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Constraints on the LAB from Seismology, Petrology and Geodynamics/Mineral Physics

Constraints on the LAB from Seismology, Petrology and Geodynamics/Mineral Physics.

bethany-ellison
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Constraints on the LAB from Seismology, Petrology and Geodynamics/Mineral Physics

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  1. Constraints on the LAB from Seismology, Petrology and Geodynamics/Mineral Physics A. Bengston, M. Blondes, M. Collier, J. Gaherty, T. Höink, M. Jiang, E. Kite, C.-T. Lee, A. Levander, J. Li, Q. Li, P. Luffi, M. Manga, M. Miller, J. Naliboff, T.-L. Tseng, D. Weeraratne, Y. Xu, T. Yano, Z. Yang, Y. Zhang www.physicalgeography.net/ fundamentals/10h.html

  2. Understanding the nature of the lithosphere-asthenoshpere boundary (LAB) Hypotheses Solid state anelastic effects Wet/damp asthenosphere Partial melting in the asthenosphere Stixrude and Lithgow-Bertelloni 2005 Hirth and Kohlstedt 1996

  3. H0: The Asthenosphere results from solid-state anelasticity. H1: The Asthenosphere is partially molten. Establish reference model for solid state (anharmonicity and anelasticity ) ? Refine estimates of Q beneath ocean basins = Petrologic constraints on the origin depths of magmas ? Seismic constraints on depth of LVZ = Geodynamics of a low viscosity channel (solid state creep reference) Dynamic Topography Modeling

  4. Melting depths vs seismic lid (also dynamic topography and surface heat flow) Observed Q vs theoretical Q Geodynamics

  5. Testing LVZ Hypotheses with Thermodynamically Calculated Seismic Velocities and Estimates of Q Input (P,T,C) Calculate equilibrium phase assemblages & elastic constants Test null hypothesis by comparing calculated seismic velocities with Q corrections to seismic observations.

  6. Null Hypothesis for LVZ • For a given composition and temperature, solid-state anhydrous processes can explain the low-velocity zone observed in some regions beneath the lithosphere. • Solid-state processes: • Attenuation related to anelasticity • Seismic anisotropy related to solid-state dislocation creep. • Estimates of attenuation in the upper mantle: • Romanowicz (1995)*, Faul and Jackson (2005), this group.

  7. Solid-State LVZ? Stixrude and Lithgow-Bertelloni, JGR 2005

  8. Estimate Q models for LVZ under West Pacific Tan&Helmberger(2007) Data Source: 30 events with intermediate depth

  9. Example of synthetic/observed seismograms with pa5_Q50 model 6 different Q models with PA5 as velocity model: Q30 Q50g Q50 (original PA5) Q70g Q70 Q90g PA5 velocity model

  10. More sensitive to Q Test of data sensitivity to Q in LVZ Synthetic SS/S ratios, relative to Q50 Observed SS/S ratios relative to Q models Preliminary Result: High Q in West Pacific?

  11. Japan

  12. Non-Plume Intraplate Magmas near Japan Motivations Partial melting in asthenosphere or plume? Hirano et al., 2006

  13. Inferred Pressure and Temperature Pressure ~ MORB Temperature ~ MORB Consistent with plate model -- Not plume

  14. Modified from Garcia-Castellanos 2000 How to get the melt Up? Current stress pattern (fps) consistent with the model prediction Extension predicted by slab pull model The extension may facilitate the melt rising up

  15. Western USA

  16. Teleseismic S wave 59 events 556 stations

  17. SRF vs PRF Sdp Moho LAB Pds Moho LAB

  18. Latitude 37 deg Moho LAB Longitude -119 deg Moho LAB Sierra drip Zandt Nature 2004

  19. Basalt whole rock data from NAVDAT database Black: all data Red: most likely to be unaffected by petrologic complexity 1) likely not highly modified 2) likely saturated only in olivine

  20. Viscous Radial Forces Acting on the Base of the Lithosphere ~ Dynamic Topography Ref Lith E c Moucha et al. (2008) c Residual Topography = Observed topography - Isostatic Elevation (E) m m m m => constant, (P,TC) Pref Pref=Plith Plith≠ Pref

  21. Variations in Isostatic Elevation Isostatic Elevation - Mean Isostatic Elevation (meters) 63 km 45 km 30 km Depleted Mantle Density (kg/m3)

  22. Compositional and Thermal Constraints Residual Topography (meters) Average Mantle Density (kg/m^3)

  23. LZ from dynamic rheology? use rheologic flow lax + simple flow = consistent computing strategy flow law simple flow: effective viscosity plume slab

  24. generic dry oceanic system (dislocation creep) solidus • prediction: • developed LZ • without melt or water • strain rate localization • anisotropy maximized • descends with age 60 Ma 1450 K

  25. generic dry continental system (dislocation creep) solidus adiabat • prediction: • strong continental lithosphere • pronounced LZ from solid state effects without melt or water surface heat flow: 41 mW/m2 crustal heat production: 0.6 W/m2

  26. The LAB is hot, weak, produces melt (at least in some places) and might be wet. A. Bengston, M. Blondes, M. Collier, J. Gaherty, T. Höink, M. Jiang, E. Kite, C.-T. Lee, A. Levander, J. Li, Q. Li, P. Luffi, M. Manga, M. Miller, J. Naliboff, T.-L. Tseng, D. Weeraratne, Y. Xu, T. Yano, Z. Yang, Y. Zhang

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