| Understanding the details of plate failure and the initiation of subduction remains a challenge
due to the complexity of mantle rocks. We carried out experiments on convection in aqueous
colloidal dispersions heated from below, and dried and cooled from above. The rheology of
these fluids depends strongly on solid particle fraction fp, being Newtonian at low fp, and
presenting memory, yield stress, elasticity, and brittle properties as fp increases. Such a
behaviour is analogue to the rheology of mantle rocks as temperature decreases. When drying
is sufficiently rapid in the laboratory, a visco-elasto-plastic skin ("lithosphere")
forms on the fluid surface. Depending on its rheology, and on the different scales
of convection existing in our laboratory mantle, we observed different modes of
one-sided subduction initiation. However, not all of them lead to continuous plate
tectonics. If subduction is definitely a necessary condition for plate tectonics, it is not
sufficient.
Amongst the different modes of subduction initiation, we observed two of them where
one-sided subduction was induced by the impingement of a hot plume under the skin, the
trench being localized on the rim of the plume impingement zone under the lithosphere. Then
depending on the lithospheric rheology, the nascent subduction can then either stop as the
result of subducted plate necking, or continue to sink smoothly. Due to the brittle
character of the skin, the subduction trench will never describe a complete circle,
but several tears and/or transform faults will develop as subduction and roll back
proceed.
Inspection of the geological record on Earth suggests that such a strong association
between plumes and subduction may have been instrumental in the nucleation and growth of
cratons, the onset of continuous plate tectonics, and present-day initiation of subduction
around some large oceanic plateaus. |