supply and fast convergence result in poor development or
absence of accretionary wedges and, instead, facilitate
subduction erosion removing hanging-wall materials
around trenches. On the contrary, abundant clastic turbidites from adjacent continents bury trenches of the Lesser
Antilles (Barbados) and eastern Aleutian subduction
zones, where sediments are being scraped off to comprise
accretionary wedges.
Forearc
Forearc regions between the trench and the volcanic front
consist of inner trench slope, forearc basin, and outer
arc. Surface slope dips greatly change between the steeper
inner trench slope and the flat forearc basin, and the
boundary between them is called as a trench-slope break.
Inner trench slopes to outer margins of forearc basins of
intraoceanic non-accretionary margins are dominated by
normal faults with stepping fault scarps and/or horsts
and grabens (Hussong and Uyeda, 1981). In Izu-BoninMariana and Tonga-Kermadec forearcs, the basement
rocks there show ophiolitic assemblage of serpentinized
peridotites, gabbro, and early arc volcanic rocks including
boninites (Ishii, 1985; Bloomer and Fisher, 1987;
Bloomer et al., 1995). Minor occurrences of mid-ocean
ridge basalts (MORB: see “Mid-ocean Ridge Magmatism
and Volcanism”) and alkali basalts of seamount origins,
chert, and limestone were presumably accreted from the
subducted Pacific plate (Johnson et al., 1991).
In outer margins of the Mariana forearc basin,
serpentinite seamounts of conical shapes are aligned parallel to the trench, in addition to serpentinite horsts of tectonic features (Fryer, 1992). Similar serpentinite
seamounts also occur in the inner trench slope of the
Bonin forearc (Fryer, 1992). The Mariana forearc
serpentinite seamounts are mud volcanoes of highly fractured and clayey serpentinites, occasionally with serpentine mud flows and cold-water vents with carbonate
chimneys (Fryer et al., 1990). These serpentinites originated from residual peridotites after extensive extraction
presumably of arc magmas and boninites (Ishii et al.,
1992) and are thought to represent forearc mantle rocks.
Forearc basins are filled by thick sediments comprising
of flat and smooth surfaces. Tectonic deformation is less
extensive compared with trench-side areas, suggesting
more rigid basements. The Moho surfaces beneath
forearcs become indistinct as seismic velocity of the mantle is reduced toward the trench (Suyehiro et al., 1996). If
successfully imaged, forearc crust is thinner than volcanic
arc crust and partly as thin as normal oceanic crust
(Kodaira et al., 2010).
Subduction interface
There has been no drill hole which penetrated intraoceanic
subduction boundaries of non-accretionary type. However, it is probable that the wedge mantle extends to or
very close to the trenches (Bloomer and Fisher, 1987;
Kamimura et al., 2002) in the cases of the TongaKermadec and Izu-Bonin-Mariana subduction zones, taking into account thin forearc crusts and common occurrences of peridotite and serpentinites on inner trench
slopes. Reduction of seismic velocity in the wedge mantle
toward the trench suggests serpentinization of peridotites
(Hyndman and Peacock, 2003). These lines of evidence
imply that serpentinites are in contact with the subducting
oceanic crust since shallow depths of these intraoceanic
subduction zones and that crust-crust interface is limited.
This may result in narrow zones of seismic coupling, in
contrasts to continental subduction zones, where contacts
of thick continental crusts and the subducted slabs comprise extensive seismogenic zones (Hyndman et al., 1997).
Serpentinized subduction interface is also supported by
the occurrences of serpentinite seamounts in the Mariana
forearc. Basaltic clasts recording high-pressure/lowtemperature metamorphism enclosed in the serpentinite
suggest the origin of the mud volcanoes from the subduction interface (Maekawa et al., 1993). Mixed occurrences
of sheared serpentinite matrix and clasts and blocks of
ophiolitic and metamorphic rocks are correlative to
serpentinite melange.
Because water fluids might be supplied from the underlying slab, a wedge mantle is expected to be serpentinized
from its base, where mechanical strength is significantly
reduced. This predicts a structure that the weak
serpentinite zone is sandwiched between more rigid
layers: the slab and the unhydrated mantle wedge
(Figure 2). Strain resulted from plate convergence concentrate to this serpentinite zone, where material flows are
channelized. The concept serpentinite channel (Guillot
et al., 2001) is applied to the zone and is regarded as a conduit for upwelling of deeply subducted rocks.
Extension and weak seismic coupling
Uyeda and Kanamori (1979) classified subduction zones
into Mariana and Chilean types in terms of stress regime
in the hanging-wall plate. The Mariana type is characterized by extensional stress evidenced by active back-arc
rifting or spreading. It is associated with steep dip subduction and tends to occur with west-dipping subduction
polarity. Large inter-plate earthquakes are scarce,
suggesting weak coupling between the two converging
plates. The Chilean type in contrast is characterized by
compression without active back-arc spreading, gently
dipping slabs dominantly of eastward subduction, and
common large inter-plate earthquakes with strong coupling. As evident from its nomenclature, typical
intraoceanic subduction zones such as Izu-Bonin-Mariana
and Tonga belong to the Mariana type. Extension and
weak inter-plate coupling is attributed to relative retreat
of the trench off the hanging-wall plate. Uyeda and
Kanamori (1979) argued several ideas to explain the
trench retreat: by sinking of the slab (trench rollback);
migration of the hanging-wall plate away from the trench
fixed to the deep mantle, and eastward migration of
trenches reflecting the global asthenosphere flow.
INTRAOCEANIC SUBDUCTION ZONE
369
absence of accretionary wedges and, instead, facilitate
subduction erosion removing hanging-wall materials
around trenches. On the contrary, abundant clastic turbidites from adjacent continents bury trenches of the Lesser
Antilles (Barbados) and eastern Aleutian subduction
zones, where sediments are being scraped off to comprise
accretionary wedges.
Forearc
Forearc regions between the trench and the volcanic front
consist of inner trench slope, forearc basin, and outer
arc. Surface slope dips greatly change between the steeper
inner trench slope and the flat forearc basin, and the
boundary between them is called as a trench-slope break.
Inner trench slopes to outer margins of forearc basins of
intraoceanic non-accretionary margins are dominated by
normal faults with stepping fault scarps and/or horsts
and grabens (Hussong and Uyeda, 1981). In Izu-BoninMariana and Tonga-Kermadec forearcs, the basement
rocks there show ophiolitic assemblage of serpentinized
peridotites, gabbro, and early arc volcanic rocks including
boninites (Ishii, 1985; Bloomer and Fisher, 1987;
Bloomer et al., 1995). Minor occurrences of mid-ocean
ridge basalts (MORB: see “Mid-ocean Ridge Magmatism
and Volcanism”) and alkali basalts of seamount origins,
chert, and limestone were presumably accreted from the
subducted Pacific plate (Johnson et al., 1991).
In outer margins of the Mariana forearc basin,
serpentinite seamounts of conical shapes are aligned parallel to the trench, in addition to serpentinite horsts of tectonic features (Fryer, 1992). Similar serpentinite
seamounts also occur in the inner trench slope of the
Bonin forearc (Fryer, 1992). The Mariana forearc
serpentinite seamounts are mud volcanoes of highly fractured and clayey serpentinites, occasionally with serpentine mud flows and cold-water vents with carbonate
chimneys (Fryer et al., 1990). These serpentinites originated from residual peridotites after extensive extraction
presumably of arc magmas and boninites (Ishii et al.,
1992) and are thought to represent forearc mantle rocks.
Forearc basins are filled by thick sediments comprising
of flat and smooth surfaces. Tectonic deformation is less
extensive compared with trench-side areas, suggesting
more rigid basements. The Moho surfaces beneath
forearcs become indistinct as seismic velocity of the mantle is reduced toward the trench (Suyehiro et al., 1996). If
successfully imaged, forearc crust is thinner than volcanic
arc crust and partly as thin as normal oceanic crust
(Kodaira et al., 2010).
Subduction interface
There has been no drill hole which penetrated intraoceanic
subduction boundaries of non-accretionary type. However, it is probable that the wedge mantle extends to or
very close to the trenches (Bloomer and Fisher, 1987;
Kamimura et al., 2002) in the cases of the TongaKermadec and Izu-Bonin-Mariana subduction zones, taking into account thin forearc crusts and common occurrences of peridotite and serpentinites on inner trench
slopes. Reduction of seismic velocity in the wedge mantle
toward the trench suggests serpentinization of peridotites
(Hyndman and Peacock, 2003). These lines of evidence
imply that serpentinites are in contact with the subducting
oceanic crust since shallow depths of these intraoceanic
subduction zones and that crust-crust interface is limited.
This may result in narrow zones of seismic coupling, in
contrasts to continental subduction zones, where contacts
of thick continental crusts and the subducted slabs comprise extensive seismogenic zones (Hyndman et al., 1997).
Serpentinized subduction interface is also supported by
the occurrences of serpentinite seamounts in the Mariana
forearc. Basaltic clasts recording high-pressure/lowtemperature metamorphism enclosed in the serpentinite
suggest the origin of the mud volcanoes from the subduction interface (Maekawa et al., 1993). Mixed occurrences
of sheared serpentinite matrix and clasts and blocks of
ophiolitic and metamorphic rocks are correlative to
serpentinite melange.
Because water fluids might be supplied from the underlying slab, a wedge mantle is expected to be serpentinized
from its base, where mechanical strength is significantly
reduced. This predicts a structure that the weak
serpentinite zone is sandwiched between more rigid
layers: the slab and the unhydrated mantle wedge
(Figure 2). Strain resulted from plate convergence concentrate to this serpentinite zone, where material flows are
channelized. The concept serpentinite channel (Guillot
et al., 2001) is applied to the zone and is regarded as a conduit for upwelling of deeply subducted rocks.
Extension and weak seismic coupling
Uyeda and Kanamori (1979) classified subduction zones
into Mariana and Chilean types in terms of stress regime
in the hanging-wall plate. The Mariana type is characterized by extensional stress evidenced by active back-arc
rifting or spreading. It is associated with steep dip subduction and tends to occur with west-dipping subduction
polarity. Large inter-plate earthquakes are scarce,
suggesting weak coupling between the two converging
plates. The Chilean type in contrast is characterized by
compression without active back-arc spreading, gently
dipping slabs dominantly of eastward subduction, and
common large inter-plate earthquakes with strong coupling. As evident from its nomenclature, typical
intraoceanic subduction zones such as Izu-Bonin-Mariana
and Tonga belong to the Mariana type. Extension and
weak inter-plate coupling is attributed to relative retreat
of the trench off the hanging-wall plate. Uyeda and
Kanamori (1979) argued several ideas to explain the
trench retreat: by sinking of the slab (trench rollback);
migration of the hanging-wall plate away from the trench
fixed to the deep mantle, and eastward migration of
trenches reflecting the global asthenosphere flow.
INTRAOCEANIC SUBDUCTION ZONE
369