LOCALIZATION MECHANISMS IN PLANETARY LITHOSPHERES

The Earth differs from the other telluric planets in its unique plate tectonics mode of mantle convection. Plate tectonics require a combination of strong internal heat generation and localizing lithosphere rheology. As the Earth and Venus have similar size and overall composition, the difference between these planets must reside in the lithosphere rheology.

The drier and hotter lithosphere environment of Venus probably explains why localization is less likely than on Earth. Here, I review the localization mechanisms most commonly proposed for conditions below the seismogenic zone and show that each one of them is less efficient under Venus-like conditions.

Shear heating requires low temperature and high activation energy of the dominant deformation mechanism. This limits its usefulness as a localization-inducing phenomenon to mantle rocks at temperatures less than 400°C. Grain size reduction is limited to high stresses or strength contrasts that appear unlikely on Venus. Foliation development and metamorphism, which are the most efficient mechanisms for terrestrial conditions, work best if there is a high strength contrast between the various mineral that compose the rock or if the reaction product is very weak. This essentially implies the presence of hydrated phases or mica in the shear zones, which are unlikely on Venus.

High temperature and the absence of water reduce the efficiency of all of these potential localization mechanisms to the point that they may not be expected to play an important role in the Venusian lithosphere. Localization on Venus appears possible only if deformation and melting occur simultaneously, limiting its likelihood to rifting environments, which are indeed more Earth-like than other Venusian tectonic features.