[S-CG58] New Developments in fluid-rock Interactions: From Surface to Deep Subduction Zone

[SCG58-P13] Challenge to rock deformation experiments under in-situ high lithostatic pressure, pore fluid pressure and high temperature conditions of deep of the Earth

*Keishi Okazaki^1,2 (1.Earth and Planetary Systems Science Program, Hiroshima University, 2.Kochi Institute for Core Sample Research, x-star, JAMSTEC)

Keywords:subduction zone, rheology, earthquake, pore fluid pressure

At subduction plate boundaries and inland active faults, most of large earthquakes (Mw >7) occur at the base of the seismogenic zone with a depth of about 10–25 km. The PT conditions of these areas corresponds to lithostatic pressures ranging 300–800 MPa and temperatures ranging 200–400 degC. In addition, seismological observations indicate that the pore fluid pressure is nearly lithostatic, meaning 300–800 MPa. The presence of high pore fluid pressure plays a key role in controlling the rock rheology, because the frictional strength of the fault is a function of the effective pressure and physico-chemical properties of fluids changes exponentially under high pressure. However, most of rock friction experiments have been conducted under pressures lower than 200 MPa. Therefore, understanding of fault rheology at the base of the seismogenic zone is limited by the extrapolations of low PT experiments and experiments using analog materials.
We recently built new solid pressure medium apparatuses at Hiroshima University. These are capable of increasing the confining pressure, the pore pressure and the temperature up to 1.5 GPa, 1.3 GPa and 1200 degC, respectively. These apparatuses can reproduce the in-situ or even deeper P-Pp-T conditions of the seismogenic zone. Our preliminary experiment on San Carlos olivine deformed under the confining pressure of 1 GPa at the temperature of 400 degC without the pore fluid showed a stick-sklip behavior. The nominal friction coefficient was 0.45 for the dry experiment. In contrast, deformation experiment on San Carlos olivine under the confining pressure of 1 GPa, the pore pressure of 480 MPa at the temperature of 462 degC indicates that the nominal friction coefficient was 0.42 with assuming fully undrained condition. The recovered sample showed the formation of chlorite-rich shear zone along the piston-sample boundary probably due to reaction among the sample, water and alumina piston. Serpentine and Fe-oxide were also observed in the sample matrix. This presentation will also report basic performances, calibration data of the apparatuses and further experimental results.