continental margins. Even if these subduction systems constitute \15 % of the
crust created (Lallemand et al. 2008), they are important areas since this is where
any eventual new crust will be accreted onto the continents.
Subduction zones are located above the inclined lithospheric slabs that plunge
under a crust of lighter density (silica-enriched and metamorphosed rocks). The
descent of the lithospheric slabs, which are about 60–90 km thick, is defined by the
traces of a seismic zone that is called the ‘‘Benioff zone’’, named after a US.
geologist named Hugo Benioff. The Benioff zone refers to a seismic area where
slabs of a plate are subducted under the continents and/or island arcs producing
deep-seated earthquakes. A bending of the oceanic plates is accompanied by their
breaking up when slabs under thrust the overriding landmasses. During subduction, earthquakes are caused by strain accumulation taking place during the plate
motions and will depend on several factors such as: plate velocity, plate age, the
length of the down-going slab, the dip angle of the Benioff-zone, and the structure
and nature of the plate. Also, the presence of earthquakes will be influenced by
fault-breaks in the subducting plate, by the degree of rock alteration, and by the
presence of large seamounts and/or sediment. All these factors have an impact on
slab-subduction. For example, the fractured oceanic crust forming a horst and
graben type of structure can develop a heterogeneous contact plane that will
decrease the strength of the subducted slab. Also, a plate’s older age, therefore
making the plate denser, also means that the plate will have a stronger tendency to
sink with less strain accumulation, thus generating smaller earthquakes. A structurally uniform subducting plate will form a more resistant slab and thus be likely
to accumulate more strength in resisting subduction. More details on the subject of
subduction are available in the work done by Hamilton (1974), Kanamori (1986)
and in a publication edited by Larter and Leat (2003).
Volcanism and Metamorphism in Subduction Zone
During subduction, the plunging slab will drag indurate sediment and altered
rocks, thereby generating frictional heating of the rocks, so they will melt and form
local pockets of magma. The magma pockets will rise, due to the effects of
buoyancy, and arrive in a shallower level in the crust to form magma pools. It is
also known that the plunging slab contains water due to its release from compacted
sediment and the alteration of ancient oceanic lithosphere. This water enters into
the lattice of hydrated minerals during metamorphism. The water content of
altered oceanic rocks (metamorphosed rocks = metabasalt, metagabbros, serpentinized peridotite) can be as much 2–6 % of a rock’s weight (H 2 O = 2–6 wt%).
This water content will lower the melting temperature of the oceanic rock to about
600° at shallow depths or will create a lower pressure in an anhydrous condition.
The depth of seawater penetration and the subsequent hydrothermal circulation
favoring rock alteration could be as deep as several kilometers (10–15 km), on the
basis of inferences of earthquake analyses defining the limit between the brittle/
Nomenclature of Subduction Zones
349
crust created (Lallemand et al. 2008), they are important areas since this is where
any eventual new crust will be accreted onto the continents.
Subduction zones are located above the inclined lithospheric slabs that plunge
under a crust of lighter density (silica-enriched and metamorphosed rocks). The
descent of the lithospheric slabs, which are about 60–90 km thick, is defined by the
traces of a seismic zone that is called the ‘‘Benioff zone’’, named after a US.
geologist named Hugo Benioff. The Benioff zone refers to a seismic area where
slabs of a plate are subducted under the continents and/or island arcs producing
deep-seated earthquakes. A bending of the oceanic plates is accompanied by their
breaking up when slabs under thrust the overriding landmasses. During subduction, earthquakes are caused by strain accumulation taking place during the plate
motions and will depend on several factors such as: plate velocity, plate age, the
length of the down-going slab, the dip angle of the Benioff-zone, and the structure
and nature of the plate. Also, the presence of earthquakes will be influenced by
fault-breaks in the subducting plate, by the degree of rock alteration, and by the
presence of large seamounts and/or sediment. All these factors have an impact on
slab-subduction. For example, the fractured oceanic crust forming a horst and
graben type of structure can develop a heterogeneous contact plane that will
decrease the strength of the subducted slab. Also, a plate’s older age, therefore
making the plate denser, also means that the plate will have a stronger tendency to
sink with less strain accumulation, thus generating smaller earthquakes. A structurally uniform subducting plate will form a more resistant slab and thus be likely
to accumulate more strength in resisting subduction. More details on the subject of
subduction are available in the work done by Hamilton (1974), Kanamori (1986)
and in a publication edited by Larter and Leat (2003).
Volcanism and Metamorphism in Subduction Zone
During subduction, the plunging slab will drag indurate sediment and altered
rocks, thereby generating frictional heating of the rocks, so they will melt and form
local pockets of magma. The magma pockets will rise, due to the effects of
buoyancy, and arrive in a shallower level in the crust to form magma pools. It is
also known that the plunging slab contains water due to its release from compacted
sediment and the alteration of ancient oceanic lithosphere. This water enters into
the lattice of hydrated minerals during metamorphism. The water content of
altered oceanic rocks (metamorphosed rocks = metabasalt, metagabbros, serpentinized peridotite) can be as much 2–6 % of a rock’s weight (H 2 O = 2–6 wt%).
This water content will lower the melting temperature of the oceanic rock to about
600° at shallow depths or will create a lower pressure in an anhydrous condition.
The depth of seawater penetration and the subsequent hydrothermal circulation
favoring rock alteration could be as deep as several kilometers (10–15 km), on the
basis of inferences of earthquake analyses defining the limit between the brittle/
Nomenclature of Subduction Zones
349