10:00 AM - 10:15 AM
[SSS05-15] Contribution of Plastic Deformation Estimated from Microstructures after Deformation Experiment on Quartz Aggregates
Keywords:Brittle-Plastic transition, Shear experiment
Brittle deformation is dominant and earthquakes can occur in the upper crust. In this region, shear strength basically follows Byerlee's law and increases with effective pressure. On the other hand, crystalline plastic deformation is dominant and earthquakes rarely occur in the lower crust. In this region, shear strength follows a flow law and decreases with temperature (e.g., Sibson, 1977; Scholz, 1988). The transition of deformation mechanism between brittle and plastic regimes is called the brittle-plastic transition (BPT). The BPT is important because it conceivably correlates with the lower limit of the seismogenic zone and large earthquakes often initiate there (e.g., Sibson, 1982). But the mechanism of rock deformation within faults in the BPT is not well understood. There have been many studies to formulate a constitutive law in the BPT (e.g., Reinen et al, 1992; Shimamoto and Noda, 2012; Chen and Spiers, 2016; Noda, 2021). Especially, Noda (2021) proposed a deformation model focusing S-C ' structures. In this study, we have two aims. One is verification of this model by comparing with shear experiments. The other is estimation of the ratio of shear strain rate due to plastic deformation to the total strain rate (contribution) in the experimental shear zone.
The shear experiments are conducted using a Griggs-type solid pressure medium apparatus installed at Hiroshima University. Quartz aggregates were used as the samples. The experiments were performed at confining pressure of 1000 MPa, shear strain rate of 2.5*10-4 /s, and various temperatures ranging from 400 to 900 degrees so that we could capture the BPT of quartz (Hirth & Tullis, 1994; Richter et al., 2018). Mechanical data was measured during the experiment, and strain of shear zone, R1 orientations and major axis orientations of grains were measured form S-C' structures of retrieved samples. contribution was estimated from these structural parameters based on the kinematic model of Noda (2021). In this estimation, initial aspect ratio and initial major axis orientation of before deformation were taken into account.
As a result, we succeeded to capture a gradual transition by quantifying contribution in the BPT from microstructure for the first time. Even if R1 planes are recognized, them may be inactive during deformation after the initial phase. We chose the model between R1 slip and Y slip based on the Byerlee's law and tractions on these planes. The mechanical data suggested the Y slip below 700 degrees, and R1 slip above it, and the estimated contribution increased significantly at the transition between them. The comparison of the model by Noda (2021) and experiments shows a discrepancy. In the model, R1 orientations increase with increasing temperature because they are assumed to be formed at the optimum orientation. But measured R1 orientations form recovered samples were almost constant. The discrepancy probably owes to too much idealization in the model for the direct comparison with the experiment. For example, the model considered an one-dimensional problem, while the experimental shear zone has the end effect. The experimental structures were analyzed assuming a steady-state deformation and using a strain ellipse in it. Deformation microstructures are often heterogeneous (e.g., Hiraga and Shimamoto, 1987). In addition, formation of new slip planes may be suppressed if R1 planes formed in the initial stage work as weak planes. These points should be verified for applying the model to experiments and natural faults.
The shear experiments are conducted using a Griggs-type solid pressure medium apparatus installed at Hiroshima University. Quartz aggregates were used as the samples. The experiments were performed at confining pressure of 1000 MPa, shear strain rate of 2.5*10-4 /s, and various temperatures ranging from 400 to 900 degrees so that we could capture the BPT of quartz (Hirth & Tullis, 1994; Richter et al., 2018). Mechanical data was measured during the experiment, and strain of shear zone, R1 orientations and major axis orientations of grains were measured form S-C' structures of retrieved samples. contribution was estimated from these structural parameters based on the kinematic model of Noda (2021). In this estimation, initial aspect ratio and initial major axis orientation of before deformation were taken into account.
As a result, we succeeded to capture a gradual transition by quantifying contribution in the BPT from microstructure for the first time. Even if R1 planes are recognized, them may be inactive during deformation after the initial phase. We chose the model between R1 slip and Y slip based on the Byerlee's law and tractions on these planes. The mechanical data suggested the Y slip below 700 degrees, and R1 slip above it, and the estimated contribution increased significantly at the transition between them. The comparison of the model by Noda (2021) and experiments shows a discrepancy. In the model, R1 orientations increase with increasing temperature because they are assumed to be formed at the optimum orientation. But measured R1 orientations form recovered samples were almost constant. The discrepancy probably owes to too much idealization in the model for the direct comparison with the experiment. For example, the model considered an one-dimensional problem, while the experimental shear zone has the end effect. The experimental structures were analyzed assuming a steady-state deformation and using a strain ellipse in it. Deformation microstructures are often heterogeneous (e.g., Hiraga and Shimamoto, 1987). In addition, formation of new slip planes may be suppressed if R1 planes formed in the initial stage work as weak planes. These points should be verified for applying the model to experiments and natural faults.