Japan Geoscience Union Meeting 2016

Presentation information

International Session (Oral)

Symbol S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Techtonophysics

[S-IT08] Structure and Dynamics of Suboceanic Mantle: Theories and Observations

Tue. May 24, 2016 3:30 PM - 5:00 PM 201A (2F)

Convener:*Jun Korenaga(Department of Geology and Geophysics, Yale University), Hitoshi Kawakatsu(Earthquake Research Institute, University of Tokyo), James Gaherty(Lamont Doherty Earth Observatory), Kiyoshi Baba(Earthquake Research Institute, University of Tokyo), Chair:Kiyoshi Baba(Earthquake Research Institute, University of Tokyo), Jun Korenaga(Department of Geology and Geophysics, Yale University)

4:45 PM - 5:00 PM

[SIT08-12] Rheological weakening via hydration reactions in a mantle shear zone: Implications for the initiation of oceanic plate subduction

*Ken-ichi Hirauchi1, Kumi Fukushima2, Masanori Kido3, Jun Muto3, Atsushi Okamoto4 (1.Department of Geosciences, Faculty of Science, Shizuoka University, 2.Department of Science, Graduate School of Integrated Science and Technology, Shizuoka University, 3.Department of Earth Science, Graduate School of Science, Tohoku University, 4.Graduate School of Environmental Studies, Tohoku University)

Keywords:oceanic mantle, subduction initiation, rheological weakening, brittle–plastic transition, frictional–viscous flow, talc

Plate tectonics on Earth is essential for mantle geochemistry and planetary habitability; however, its initiation remains controversial and previous geodynamic models require a preexisting zone of weakness (average stress less than 30 MPa) in the oceanic lithosphere. Although the operation of grain-sensitive creep (e.g., diffusion creep) causes a reduction in stress, fault strength near the brittle–ductile transition (BDT) remains remarkably high (1500 MPa), even when assuming olivine diffusion creep with an anomalously small grain size (1 μm) and a slow strain rate (10–15 s–1).
Although the oceanic lithosphere is considered to be dry, infiltration of seawater into a preexisting fault zone (e.g., fracture zones) will lead to the formation of hydrous phyllosilicates (e.g., amphibole, serpentine, and talc). To investigate hydration-induced rheological weakening effects on preexisting faults in intra-oceanic settings, we conducted high-pressure friction experiments on peridotite gouge under hydrothermal conditions. We find that increasing strain and reactions lead to the development of localized talc-rich shear zones, which induce an order-of-magnitude reduction in stress. The rate of reaction is strongly dependent on the degree of cataclastic deformation, rather than time.
Our laboratory experiments demonstrate that the operation of frictional–viscous flow, controlled by pressure-solution-accommodated frictional sliding on weak hydrous phyllosilicates, leads to a drastic reduction (down to 40 MPa) in the high stresses near the BDT within the oceanic lithosphere. Our results also suggest that the existence of oceans is a prerequisite for the initiation of plate tectonics on terrestrial planets (e.g., Earth); otherwise, stagnant lid convection operates in the mantle (e.g., Venus).