Seismic and Rheological Behavior of the Asthenosphere Beneath Intraplate
Seamount Chains in the South Pacific

Dayanthie S. Weeraratne

Intraplate seamount chains and volcanic ridges are the surface expression of
the dynamic interaction between the oceanic asthenosphere and a mobile tectonic
plate. One class of these volcanic features seems to be associated with gravity
lineations west of the East Pacific Rise that are aligned in the direction of
Pacific plate motion. Rayleigh wave data from the recent GLIMPSE marine
expedition and new fluid dynamic experiments of Saffman-Taylor instabilities in
the oceanic asthenosphere are presented to investigate the origin and formaton
of these seamount chains. A tomographic inversion method is used which takes
advantage of a high volume of crossing ray paths and accounts for deviations
from the great circle paths by using a finite frequency approximation that
includes scattering effects in the incoming wavefield. Phase velocity data at
periods up to 100 s indicate anomalously low velocities are observed in the
upper mantle beneath the Sojourn Ridge, the Brown Ridge, and the Hotu Matua
volcanic complex with high velocities between the ridges. Shear wave velocity
inversions indicate sensitivity to the seismic low velocity zone (LVZ)
including resolution of the high velocity lithosphere and a steep positive
velocity gradient below the LVZ identifying the base of the asthenosphere.
Geophysical results from this study are compared with predictions from previous
models suggested for seamount formation including lithospheric extension, plate
bending, and small scale convection. A new model is presented from laboratory
fluid experiments which considers asthenospheric flow between an off-axis
mantle plume and a spreading ridge in the form of viscous fingering known as
Saffman-Taylor instabilities. Scaling relationships are developed which
consider rheological behavior, fingering length scales, and plate motion in the
context of the oceanic upper mantle. The viscous fingering model is consistent
with the seismic, petrologic and geochemical observations obtained in this area
and is an intuitively appealing mechanism to explain the formation of
intraplate seamount chains in the south Pacific. 

REFERENCE: Weeraratne, D. S., D. W. Forsyth, Y. Yang, and S. C. Webb (2007),
Rayleigh wave tomography beneath intraplate volcanic ridges in the South
Pacific, J. Geophys. Res., 112, B06303, doi:10.1029/2006JB004403.