Japan Geoscience Union Meeting 2015

Presentation information


Symbol S (Solid Earth Sciences) » S-CG Complex & General

[S-CG59] Rheology, fracture and friction in Earth and planetary sciences

Wed. May 27, 2015 4:15 PM - 6:00 PM 106 (1F)

Convener:*Osamu Kuwano(Japan Agency for Marin-Earth Science and Technology), Tomohiro Ohuchi(Geodynamics Research Center, Ehime University), Ichiko Shimizu(Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo), Hidemi Ishibashi(Graduate School of Science, Shizuoka University), Chair:Ichiko Shimizu(Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo)

5:54 PM - 5:57 PM

[SCG59-P10] High-pressure deformation experiments on olivine-orthopyroxene aggregates under hydrothermal conditions

3-min talk in an oral session

*Kumi FUKUSHIMA1, Ken-ichi HIRAUCHI1, Masanori KIDO2, Jun MUTO2 (1.Department of Geosciences, Graduate School of Science, Shizuoka University, 2.Department of Earth Sciences, Tohoku University)

Keywords:olivine, orthopyroxene, talc, hydration reaction, strength weakening, subduction initiation

For plate tectonics to operate on a terrestrial planet, the strength of faults within the oceanic lithosphere must be low, with the coefficient of friction below 0.05. However, standard strength profile using olivine flow law far exceeds this threshold value, particularly at depths of 20 to 40 km, where fluids passed through the faults may interact with peridotites to form hydrous minerals (e.g., serpentine). Here, we conducted deformation experiments on harzburgitic olivine-orthopyroxene aggregates under hydrothermal conditions, at a temperature of 500℃, a confining pressure of 1.0 GPa, and shear strain rates of 5.9 x 10-5 to 4.3 x 10-6 s-1. All experiments showed a peak shear strength (about 400 MPa) at shear strains of 0.7, followed by a large stress drop (up to 150 MPa), after which steady-state sliding was observed until significant strain weakening started to occur at shear strains of 1.5. The drop in shear stress was initially caused by unstable slip, which resulted from the development of localized shear planes (Riedel or boundary shears) after yielding. The strain weakening after shear strains of 1.5 is related to shearing of newly formed talc along the shear planes. Talc may form from preferential dissolution of orthopyroxene rather than olivine. The final shear strength (down to 30 MPa) decreased with decreasing shear strain rates, reflecting widening of the talc layer along the shear planes. These results suggest that hydrothermal alteration of peridotites along the deep faults play an important role in forming the extremely weak zone for subduction initiation.