How Does Viscosity Contrast Influence Phase Mixing and Strain Localization?

Abstract

Ultramylonites—intensely deformed rocks with fine grain sizes and well‐mixed mineral phases—are thought to be a key component of Earth‐like plate tectonics, because coupled phase mixing and grain boundary pinning enable rocks to deform by grain‐size‐sensitive, self‐softening creep mechanisms over long geologic timescales. In isoviscous two‐phase composites, “geometric” phase mixing occurs via the sequential formation, attenuation (stretching), and disaggregation of compositional layering. However, the effects of viscosity contrast on the mechanisms and timescales for geometric mixing are poorly understood. Here, we describe a series of high‐strain torsion experiments on nonisoviscous calcite‐fluorite composites (viscosity contrast, ηca/ηfl ≈ 200) at 500°C, 0.75 GPa confining pressure, and 10−6–10−4 s−1 shear strain rate. At low to intermediate shear strains (γ ≤ 10), polycrystalline domains of the individual phases become sheared and form compositional layering. As layering develops, strain localizes into the weaker phase, fluorite. Strain partitioning impedes mixing by reducing the rate at which the stronger (calcite) layers deform, attenuate, and disaggregate. Even at very large shear strains (γ ≥ 50), grain‐scale mixing is limited, and thick compositional layers are preserved. Our experiments (1) demonstrate that viscosity contrasts impede mechanical phase mixing and (2) highlight the relative inefficiency of mechanical mixing. Nevertheless, by employing laboratory flow laws, we show that “ideal” conditions for mechanical phase mixing may be found in the wet middle to lower continental crust and in the dry mantle lithosphere, where quartz‐feldspar and olivine‐pyroxene viscosity contrasts are minimized, respectively.

Crystallographic Preferred Orientation (CPO) Development Governs Strain Weakening in Ice: Insights From High‐Temperature Deformation Experiments

“…In order to accurately model plate‐scale lithospheric deformation, we therefore need a comprehensive understanding of the microphysical processes that produce strain weakening across a broad range of conditions in rock‐forming minerals. Many studies have linked strain weakening at relatively low homologous temperatures ($${T}_{h}$$ <0.7) to mechanisms such as grain size evolution (Karato et al., 1986; Rutter, 1995), CPO development (Schmid et al., 1987; Urai et al., 1986), partial melting (Burlini & Bruhn, 2005; Hirth & Kohlstedt, 1995), metamorphic reactions and transformations (Gordon, 1971; Poirier, 1982), water weakening (Griggs & Blacic, 1965; Karato et al., 1986; Kronenberg & Tullis, 1984) and mineral phase mixing and/or layering (Bons & Cox, 1994; Cross et al., 2020). However, we lack a comprehensive understanding of the processes that produce strain weakening in rocks and minerals at very high temperatures ($${T}_{h}$$ >0.7).…”

Effect of Low Viscosity Contrast between Quartz and Plagioclase on Creep Behavior of the Mid-Crustal Shear Zone

“…Some entrained grains developed parallel to mylonitic foliation. These observations suggest that the compositional layers became severely attenuated and eventually disaggregated to form mixed aggregates [18]. To characterize each compositional layer, we used an EBSD map to select 20 subdomains (Figures 7b and 8; see also Figures S1 and S2 for all subdomains), for which we calculated the mineral modes of quartz and plagioclase, as well as their grain sizes and CPOs (Figures 9 and 10; see also Figures S3-S6 for all the subdomains).…”

“…As quartz-plagioclase phase mixing is likely in Figure 8 and Figure S2, we calculated the viscosity contrast as a function of stress, grain size, and temperature for the quartzplagioclase composites under wet conditions, as proposed by Cross et al [18] (Figure 11). The viscosities of the quartz-and plagioclase-rich layers were calculated using laboratoryderived flow laws for wet quartz [11,63,64] and wet plagioclase (Ab 100 ) ( [65] (Table 1).…”

“…However, there remains continued discussion over the precise mechanisms responsible for grain-scale phase mixing (e.g., [14][15][16][17]). More recently, Cross et al [18] has proposed that the ideal conditions for mechanical phase mixing under wet middle-to lower-crust conditions are 400-800 • C, 10-100 MPa stress, and 10-100 µm grain size, where the quartz-plagioclase viscosity contrast is minimized to <10. Here, we attempted to assess whether these conditions apply to natural rocks, such as ultramylonite, in a shear zone.…”

The Effect of Secondary‐Phase Fraction on the Deformation of Olivine + Ferropericlase Aggregates: 1. Microstructural Evolution

“…Similar effects have been demonstrated theoretically (Bercovici et al., 2023; Bercovici & Mulyukova, 2021) and experimentally in samples composed of Ol + Opx (Tasaka et al., 2017, 2020), calcite + anhydrite (Cross & Skemer, 2017), and quartz + albite (Cross et al., 2017). Additionally, microstructural developments from experimentally deformed samples have been used to investigate possible mechanisms responsible for efficiently mixing two or more mineral phases in samples in which pinning is effective (Cross et al., 2020; Cross & Skemer, 2017; Tasaka et al., 2017). However, many of these experimental studies focused on two‐phase samples with only one to three different proportions of the two phases.…”