[S-CG46] Shallow and intermediate depth intraslab earthquakes: seismogenesis and rheology of the slab

[SCG46-10] Deformation of metastable olivine under conditions of the lower part of the mantle transition zone

*Kohei Matsuda¹, Tomohiro Ohuchi¹, Yuji Higo² (1.Geodynamics Research Center, Ehime University, 2.Japan Synchrotron Radiation Research Institute)

Keywords:deep earthquakes, olivine phase transformation , weak gouge, in situ measurements

Intraslab earthquakes are divided into shallow (<40 km depth), intermediate-depth (>40 km), and deep-focus (>300km) ones. Seismicity decreases at greater depths due to the positive pressure dependency of frictional strength (i.e., Byerlee’s rule), which inhibits the rupture or faulting at higher pressures. Surprisingly, seismicity increases with depths in the mantle transition zone (e.g., Florich, 1989), suggesting that the mechanism of deep-focus earthquakes is different from that of shallower earthquakes. It has been proposed that the pressure-induced phase transition of olivine may trigger the transformational faulting based on the deformation experiments using germinate olivine at 1-2 GPa (Mg₂GeO₄: Green and Burnley, 1989; Wang et al., 2017) or fayalite at 4-9 GPa (Fe₂SiO₄: Officer and Secco, 2020). Recently, deformation experiments on mantle olivine at 11-17 GPa have been successfully performed (Ohuchi, et al., 2022). Ohuchi, et al. (2022) reported that the formation of a weak gouge filled with nanocrystalline metastable olivine and wadsleyite (and/or ringwoodite) could trigger faulting aided by shear heating in the interior of the subducted slab. However, the detailed process of the formation of week gouge layers via the phase transition of metastable olivine is still unclear. Thus, we performed deformation experiments on olivine under the pressure and temperature condition of lower part of the mantle transition zone.
We conducted in situ uniaxial deformation experiments on as-is olivine aggregates at pressures of 16-20 GPa and temperatures of 970-1120 K with a constant strain rate using a D-DIA apparatus at BL04B1/SPring-8. Pressure, stress, and strain were determined by using x-ray diffraction patterns and radiographs. Acoustic emissions (AEs) were also recorded by using six sensors glued on the sides of the second-stage anvils, and the then hypocenters of AEs were determined. We also conducted some off-line shear deformation experiments on olivine single-crystal samples at 14.5 GPa and temperatures of 1020-1220 K at GRC, Ehime Univ. Crystallographic orientations of grains of olivine and high-pressure phases in the recovered samples were examined using a FE-SEM equipped with a EBSD camera.
We observed semi-brittle deformation of olivine samples associating some weak AEs, and strain reached to 0.15-0.20. The microstructures of the recovered samples showed developments of a few straight damage zones on mode-Ⅱ cracks at an angle of 20-40° to the direction of compression. The damage zones were filled with nanocrystalline olivine and wadsleyite (and/or ringwoodite) without a significant slip. The observed microstructures of the damages zone are similar to the ‘weak gouge’ layers on the faults observed in previous studies (Wang et al., 2017; Ohuchi, et al., 2022). Off-line shear deformation experiments demonstrated nucleation of nanocrystalline wadsleyite lamellae parallel to olivine (100) even at low temperatures (i.e., 1020K). This would be explained by the martensitic transformation of olivine on the (100) plane (Madon and Poirier, 1983). Our results suggest that weak layers inducting shear localization (i.e., faulting) could be formed via the martensitic transformation of olivine, which would be effective at low temperature corresponding to those of the surface of the metastable olivine wedge in the subducted slabs.