EVIDENCE THAT RIDGES ARE FED FROM THE TRANSITION ZONE BUT NOT AS CONTINUOUS UPWELLING SHEETS. WHEN A TZ UPWELLING COMES UP DIRECTLY UNDER A RIDGE IT IS CALLED A RIDGE-CENTERED HOTSPOT AND IT IS MORB-LIKE. WHEN IT COMES UP UNDER A PLATE IT GETS CONTAMINATED BY THE SURFACE BOUNDARY LAYER AND IS CALLED A PLUME. MOST HOTSPOTS HAVE TOMOGRAPHIC ANOMLAIES THAT TERMINATE ABOVE 200 KM DEPTH. ONLY “NEAR- RIDGE” HOTSPOTS EXTEND DEEPER. THESE MAY ACTUALLY BE RIDGE FEEDERS. DON L. ANDERSON 8/2/12

Compositions of partial melts from G2 eclogite partial melting experiments. PERTERMANN M, HIRSCHMANN M M J. Petrology 2003;44: © Oxford University Press 2003; all rights reserved

Global Azimuthal Anisotropy in the Transition Zone Jeannot Trampert1 * and Hendrik Jan van Heijst2. Science 296, 1297 (2002); Surface wave dispersion measurements carry evidence of azimuthal anisotropy in the transition zone (410 to 650 km depth)...robust long-wavelength azimuthally anisotropic velocity structure...this anisotropy may result from aligned minerals, tilted laminated structures, or organized pockets of fluid inclusions. The anisotropy in the TZ is consistent with flow toward most midocean ridges and upward flow at shallower depths, consistent with the source depth for MORB being in the TZ.

DOI: /science Science 296, 1297 (2002); Jeannot Trampert and Hendrik Jan van Heijst Global Azimuthal Anisotropy in the Transition Zone Observed surface wave anisotropy is consistent with flow in the transition region from South America towards the S.Atlantic and EPR and from subduction zones towards the interiors of plates., from under North America toward the Atlantcic rise Flow appears to be along the Indian Ocean-Antarctic rise, and from Africa toward the SW Indian rise.. To explain observed transverse anisotropy in and below the transition zone, Karato (24) favors laminatedstructures (SPO). If these structures are tilted, azimuthal anisotropy could result. SPOof partial melt inclusions has been evoked to explained observed anisotropy in the lowermost mantle (26), but the presence of partial melt in the transition zone is not as likely (27)…one is tempted to infer that mantle flow should have some horizontal component in the transition zone. Shorter period surface wave modes generally indicate shallow plate scale flow away from ridges

600 km depth

“red” indicates vertical flow; note ridges.

upwellings

Note that on average sub-ridge structures are high wavespeed but they have an anisotropy that is consistent with upwelling…

140 Ma 40 Ma

Extensional regions (red) correlate with magmatism