Marine Ecosystems Research Department - jamstec japan agency ...

JAMSTEC 2002 Annual Report
Institute for Frontier Research on Earth Evolution (IFREE)
Density (g/cm 3 )
6.0
5.8
5.6
5.4
5.2
5.0
4.8
4.6
Mg-perovskite
4.4
20 40 60 80 100 120 140
Pressure (GPa)
crust. The chemical composition of phases also seems
to be sensitive to the P-T conditions. Therefore, in
order to estimate the density of subducted MORB, the
effects of whole rock composition and P-T condition
need to be considered.
Research Program for Plate Dynamics
1. Research Overview
Mg-perovskite
Mg-perovskite
CaCl 2
-type SiO 2
Ca-perovskite
Hexagonal aluminous phase
Calcium-ferrite type aluminous pahse
Stishovite
Fig.16 Pressure-density relations for high-pressure minerals in
MORB. Comparison of pressure-density relations for highpressure
phases in the subducted MORB at room temperature.
Densities were calculated using the chemical compositions
of minerals in the subducted MORB and the
parameters of the equations of state of minerals obtained
in this study.
The objective of this program is focused on subduction
zones, where observations can be made on various
aspects of plate structure and deformation such as
those from rapid, large-scale rupture of plates to
micro-fractures in fault rocks. Research into lithospheric
structure, seismogenic zone material science,
and plate dynamics modeling will be integrated into a
comprehensive understanding of plate behavior and
surface phenomena of the Earth. The research will
involve structural research, sampling, and modeling
analysis using the Earth Simulator (ES).
The main research activities in were as follows:
() The lithospheric structure research group investigated
the ridge subduction system through seismic
imaging of the central Japan convergent margin off
Tokai. The new field experiment, integrating onshoreoffshore
wide-angle seismic surveys, was carried out
from the Shikoku district to the Japan Sea off Tottori
to image the subducting Philippine Sea plate where it
subducts beneath SW Japan.
() The seismogenic zone material science group sampled
and analyzed exhumed ancient fault rocks in the
Shikoku district to better understand the microdynamic
systems in seismogenic zones. As a result,
we elucidated that a seismogenic fault containing
pseudotachylyte was developed along the roof thrust
of a duplex structure.
() The plate dynamics modeling group produced
results of modeling of regions around the Nankai
trough and the Japan trench, using mainly the results
from the structure research group. The coding for the
ES has been developed, and some large scale simulations,
such as crustal activity, earthquake cycles, and
mantle convection have been implemented on the ES.
Details of the research activities are summarized in
the following.
2. Lithospheric Structure Research Group
2.1. Outline
Toward understanding both variety and universality
of plate dynamics in various spatial-temporal scales,
the lithospheric structure research group strives to
clarify structural factors constraining tectonic processes
from the evolution of trench-arc-backarc systems to
the generation of great subduction earthquakes.
2.2. Results
() Research on the Nankai Trough seismogenic zone
(i) Subduction dynamics of an oceanic ridge imaged in
the central Japan convergent margin
Recent seismic reflection and refraction data reveal
a trough-parallel subducted oceanic ridge that is
attached to the descending plate beneath the accretionary
wedge of the overriding plate and spans the
two segments (Fig.). The subducted ridge is esti-
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