Dix Seismo Lab Seminar
Beno Gutenberg's Low Velocity Zone Revisited; the roles of anisotropy, adiabaticity, anelasticity, anharmonicity, artefacts, advection, absolute wavespeeds, asthenosphere and ambient mantle
When the canonical 1988 Cambridge geotherm, upon which much of mantle petrology is based, is corrected for physics and seismological oversights, both the possibility and need for deep mantle sources and narrow upwellings disappear. As Francis Albarede put it, this turns the rational behind canonical models of geodynamics and geochemistry upside down. In fact, surface cooled systems, including mantle dynamics, are driven by the downward sinking of the upper layers (The Up-Side Goes Down), accompanied by broad passive upwellings from the transition region, the region (410-650-km depth) that Francis Birch (1952) called the key to a number of geophysical problems. He also suggested that eclogite was concentrated in the transition region, which solves some of these problems. The canonical models of mantle geochemistry can be labeled the Down-Side Up hypothesis, which is appropriate for strongly bottom heated systems with constant properties, no secular cooling and no radioactivity. The importance of surface waves and other horizontal rays in eliminating seismological mirages is illustrated. A physics based Earth model, based on a broad spectrum of geophysical data, reconciles geochemical, petrological and seismological data. But it is important to use surface waves, waveforms, scattered waves, absolute velocities and anisotropy as well as teleseismic arrivals.
http://www.mantleplumes.org/LLAMA.html, and Physics Today, October 2012, pages 10-12.