The Effect of Composition on Shear Localization in Planetary Lithospheres

Earth's particular style of plate-tectonics—characterized by localized deformation along dynamic plate boundaries and long-lived stable plate interiors—appears to be unique among rocky objects in the solar system. However, it is entirely unknown how common plate tectonics and related lithospheric phenomena are among the vast population of exoplanets discovered astronomically or assumed to exist throughout the Universe. In this study, we explore the effect of planetary composition on mylonitization—a set of microphysical processes that is commonly associated with shear localization and plate boundary deformation on Earth. A model for planet compositions, based on stellar spectroscopy, is used to define a plausible range of theoretical mineral abundances in the mantles of rocky Earth-sized exoplanets. These mineral abundances, along with experimental rock rheology, are used to model microphysical evolution with two-phase mixing. The model is then used to determine the effect of composition on the time-scales for shear zone formation. We demonstrate that lithospheres composed of sub-equal proportions of two mineral phases will form shear zones over relatively short time-scales, a more favorable condition for forming Earth-like plate boundaries. In contrast, lithospheres that are nearly monomineralic may require unrealistically long time-scales to form plate boundary shear zones. Using this approach, we identify specific nearby stars with the optimal range of compositions to be targeted by future astronomical missions, including the Habitable Worlds Observatory.