On The Robustness of Asthenosphere Plug Flow in Mantle Convection Models
With Plate-Like Behavior
The question of what drives tectonic plates has been revitalized by
seismic observations that cannot be explained by conventional
plate-driving forces. The observations, designed to constrain flow in
the asthenosphere, are consistent with the asthenosphere locally flowing
faster than the plate above and in a direction offset from plate motion.
These inferences are not consistent with plates being driven exclusively
by slab-pull and/or ridge-push forces. Mantle convection models were put
forth to argue that pressure-driven flow, interacting with a
non-Newtonian upper mantle viscosity, could explain these observations.
To test the robustness of those results, we expand the models to allow
for the development of weak plate margins and associated plate-like
behavior. We find that with weak margins, the overall component of
slab-driven flow becomes stronger while pressure driven asthenosphere
flow remains active. Locally, the asthenosphere can lead plates and
there are rotations in the direction of asthenosphere flow with depth.
The balance of plate driving forces (i.e., the ratio of slab-pull to
asthenosphere flow) is found to depend on plate margin strength. The
models also indicate that a non-Newtonian upper mantle allows for a
hysteresis effect such that, depending on initial conditions,
single-plate and plate-tectonic modes can exist at the same parameter