Fabric Development , Shear Zone Formation , and the Possibility of Plate Tectonics on Earth

The Earth stands out in the solar system as the only planet with plate tectonics. This peculiarity is due not only to strong internal heating but also a lithosphere rheology characterized by brittle failure and localization, as a comparison with Venus clearly show. Localization in the ductile regime are manifested on Earth by ductile shear zones, which have proven difficult to explain theoretically. The difference with Venus may be the absence of these shear zones, which would prevent plate boundaries to form. Here, I show that the development of layer in a shearing material which is a hallmark of terrestrial shear zones, can produce enough weakening to enable localization, and therefore plate tectonics. However, the associated weakening is virtually inexitant in the absence of water. Therefore, localization on dry Venus is unlikely, except in settings undergoing partial melting. This explains why Venus displays Earth-like tectonics in rift zones but not in other tectonic settings, and no global plate tectonics. Ductile shear zones as a requirement for plate tectonics: A recent analysis by O’Neill et al. [1] showed that every planet and icy satellite is expected to be at or below the transition from stagnant to mobile lid convection. For the Earth to exceed this transition, i.e., to have plate tectonics, the coefficient of friction of the lithosphere must be reduced to ~0.15. Similarly low coefficients of friction appear also necessary to produce double-sided subduction [2]. Although O’Neill et al. [1] justify this modification based on the presence of free water, it is not clear that most fault have a such a low coefficient of friction [3]. Moreover, Solomatov [4] showed that the yield strength required to allow subduction is only 3 MPa. All these study rely on a significant reduction the strength of the lithosphere compared to standard models [5] to enable plate tectonic. Although in the laboratory, ductile rheology is, per definition, non-localized, there is ample evidence for ductile shear zones in the field [6]. Deformation is strongly enhanced in these shear zones, which requires a reduced strength compared to the host rocks [7]. In spite of decades of research [8], what causes this strength reduction remains uncertain as traditional mechanisms such as shear heating and grain size reduction are shown to be inefficient under natural lithospheric conditions [7]. Explaining shear zones has the double benefit that not only the strength of the lithosphere in deformation zones is reduced, but deformation is localized, as may be expected for a narrow plate boundary. Explaining the difference between the Earth and Venus may therefore hinge on identifying a process that enables shear zone formation on Earth, but would be inactive on Venus. I propose that the development of layers is such a process.