4. Observed Predicted Carl Tape

  5. Study of the Japan slab structure Min Chen

  6. 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

  7. Study of the Japan slab structure FD model of SH-waves Min Chen

  8. 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

  9. pHMELTS: Asimow et al. (2004); based on pMELTS (Ghiorso et al., 2002) and with the adiabat_1ph front-end by Smith & Asimow (2005) full coupling between two separate models: 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 I. 2-D ConMan variable viscosity thermal flow model solves conservation of mass, energy, and momentum achieved through an iterative, particle-based feedback mechanism: initial distribution of 40,000 Lagrangian particles captures steady-state thermal and velocity conditions Laura Baker & Paula Smith

  10. Laura Baker & Paula Smith

  11. Southeastern Costa Rica Subduction Zone: 90.0 mm/yr convergence rate, 30 degree slab dip, 15 Ma slab thermal age, 30 km over-riding lithosphere Laura Baker & Paula Smith

  12. 200 km 5-20 km Laura Baker & Paula Smith

  13. Subduction zone evolution and low viscosity wedges and channels Vlad Manea

  14. Low Viscosity Wedges (LVW) versus No LVW Vlad Manea

  15. Vlad Manea

  16. Low viscosity channels (LVC) have much the same effects as LVW Vlad Manea

  17. If the weakening is confined to shallow depths, then slab dip decreases and leads to flat lying subduction Vlad Manea

  18. 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

  19. Extra Slides

  20. Carl Tape

  21. Observed Predicted Carl Tape

  22. Study of the Japan slab structure From Min Chen

  23. Central Costa Rica Subduction Zone: 87.0 mm/yr convergence rate, 45 degree slab dip, 18 Ma slab thermal age, 30 km over-riding lithosphere Laura Baker & Paula Smith

  24. Vlad Manea

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