Crustal growth
Magma supply from the mantle in subduction zones (see
“Island Arc Volcanism, Volcanic Arcs” and “Magmatism
at Convergent Plate Boundaries”) progressively thickens
arc crusts. Continental arcs develop overprinting the
preexisting continental crust of long histories and complex
structures, whereas intraoceanic arcs are more juvenile
(less than several tens m.y. old) with simpler starting materials of hanging-wall oceanic lithosphere. They can thus
provide more elementary models for magma genesis and
crustal growth, whose processes are referred to as subduction factory.
Intraoceanic arc crusts are 10–30 km thick with significant variation even in a single arc (e.g., Kodaira et al.,
2007). They are intermediate between those of oceanic
and continental crusts in terms of thickness and composition. The Izu-Bonin-Mariana arc characteristically has the
middle crust layer of 6–6.5 km/s (Suyehiro et al., 1996)
not found in oceanic crusts. It is assigned to felsic plutonic
rocks such as tonalite (low-K variety of granitoids), compared with crustal section exposed in the Izu collision
zone (Tanzawa Mountains) scraped off the Izu arc
(Kawate and Arima, 1998).
Magma input from the mantle is dominantly basaltic,
whereas the bulk continental crusts are estimated as andesitic (Turcotte, 1989). The transformation mechanism from
basaltic ingredients to more felsic products is a major subject on subduction factory. Removal of mafic components
from the differentiated crusts via delamination is one of
the convincing candidates (Turcotte, 1989; Kay and Kay,
1993). Tatsumi et al. (2008) tested several hypotheses
for crustal differentiation with volumetric simulation comparing to the Izu-Bonin-Mariana arc. They showed that
remelting (anatexis) of basaltic crust producing a tonalitic
middle crust and a high-density basal layer of mafic
restites and cumulates is the most compatible model with
the present crustal structure, and the low-Vp mantle
(crust-mantle transition zone) observed in seismic refraction profiles (e.g., Kodaira et al., 2007; Takahashi et al.,
2008) is assigned to the basal layer. Comparable restites
occur as mafic garnet granulite at the base of the Kohistan
arc crustal section (Garrido et al., 2006), a fossil
intraoceanic arc exposed on land. If such basal layer is
delaminated from the overlying crust, average composition of the rest crust could be more felsic.
Earliest stage of subduction
It is generally accepted that the an intraoceanic subduction
begins at a fracture zone or a transform fault among
preexisting oceanic crust, since Uyeda and Ben-Avraham
(1972) considered for the genesis of the Philippine Sea
plate by an idea that an oceanic transform boundary in
the Pacific switched to be convergent when Pacific plate
motion changed in the Eocene. Stern (2004) synthesized
possible processes of subduction initiation into two major
types. One is induced by compression as exampled by the
Solomon subduction system, where the collided Ontong
Java Plateau jammed the previous trench to the north of
the arc and new subduction initiated facing the south.
Another is spontaneous slab sinking resulting from a
high-density contrast between the adjacent oceanic lithosphere and characterized by extension and eruption of
boninites in the forearc as exampled in the Izu-BoninMariana subduction zone.
Boninites are high-magnesian varieties of basaltic
andesite characteristically occurring in not all but some
intraoceanic subduction zones and ophiolites. It suggests
anomalously high temperature at shallow parts of the mantle above the slab (Crawford et al., 1989; Pearce et al.,
1992). Stern and Bloomer (1992) assumed the spontaneous sinking model of the colder oceanic lithosphere at a
transform boundary, initially without convergence. They
argued that the asthenosphere might inject to fill the
vacancy above the sinking slab, promoting a preferable
condition for the boninitic genesis. This model is
supported by a numerical simulation by Hall et al. (2003),
in which however initial push to help bend the thick lithosphere was necessary. Recently, Reagan et al. (2010) and
Ishizuka et al. (2011) found occurrences of MORB-like
basalts (FAB: forearc basalts) underlying boninites in the
Mariana and Bonin forearcs, respectively. They attributed
them to the first injection of asthenosphere prior to the
boninite stage with major supply of water from the slab.
They also explained a common ophiolite succession,
where boninite magmatism followed just after formation
of MORB-like oceanic crusts, by analogy of the Mariana
and Bonin forearc succession.
Summary
Although most of the system is immersed in deep water,
an intraoceanic subduction zone may be one of the most
dynamic regions on the Earth’s surface. Lateral convergence and divergence, sinking and uprising, and accretion
and removal concurrently occur in a single system of
adjoining areas. Intraoceanic subduction zone could also
give hints to consider how Earth’s simplest layering of
the mantle, oceanic crusts, marine water, and atmosphere
interacted and evolved to more diverse and complicated
structure and chemistry as seen in the present. Comprehensive understanding of such dynamics and evolution is
still in progress, and further investigations are necessary
heavily depending on oceanic surveys, as well as prediction and testing by numerical modeling recently significantly developed (Gerya, 2011).
Bibliography
Bloomer, S. H., and Fisher, R. L., 1987. Petrology and geochemistry
of igneous rocks from the Tonga Trench – a non-accreting plate
boundary. Journal of Geology, 95, 469–495.
Bloomer, S. H., Taylor, B., MacLeod, C. J., Stern, R. J., Fryer, P.,
Hawkins, J. W., and Johnson, L., 1995. Early arc volcanism
and the ophiolite problem: a perspective from drilling in the
Western Pacific. In Taylor, B., and Natland, J. (eds.), Active Margin and Marginal Basins of the Western Pacific. Washington,
DC: AGU, pp. 1–30.
370
INTRAOCEANIC SUBDUCTION ZONE
Magma supply from the mantle in subduction zones (see
“Island Arc Volcanism, Volcanic Arcs” and “Magmatism
at Convergent Plate Boundaries”) progressively thickens
arc crusts. Continental arcs develop overprinting the
preexisting continental crust of long histories and complex
structures, whereas intraoceanic arcs are more juvenile
(less than several tens m.y. old) with simpler starting materials of hanging-wall oceanic lithosphere. They can thus
provide more elementary models for magma genesis and
crustal growth, whose processes are referred to as subduction factory.
Intraoceanic arc crusts are 10–30 km thick with significant variation even in a single arc (e.g., Kodaira et al.,
2007). They are intermediate between those of oceanic
and continental crusts in terms of thickness and composition. The Izu-Bonin-Mariana arc characteristically has the
middle crust layer of 6–6.5 km/s (Suyehiro et al., 1996)
not found in oceanic crusts. It is assigned to felsic plutonic
rocks such as tonalite (low-K variety of granitoids), compared with crustal section exposed in the Izu collision
zone (Tanzawa Mountains) scraped off the Izu arc
(Kawate and Arima, 1998).
Magma input from the mantle is dominantly basaltic,
whereas the bulk continental crusts are estimated as andesitic (Turcotte, 1989). The transformation mechanism from
basaltic ingredients to more felsic products is a major subject on subduction factory. Removal of mafic components
from the differentiated crusts via delamination is one of
the convincing candidates (Turcotte, 1989; Kay and Kay,
1993). Tatsumi et al. (2008) tested several hypotheses
for crustal differentiation with volumetric simulation comparing to the Izu-Bonin-Mariana arc. They showed that
remelting (anatexis) of basaltic crust producing a tonalitic
middle crust and a high-density basal layer of mafic
restites and cumulates is the most compatible model with
the present crustal structure, and the low-Vp mantle
(crust-mantle transition zone) observed in seismic refraction profiles (e.g., Kodaira et al., 2007; Takahashi et al.,
2008) is assigned to the basal layer. Comparable restites
occur as mafic garnet granulite at the base of the Kohistan
arc crustal section (Garrido et al., 2006), a fossil
intraoceanic arc exposed on land. If such basal layer is
delaminated from the overlying crust, average composition of the rest crust could be more felsic.
Earliest stage of subduction
It is generally accepted that the an intraoceanic subduction
begins at a fracture zone or a transform fault among
preexisting oceanic crust, since Uyeda and Ben-Avraham
(1972) considered for the genesis of the Philippine Sea
plate by an idea that an oceanic transform boundary in
the Pacific switched to be convergent when Pacific plate
motion changed in the Eocene. Stern (2004) synthesized
possible processes of subduction initiation into two major
types. One is induced by compression as exampled by the
Solomon subduction system, where the collided Ontong
Java Plateau jammed the previous trench to the north of
the arc and new subduction initiated facing the south.
Another is spontaneous slab sinking resulting from a
high-density contrast between the adjacent oceanic lithosphere and characterized by extension and eruption of
boninites in the forearc as exampled in the Izu-BoninMariana subduction zone.
Boninites are high-magnesian varieties of basaltic
andesite characteristically occurring in not all but some
intraoceanic subduction zones and ophiolites. It suggests
anomalously high temperature at shallow parts of the mantle above the slab (Crawford et al., 1989; Pearce et al.,
1992). Stern and Bloomer (1992) assumed the spontaneous sinking model of the colder oceanic lithosphere at a
transform boundary, initially without convergence. They
argued that the asthenosphere might inject to fill the
vacancy above the sinking slab, promoting a preferable
condition for the boninitic genesis. This model is
supported by a numerical simulation by Hall et al. (2003),
in which however initial push to help bend the thick lithosphere was necessary. Recently, Reagan et al. (2010) and
Ishizuka et al. (2011) found occurrences of MORB-like
basalts (FAB: forearc basalts) underlying boninites in the
Mariana and Bonin forearcs, respectively. They attributed
them to the first injection of asthenosphere prior to the
boninite stage with major supply of water from the slab.
They also explained a common ophiolite succession,
where boninite magmatism followed just after formation
of MORB-like oceanic crusts, by analogy of the Mariana
and Bonin forearc succession.
Summary
Although most of the system is immersed in deep water,
an intraoceanic subduction zone may be one of the most
dynamic regions on the Earth’s surface. Lateral convergence and divergence, sinking and uprising, and accretion
and removal concurrently occur in a single system of
adjoining areas. Intraoceanic subduction zone could also
give hints to consider how Earth’s simplest layering of
the mantle, oceanic crusts, marine water, and atmosphere
interacted and evolved to more diverse and complicated
structure and chemistry as seen in the present. Comprehensive understanding of such dynamics and evolution is
still in progress, and further investigations are necessary
heavily depending on oceanic surveys, as well as prediction and testing by numerical modeling recently significantly developed (Gerya, 2011).
Bibliography
Bloomer, S. H., and Fisher, R. L., 1987. Petrology and geochemistry
of igneous rocks from the Tonga Trench – a non-accreting plate
boundary. Journal of Geology, 95, 469–495.
Bloomer, S. H., Taylor, B., MacLeod, C. J., Stern, R. J., Fryer, P.,
Hawkins, J. W., and Johnson, L., 1995. Early arc volcanism
and the ophiolite problem: a perspective from drilling in the
Western Pacific. In Taylor, B., and Natland, J. (eds.), Active Margin and Marginal Basins of the Western Pacific. Washington,
DC: AGU, pp. 1–30.
370
INTRAOCEANIC SUBDUCTION ZONE