Rheological evolution of oceanic lithosphere: current status and future challenges

Plate tectonics is what makes the Earth a unique planet in the solar system, but the physics of this particular style of mantle convection is still largely unresolved. We still do not know, for example, under what conditions plate tectonics takes place on a terrestrial planet. The inability to answer this basic question creates a considerable bottleneck in developing the general theory of planetary evolution; such a theory would be extremely valuable for ongoing exoplanetary research, or more broadly speaking, our quest for the origins of life in the universe. As the defining characteristic of plate tectonics is the subduction of oceanic lithosphere, one promising direction is to improve our understanding of the oceanic upper mantle. The oceanic upper mantle is perhaps the best understood part of the Earth's mantle. To first order, its evolution is expected to be a simple function of seafloor age, and its chemical state is generally considered to be relatively homogeneous. The rheology of olivine, which is the dominant phase of the upper mantle, has been studied extensively. However, if we try to quantify how surface environment is controlled by mantle dynamics, it becomes evident that there exist a few major gaps in our understanding of the oceanic upper mantle. In this contribution, I will summarize available constraints on the rheology of the upper mantle materials, a good understanding of which is indispensable when discussing the dynamics of the oceanic upper mantle, and provide some suggestions for future research in relevant disciplines, including marine geophysics, rock mechanics, and computational geodynamics.