Presentation on theme: "Constraints on the LAB from Seismology, Petrology and Geodynamics/Mineral Physics www.physicalgeography.net/ fundamentals/10h.html A. Bengston, M. Blondes,"— Presentation transcript:
Constraints on the LAB from Seismology, Petrology and Geodynamics/Mineral Physics www.physicalgeography.net/ fundamentals/10h.html 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
Understanding the nature of the lithosphere- asthenoshpere boundary (LAB) Partial melting in the asthenosphere Wet/damp asthenosphere Solid state anelastic effects Stixrude and Lithgow-Bertelloni 2005 Hirth and Kohlstedt 1996 Hypotheses
H 0 : The Asthenosphere results from solid-state anelasticity. H 1 : 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
Melting depths vs seismic lid (also dynamic topography and surface heat flow) Observed Q vs theoretical Q Geodynamics
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.
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.
Stixrude and Lithgow-Bertelloni, JGR 2005 Solid-State LVZ?
Tan&Helmberger(2007) Estimate Q models for LVZ under West Pacific Data Source: 30 events with intermediate depth
6 different Q models with PA5 as velocity model: Q30 Q50g Q50 (original PA5) Q70g Q70 Q90g Example of synthetic/observed seismograms with pa5_Q50 model PA5 velocity model
Preliminary Result: High Q in West Pacific? Test of data sensitivity to Q in LVZ Synthetic SS/S ratios, relative to Q50 More sensitive to Q Q30Q50gQ50Q70gQ70Q90g Residual Sum 126.96.36.199.77.47.8 Observed SS/S ratios relative to Q models
Non-Plume Intraplate Magmas near Japan Motivations Partial melting in asthenosphere or plume? Hirano et al., 2006
Pressure ~ MORB Temperature ~ MORB Consistent with plate model -- Not plume Inferred Pressure and Temperature
How to get the melt Up? Modified from Garcia-Castellanos 2000 Current stress pattern (fps) consistent with the model prediction Extension predicted by slab pull model The extension may facilitate the melt rising up
Viscous Radial Forces Acting on the Base of the Lithosphere ~ Dynamic Topography Moucha et al. (2008) P ref P lith ≠ P ref P ref =P lith E cc cc mm mm mm Residual Topography = Observed topography - Isostatic Elevation (E) m => constant, (P,TC) RefLith
Variations in Isostatic Elevation Depleted Mantle Density (kg/m 3 ) Isostatic Elevation - Mean Isostatic Elevation (meters) 63 km 45 km 30 km
Compositional and Thermal Constraints Average Mantle Density (kg/m^3) Residual Topography (meters)
L Z from dynamic rheology? flow law simple flow: slab plume use rheologic flow lax + simple flow = consistent computing strategy effective viscosity
generic dry oceanic system (dislocation creep) 60 Ma 1450 K solidus prediction: developed L Z without melt or water strain rate localization anisotropy maximized descends with age
generic dry continental system (dislocation creep) prediction: strong continental lithosphere pronounced L Z from solid state effects without melt or water solidusadiabat surface heat flow: 41 mW/m 2 crustal heat production: 0.6 W/m 2
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