[SIO GP Seminars] TODAY: 3:00 PM: Nick Harmon, IGPP

[Robin Matoza]

Geophysics Seminar Reminder-


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Friday, January 19, 3:00 PM
   (refreshments served at 2:45 PM)
   Munk Conference Room

													Nick Harmon, IGPP

   "Constraints on the Origin of the Cross-Grain Gravity Lineations  
in the
Pacific from the GLIMPSE and MELT Experiments"


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ABSTRACT
The GLIMPSE and MELT experiments near the southern East Pacific Rise
include the beginnings of the cross-grain gravity lineations that have
been mapped by satellite altimetry, as well as some of the volcanic
ridges that are associated with the lineations. We use shipboard,
multi-beam bathymetry to distinguish between free-air anomalies created
by dynamic warping of the surface topography by lithospheric stresses
and those associated with subsurface density anomalies. After
subtracting the attraction of the surface topography, the effects of
thickened crust beneath the ridges and the trends with seafloor age,
significant residual mantle Bouguer anomalies (rMBA) remain that are
inconsistent with the origin of the lineations through thermoelastic
bending and cracking of the lithosphere. We also measure P and S delays
for the GLIMPSE study area and perform Rayleigh wave tomography for the
combined GLIMPSE and MELT region. Changes in Rayleigh wave phase
velocities at periods of 33 s and less, sensitive to shear velocities in
the uppermost mantle, are dominated by the cooling of the oceanic
lithosphere, but at longer periods, there is a better correlation
between velocity and the rMBA than the age of the seafloor. The
correlation is significant at the 99% confidence level, with lateral
variations in velocity as large as 10% beneath 7 Ma seafloor at
asthenospheric depths. The gravity and seismic velocity anomalies show
that there are low densities and low velocities underlying the lows in
the free-air lineations, which is consistent with a small-scale
convection origin provided there is a relatively stiff, overlying plate.
Because the rMBA, seismic anomalies, and volcanism continue nearly all
the way to the East Pacific Rise, a simple origin from convective
instability of the lower lithosphere is not sufficient. There may be
associated compositional anomalies triggering convection or the
convection could be swept back toward the spreading center by return
flow in the asthenosphere.


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