1. GEO 5/6690 Geodynamics 03 Dec 2014 © A.R. Lowry 2014 Read for Fri 5 Dec: T&S 410-427 Last Time: Buoyancy (Tectosphere) vs Rheology The isopycnic hypothesis holds that buoyancy of melt-depleted cratonic lithosphere (tectosphere) prevents Rayleigh-Taylor instability, allows cooling to greater depths, offsets negative thermal buoyancy  neutral elevation, and stabilizes continental lithosphere. Supporting obs include: Xenolith evidence for geotherms, buoyancy under cratons High seismic velocity to 200-300 km depths However: Melt-depletion buoyancy from lab experiments is smaller than generally assumed and has a narrow depth range from ~100-200 km Uppermost mantle temperature from Pn does not vary as much as generally assumed Simulations  buoyancy doesn’t matter, but viscosity does!

2. Wang et al. EPSL 2014 Most-recent dynamical modeling suggests compositional buoyancy helps resist RT-instability but does not prevent it by itself… And no compositional buoyancy is needed to keep cratons stable if there is a 10x difference in compositional viscosity! No viscosity difference; yes chemical buoyancy  = 3x; yes chemical buoyancy  = 3x; no chemical buoyancy  = 10x; no chemical buoyancy

3. A bit more on geophysical LAB: It’s observed both in seismic and electrical conductivity structure (but are they the same thing?) Here, MT image suggests partial melt &/or free water under subducting Cocos plate. Naif et al. Nature 2012

4. There are as many (or more) candidate mechanisms and related processes as there are observations that relate to them… Leaving the answer somewhat nebulous. Eaton et al. Lithos 2009 Mierdel et al. Science 2007

5. (Levander & Miller, G 3, 2012) Negative Velocity Gradients in seismic impedance imaging (faster over slower) are called “lithosphere/asthenosphere boundary (LAB)”; but also “mid-lithosphere discontinuity (MLD)”… Here both shown blue. Interpreted as partial melt, water, hydrated layer, ?

6. Foster et al. EPSL 2013 WYNEIA This image shows MLD a bit better, along with its relation to seismic shear wave velocity and anisotropy… Which also may be mechanism for generating an impedance contrast…

7. (Levander & Miller, G 3 2012)(Lowry & Pérez-Gussinyé, 2011) Linear regression has correlation coefficient R = 0.7 !

8. T e sensitivity… Could L&M (2012) negative velocity gradients “sense”, e.g., anelasticity change near the base of the mechanical boundary layer? If so, the geotherms & lithologies that best-match P n should best-match LAB in similar locations!

9. Wet Quartz over Wet Olivine Dry Anorthite over Dry Olivine Mechanical boundary layer and Sp NVG are in the same vicinity… But if the NVG is a hydration front, the relationship to mean hydration is not simple.