Ultramylonite generation via phase mixing in high‐strain experiments

Abstract

Dynamic recrystallization and phase mixing are considered to be important processes in ductile shear zone formation, as they collectively enable a permanent transition to the strain‐weakening, grain‐size sensitive deformation regime. While dynamic recrystallization is well understood, the underlying physical processes and timescales required for phase mixing remain enigmatic. Here, we present results from high‐strain phase mixing experiments on calcite‐anhydrite composites. A poorly mixed starting material was synthesized from fine‐grained calcite and anhydrite powders. Samples were deformed in the Large Volume Torsion apparatus at 500°C and shear strain rates of 5 × 10−5 to 5 × 10−4 s−1, to finite shear strains of up to γ = 57. Microstructural evolution is quantified through analysis of backscattered electron images and electron backscatter diffraction data. During deformation, polycrystalline domains of the individual phases are geometrically stretched and thinned, causing an increase in the spatial density of interphase boundaries. At moderate shear strains (γ ≥ 6), domains are so severely thinned that they become “monolayers” of only one or two grain's width and form a thin compositional layering. Monolayer formation is accompanied by a critical increase in the degree of grain boundary pinning and, consequently, grain‐size reduction below the theoretical limit established by the grain‐size piezometer or deformation mechanism field boundary. Ultimately, monolayers neck and disaggregate at high strains (17 <γ <57) to complete the phase mixing process. This “geometric” phase mixing mechanism is consistent with observations of mylonites, where layer (i.e., foliation) formation is associated with strain localization, and layers are ultimately destroyed at the mylonite‐ultramylonite transition.

The Generation of Plate Tectonics From Grains to Global Scales: A Brief Review

“…At lithosphere like stress and temperature conditions, the model predicts the formation of rapidly deforming strongly localized ultramylonitic bands embedded within coarser grained, poorly mixed or monomineralic, slowly deforming regions (Figure c). The coexistence of the well‐mixed weak regions and the poorly mixed strong regions is in agreement with the hysteretic branches predicted by Bercovici and Ricard (), as well as the experimental observations of Cross & Skemer (; see also Figure d) and Wiesman et al ().…”

“…(C) A theory for diffusive grain mixing coupled to two‐phase grain damage (Bercovici & Mulyukova, ) uses a micromechanical model for mechanical grain mixing (Bercovici & Skemer, ) to model diffusivity; it predicts that two deformation or “piezometric” branches arise, similar to the hysteresis grain damage model of Bercovici and Ricard (), in which well‐mixed polyphase regions are charactized by small grains deforming via diffusion creep typical of ultramylonites, while poorly mixed, monophase regions contain large grains deforming in dislocation creep. (d) Deformation map for calcite‐anhydrite experiments of Cross and Skemer (), showing that when calcite is unmixed with anhydrite it follows a normal large grain size calcite piezometer, but when well mixed it follows a branch with smaller grain sizes, similar to that predicted by theory.…”

“…The effectiveness of Zener pinning, however, is controlled by the degree to which the different phases are interspersed or mixed with each other at the grain scale. The process of phase mixing, and its effect on rock rheology and shear localization, have been the focus of numerous recent experimental and theoretical studies (Bercovici & Mulyukova, ; Bercovici & Ricard, ; Bercovici & Skemer, ; Cross & Skemer, ; Tasaka et al, ; Tasaka et al, ; Wiesman et al, ). At low pressures (i.e., <1 GPa), intergrain mixing may occur by chemical or metamorphic reactions (Platt, ; Czertowicz et al, ; Précigout & Stünitz, ) and brittle processes (Dimanov et al, ), all of which typically involving cavitation.…”

Weaker Than Weakest: On the Strength of Shear Zones

“…We therefore propose that as sufficient amounts of ultramylonitic mixture coalesce to form interconnected layers, it becomes the dominant control on bulk shear zone rheology. This transition coincides with dismemberment of the interconnected phyllosilicate layers, possibly by progressive flow of the weaker, fine-grained mixture or simply by thinning and extension of lithologic layering with progressive shear strain (Figure 3b; Cross & Skemer, 2017). Once the fine-grained mixture is formed, partly in the porphyroclast pressure shadows (Figures 2a and 3b), grain size sensitive deformation mechanisms will retain, sustain, and maintain the weakness.…”

The ability of significant tidal stress to initiate plate tectonics

“…Damage theory, which models the reduction and growth of grains in a deformed lithosphere, is a plausible way to obtain plate-like behavior with a realistic rheology, without invoking surface water to weaken the Earth's lithosphere (Bercovici and Ricard, 2003;Foley et al, 2012). Recent experimental (Cross and Skemer, 2017) and theoretical (Bercovici and Skemer, 2017) work argues the deformation of two-phase materials which make up the Earth's lithosphere reduce grain sizes, which lead to localized weak zones forming plate boundaries. The mixing induced by damage is active in the mid lithosphere and efficient in the deep lithosphere, but is ineffective in the shallowest portions of the Earth's lithosphere (Bercovici and Skemer, 2017).…”