Compositional effects on shear localization in planetary lithospheres

For planets to develop narrow, dynamic plate boundaries that resemble Earth’s, the rocks that make up the lithosphere must be able to localize deformation. Decades of field studies have shown that plate boundary deformation manifests as frictional faults at shallow depths and mylonitic ductile shear zones below the brittle-plastic transition, with individual strands as narrow as 10-100s of meters. The physical mechanisms that produce mylonites from a primary lithosphere are of considerable interest since it is presumably impossible to create or sustain Earth-like plate tectonics without them. Experimental studies demonstrate that the characteristic microstructures in mylonites form through the serial processes of dynamic recrystallization and phase mixing.  However, the rapidity with which this occurs depends on temperature, grain-size, and composition, and the volume fraction and viscosity contrast between constituent mineral phases. As such, the mineralogical composition of a rocky planet will determine whether the planet can (a) localize deformation, and (b) initiate and sustain Earth-like plate tectonics. This contribution will review experimental evidence for the onset of mylonitization and show the results of models that predict the time scales (and therefore ease) with which planets of different compositions can localize deformation.  Drawing on data from the Hypatia catalog of exoplanets, these models identify specific stars with exoplanets that may be most amenable to forming Earth-like plate tectonics.