2:30 PM - 2:45 PM
[SSS06-14] The brittle-plastic transition of simulated quartz shear zone : insight from microstructual observation
Keywords:Brittle-Plastic transition, Microstructures, Fault rheology, Shear experiment
The rheology of fault materials depends on depth (i.e., temperature and confining pressure). In a shallow region, earthquakes occur because brittle deformation is dominant. In this region, shear strength follows basically Byerlee's law and increases as increasing confining pressure increases with depth. On the other hand, in a deeper region, earthquakes don’t occur because crystal plastic deformation is dominant. In this region, shear strength follows a flow law and decreases with increasing temperature. At a certain depth, the dominant deformation mechanism switches from brittle to plastic. This translation is called brittle-plastic transition (BPT). The depth limit of the seismogenic zone is constrained by the BPT and hypocenters of huge earthquakes often locate at the BPT because where is the deepest part of brittle region and thus the shear strength is the maximum. However, the rheology of rocks at the BPT, especially how the dominant deformation mechanism transit from brittle to plastic deformation is not well understood.
In this study, shear experiments were conducted at the BPT, then the strain partitioning between brittle and plastic strains were estimated across the BPT by microstructures of recovered samples and compared with previous models by mechanical data. Shear experiments were conducted using a Griggs-type solid pressure medium apparatus installed at Hiroshima University. Quartz aggregates which are major minerals in the continental crusts and an analog of marine sediments, were used as the samples. The experimental conditions were constant of confining pressure of 1000 MPa, shear strain rate of 2.5*10-4 /s, and temperature range of 400-1000 ℃, which is the conditions of the BPT of quartz. Recovered sample were processed into thin sections and analyzed for microstructure under an optional microscope and with an original image analysis program. In the analysis, long axis directions and aspect ratios of grains were measured to estimate strains accommodated by the plastic deformation, and amount of plastic strain and brittle strain are calculated by these microstructural observations using methods of Reinen et al., (1992) and Noda (2021).
From both the mechanical data and microstructural, we succeeded to capture a gradual transition that the dominant deformation changes from brittle to plastic mechanisms at the BPT. The mechanical data show that strengths follow Byerlee's law at 400-700 ℃ and strengths follow the flow law at 800-1000 ℃ indicating the BPT is around 700-800 ℃. These results are narrower temperature range and higher shear strength than the model suggested by Noda and Shimamoto (2014), while these results are somewhat consistent with models by Noda (2021) and Reinen et al., (1992). On the other hand, microstructural analysis indicates that BPT is not completed within 400-1000 ℃ at the experimental conditions. A small fraction of plastic deformation was observed from the deformation microstructure at 400 ℃, and a component of brittle deformation is not vanished even at the temperature of 1000 ℃, although calculated values are different between using methods by Reinen et al., (1992) and Noda (2021). The BPT of quartz aggregates estimated from microstructural analyses is wider than that estimated from the mechanical data and previous models. This indicates that the microstructural evolution could capture the BPT in more detail than the mechanical data. These results could indicate two extreme cases of the width of the seismogenic zone: the first is that the seismgenic zone can be limited only at the fully brittle regime shallower than the BPT; the second is that the seismgenic zone can be extended to even deeper where the strength is almost identical with the flow strength. Further research on the strain-rate dependance of the strength at the BPT is highly required to constrain the depth limit of the seismogenic zone.
In this study, shear experiments were conducted at the BPT, then the strain partitioning between brittle and plastic strains were estimated across the BPT by microstructures of recovered samples and compared with previous models by mechanical data. Shear experiments were conducted using a Griggs-type solid pressure medium apparatus installed at Hiroshima University. Quartz aggregates which are major minerals in the continental crusts and an analog of marine sediments, were used as the samples. The experimental conditions were constant of confining pressure of 1000 MPa, shear strain rate of 2.5*10-4 /s, and temperature range of 400-1000 ℃, which is the conditions of the BPT of quartz. Recovered sample were processed into thin sections and analyzed for microstructure under an optional microscope and with an original image analysis program. In the analysis, long axis directions and aspect ratios of grains were measured to estimate strains accommodated by the plastic deformation, and amount of plastic strain and brittle strain are calculated by these microstructural observations using methods of Reinen et al., (1992) and Noda (2021).
From both the mechanical data and microstructural, we succeeded to capture a gradual transition that the dominant deformation changes from brittle to plastic mechanisms at the BPT. The mechanical data show that strengths follow Byerlee's law at 400-700 ℃ and strengths follow the flow law at 800-1000 ℃ indicating the BPT is around 700-800 ℃. These results are narrower temperature range and higher shear strength than the model suggested by Noda and Shimamoto (2014), while these results are somewhat consistent with models by Noda (2021) and Reinen et al., (1992). On the other hand, microstructural analysis indicates that BPT is not completed within 400-1000 ℃ at the experimental conditions. A small fraction of plastic deformation was observed from the deformation microstructure at 400 ℃, and a component of brittle deformation is not vanished even at the temperature of 1000 ℃, although calculated values are different between using methods by Reinen et al., (1992) and Noda (2021). The BPT of quartz aggregates estimated from microstructural analyses is wider than that estimated from the mechanical data and previous models. This indicates that the microstructural evolution could capture the BPT in more detail than the mechanical data. These results could indicate two extreme cases of the width of the seismogenic zone: the first is that the seismgenic zone can be limited only at the fully brittle regime shallower than the BPT; the second is that the seismgenic zone can be extended to even deeper where the strength is almost identical with the flow strength. Further research on the strain-rate dependance of the strength at the BPT is highly required to constrain the depth limit of the seismogenic zone.