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[S-IT20]
[SIT20-P05]
©Japan Geoscience Union 
Japan Geoscience Union Japan Geoscience Union Meeting 2018 
[EE] Evening  Poster | S (Solid Earth Sciences) | S-IT Science of the Earth's Interior & Tectonophysics
Structure and Dynamics of Earth and Planetary Mantles
convener:Takashi Yoshino(Institute for Planetary Materials, Okayama University), Dapeng Zhao(Department
of Geophysics, Tohoku University), Takashi Nakagawa(海洋研究開発機構数理科学・先端技術研究分野)Mon. May 21, 2018 5:15 PM - 6:30 PM  Poster Hall (International Exhibition Hall7, Makuhari Messe)
Interdisciplinary approach can lead to a better understanding of dynamics and evolution of the deep interiors
of the Earth and planets. We welcome any submissions of recent results in observational, theoretical and
experimental studies on seismology, geomagnetism, mineral physics, dynamics of deep interiors, and any
other relevant fields from researchers in many countries. Integration of such results is also welcome. In
particular, we encourage any contributions focusing on "plate and mantle dynamics in Earth and terrestrial
planets". 
Importance of a thin, low viscosity layer on top of the slab for
rock deformation and fluid migration in subduction zones
*Manabu Morishige1 (1.Japan Agency for Marine-Earth Science and Technology)
Keywords:subduction zones, low viscosity layer, volcano distribution, fluid migration, short-term slow slip
events 
In subduction zones the existence of a thin, low viscosity layer (LVL) on top of the slab has been proposed to
explain a low surface heat flow in the fore-arc region. In this presentation I will show some possible
implications of LVL for other geophysical observations (Morishige, 2015; Morishige &van Keken, 2017). 
It is well known that LVL effectively decouples the movement between the slab and the mantle just above it,
producing a cold fore-arc mantle. However, the discussion has been limited to the across-arc cross section.
We find that when the viscosity inside LVL is sufficiently low, along-arc component of the flow arises in and
around LVL and it leads to a large along-arc temperature variation. This type of 3D flow arises due to the
temperature dependence of viscosity. We also find that the wavelength of along-arc temperature variation
increases with the thickness of LVL. When the thickness of LVL is 6 km, the obtained wavelength (~80 km) is
similar to that of volcano clustering observed in Tohoku, Japan. 
Although it is still an open question what controls the formation of LVL, several previous studies have
suggested that weak hydrous minerals such as serpentine may play essential roles for it. When strongly
sheared by subducting slab serpentinite layer has permeability anisotropy, that is, fluid can migrate faster in
the direction subparallel to the slab surface. We find that when the effect of permeability anisotropy is
combined with 3D slab geometry, 3D fluid focusing occurs where the slab bends away from the trench and it
leads to the increase in porosity (volume fraction of fluid) there. It may help explain the along-arc variation in
average slip rate of short-term slow slip events observed in SW Japan and Cascadia. The flow inside LVL
produces dynamic pressure gradient inside it, which will further enhance the fluid flow subparallel to the slab
surface. 
Reference 
Morishige, M., A new regime of slab-mantle coupling at the plate interface and its possible implications for
the distribution of volcanoes, Earth and Planetary Science Letters, 427, pp262-271,
doi:10.1016/j.epsl.2015.07.011, 2015. 
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©Japan Geoscience Union 
Japan Geoscience Union Japan Geoscience Union Meeting 2018 
Morishige, M. and van Keken, P.E., Along-arc variation in short-term slow slip events caused by 3D fluid
migration in subduction zones, Journal of Geophysical Research Solid Earth, 122, 1-15,
doi:10.1002/2016JB013091, 2017.