Marine Ecosystems Research Department - jamstec japan agency ...

JAMSTEC 2002 Annual Report
Institute for Frontier Research on Earth Evolution (IFREE)
time-depend increase of
cementation area
asperities
DSR
roof thrust
duplex structure
fluid flow
subducting crust
authigenic clays. Crosscutting relations of microstructures
indicate that the décollement zone records two
compactive deformations. The early compactive
deformation involved destruction of porous cemented
structure, probably caused by fluid pressure fluctuation.
The late compactive deformation was characterized
by clay-particle rotation and porosity collapse
along sets of slip surfaces, resulting in zones of preferred
orientation of clay particles. These compactive
deformations led to significantly higher bulk densities
within the décollement zone compared to the compaction
trend of the overlying prism sediments.
Elevated fluid pressure following compactive deformation
induced an overconsolidated state within the
décollement zone, with fluid-filled dilatant fractures.
Bulk density abruptly decreases at the top of the
underthrust sediments, but there is no microstructural
evidence for cementation. Fluids in the dilated fractures
and underconsolidated underthrust sediments are
potential sources for the elevated fluid pressure in and
below the décollement zone, resulting in a mechanical
decoupling of the accretionary prism from underthrust
sediments. The fault-fluid interactions in the Muroto
region may be applicable to other convergent plate
margins where high temperatures associated with the
subduction of a spreading ridge or hot, young oceanic
crust enhance diagenesis and cementation.
() Seismogenic zone
(i) Fault rock analysis
The lateral heterogeneity of seismogenic fault rock
has been surveyed by comparing two different sites
within the same melange zone of the Cretaceous Mugi
and Okitsu Melanges, Shimanto accretionary complex,
Shikoku. Geologic survey, strain analysis, pressuretemperature
estimation, and permeablity measurements
have been done. The strain fabric of the sediment
changed from the flattening oblate to prolate
type at the shear zone, and % of dissoluble materials
were transported with a fluid during pressure-solution
deformation. Subsequently, the lithified sedimentary
rock and oceanic crust suffered ultra-cataclastic
failure at seismogenic depth. The Okitsu Melange was
composed of a duplex structure of the oceanic stratigraphic
sequence, and a seismogenic fault, including
pseudotachylyte, developed along the roof thrust of
the duplex structure (Fig.). The Okitsu Melange was
buried to a depth of ˚C in temperature, and the
estimated pressure temperature conditions of the Mugi
Melange was MPa and ˚C. These pressuretemperature
conditions are consistent with the seismogenic
zone in the present Nankai trough. The
anisotropy of the permeability of the shale was recognized
in the Mugi Melange, and this result explains
the occurrence of vein mineral concentrations in the
shale layers. The greatest concentration of vein minerals
was found along a part of the seismogenic fault in
the Okitsu Melange. The vein minerals were deposited
between clasts within the fault breccia, and the vein
minerals may therefore, cement the fault rock. The
local cementation of vein minerals along the fault
implies heterogeneity in the strength of the fault
derived from localised fluid flow and vein mineral
precipitation along the fault.
(ii) Laboratory experiment
alternative deformation of
pressure solution creep and
pseudotachylyte formation
dynamic slip
quasistatic slip
Fig.22 Tectonic setting of the pseudotachylite bearing fault.
We conducted permeability measurements of basalt
sampled from an exhumed ancient fault zone in the
Cretaceous Shimanto accretionary complex in Japan,
in order to investigate the permeability structure and
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