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.