Can the crustal strength in the brittle-plastic transition zone be estimated from the flow stress of calcite mylonite?

Quantifying crustal strength is essential to understanding lithosphere strengths and tectonic processes, such as long-term fault movements caused by plate motions. In this study, we estimated the strength of granitic upper crust using recrystallized grain size piezometer of calcite mylonite intercalated in the Cretaceous granitic Abukuma Mountains. In addition, Raman carbonaceous material thermometer was used to constrain the deformation temperature. Calcite mylonites are originated from Late Carboniferous Tateishi Formation and locate along Shajigami shear zone at eastern margin of Abukuma Mountains, Northeastern Japan. Shajigami shear zone is a strike-slip shear zone active during the Middle Cretaceous. Along Shajigami shear zone, calcite mylonite and granitic cataclasites expose.

Calcite grains are well recrystallized, and the grain size are determined by electron backscattered diffraction (EBSD) mapping with the step sizes of 2-2.5µm. The mean grain sizes are 17-26 µm. The differential stress estimated by recrystallized grain size piezometer of calcite aggregate (Platt and De Bresser, 2017) is 35-80 MPa. The estimated metamorphic temperature using the Raman carbonaceous material thermometer (Kouketsu et al., 2014) is 340-250 ˚C. The difference in estimated metamorphic temperature is attributed to the thermal effects of the Cretaceous granitoids that penetrated along the calcite mylonite. This is because the estimated metamorphic temperature is higher the closer to the granitoid. Because well dynamically recrystallized calcite grains indicate that the deformation temperature exceeding 200˚C, the estimate by Raman carbonaceous material thermometer is the upper bound for the deformation temperature.

The calcite mylonite and the granitic cataclasite are thought to have formed at the same time in the Shajigami shear zone (Watanuki et al., 2020). Although there is a slight temperature gradient near the granite, widespread deformation has occurred in this area. The deformation temperature obtained in this study is the deformation around the brittle-plastic transition zone of the upper crust. Hence, the collecting flow stress estimated from calcite mylonite intercalated in brittle granitic shear zone may be possible to constrain the stress magnitude of the shear zone data near the brittle-plastic transition at 200-300°C.

 

References

Platt and De Bresser, 2017, J. Struct. Geol., 105, 80-87.

Kouketsu et al., 2014, Island arc, 23, 33-50.

Watanuki et al., 2020, J. Struct. Geol., 137, 104046.