Monday, November 27, 2017

I missed this one...

While doing a simple search, I ran across an abstract from 1977, authored by M. Kumazawa and Y. Fukao, in a conference Proceedings volume  
(emphasis is mine):

The physical properties of materials in the vicinity of the 650-km seismic discontinuity are studied based on (1) phase relations (solid-solid, and solid-liquid) inferred from recent high-pressure experiments; (2) possible distribution of mineralogical and chemical compositions; and (3) shear softening in γ-(Mg, Fe)2SiO4 and its effect on melting temperatures and flow properties. It is predicted that the horizon just above the 650-km discontinuity is a low-velocity zone, in particular in shear wave velocity, a layer of local low melting temperature, and a layer of the active chemical migration and fractionation. At the same time this horizon is also identified as a low-viscosity layer (lower LVZ).
These features lead to a concept of dual plate tectonics models. The layer between 200- and 550-km depths is sandwiched between two relatively soft layers (upper and lower LVZs) and is expected to behave as a rigid plate (mesoplate). The interaction of two groups of plates (lithoplate and mesoplate) is discussed as a particular type of convective motion in the mantle. The upper LVZ (100 − 200-km depth) is not a zone of counter flow of lithoplate motion, but is a shear zone decoupling the lithoplate from the mesoplate. The mesoplates are spreading away from the trench, where the lithospheric materials diffuse into the lower LVZ and the mesoplates. The diapirism for generating hot spots is supposed to originate from the lower LVZ just above the 650-km depth.

I so wish I had found this twenty years ago, while I was preparing my monograph, Geokinematics: Prelude to Geodynamics, for publication, or even forty years ago, one year into my Assistant Professorship at LSU. Kumazawa and Fukao's definition and usage is so very close to mine, even though they arrive at the concept from entirely different directions from mine. Compare the above with this