The rheological parameters are the most important
parameters in the dynamics of the upper mantle. A comparison of
seismic and numerical modeling studies of the oceanic upper
mantle dynamics provides important, independent constraints
on the rheological parameters.
Recent surface wave tomography results show significant reheating
or reduced cooling between 70 Ma and 110 Ma for the Pacific lithosphere.
This correlates with the well-known observations of reduced topography
and enhanced heat flux at old seafloor relative to the half-space
cooling model. Sublithospheric small scale convection (SSC)
has been proposed to explain the observed topography and heatflow.
The process of SSC is largely controlled by gravitational instability of
the bottom part of lithosphere, which is determined by the rheology
of lithosphere and mantle.
We formulated 3D numerical models to simulate the SSC process. By
comparing the thermal structure of lithosphere above SSC to the
cooling halfspace model, we determine a 'lithospheric thermal age'
that may deviate from the real plate age due to SSC. Comparison of
numerical models of the SSC with a seismic surface wave
tomography model shows striking agreement, clearly supporting the
existence of SSC. This can also provide
independent constraints on the rheological parameters of the upper
mantle. Lower activation energy will lead to larger temperature
variation associated with SSC, which gives mechanical erosion of a
larger part of the lithosphere, an thus more thinning. This is again
reflected in the thermal age. Furthermore, dislocation creep
calculations show a much stronger thermal erosion than diffusion
creep does for the same activation energy, and therefore a much
larger effect on the thermal state of the lithosphere.
For diffusion creep, an activation energy of 120 kJ/mol or
lower shows a good fit to the seismic data. For dislocation
creep, much larger activation energy values, which are more in
agreement with laboratory measurements, fit the seismic
data best. This suggests that dislocation creep is the
dominant mechanism in upper mantle dynamics, and confirms
previous similar conclusions based of laboratory
experiments or seismic anisotropy.