THE IMPORTANCE AND ORIGIN OF ULTRAMYLONITES: AN EXPERIMENTAL PERSPECTIVE (Invited Presentation)

Ultramylonites—intensely deformed rocks with small grain sizes and well-mixed mineral phases—are found throughout the high-strain interiors of ductile shear zones, from the meter to plate boundary scale. In laboratory experiments and numerical simulations, coupled grain size reduction and phase mixing produce strain localization and long-lived rheological weakening. Thus, ultramylonites are thought to be an essential component of Earth-like plate tectonics, which requires long-lived, self-softening deformation. However, while the processes responsible for grain size reduction (e.g., dynamic recrystallization) are well understood, comparatively little is known about phase mixing mechanisms. This talk will examine a series of high-strain torsion experiments on poorly-mixed two-phase composites. Samples of calcite-anhydrite and calcite-fluorite were deformed in a Large Volume Torsion (LVT) apparatus at 500°C, 0.75–1.5 GPa confining pressure, and 10^-6–10^-4 s^-1 shear strain rate, to very large shear strains (γ > 50). Through microstructural (SEM, EBSD) analyses, we will show how ultramylonites form over long transient intervals by the shearing, stretching, and rending of compositional domains. We will discuss the influence of starting microstructure, viscosity contrast, and strain partitioning on phase mixing, and also examine the timescales and conditions required for ultramylonite formation in Earth’s lithosphere.