Title: How does viscosity contrast influence phase mixing and strain localization?

Type Dataset Cross, Andrew J., Skemer, Philip, Couvy, Hélène, Olree, Elizabeth (2020-07-07): How does viscosity contrast influence phase mixing and strain localization?. Woods Hole Oceanographic Institution. Dataset. https://darchive.mblwhoilibrary.org/handle/1912/25940

Authors: Cross, Andrew J. ; Skemer, Philip ; Couvy, Hélène ; Olree, Elizabeth ;

Links

• Item record in Woods Hole Open Access Server
• Digital object URL

Summary

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.

More information

• URI: https://hdl.handle.net/1912/25940
• DOI: 10.26025/1912/25940

Dates

• accessioned: July 07, 2020
• available: July 07, 2020
• Publication date: July 07, 2020

Funding Information

Format

electronic resource

Relateditems