thickened by formation of a mountain root during collision. These plutons record a strong interaction and
mechanical feedback with deformation in major shear
zones (e.g., Schneider et al., 1999; Rosenberg, 2004).
(iv) A suture zone belt marks the original contact between
two plates that have collided or accreted and is therefore of central interest in order to determine the timing
and the nature of collision processes (Haynes and
McQuillan, 1974; Klootwijk et al., 1984; Colchen
et al., 1986; Agard et al., 2005; Anders et al., 2005;
Schmid et al., 2008b; Jolivet and Brun, 2010). In
orogen perpendicular cross sections, suture zones
are generally steeply dipping zones, which are characterized by an asymmetry inherited from subduction
separating block with a marked different geological
history. A collisional suture marks the site of
a subducted oceanic plate, which separated the collided
continents. Therefore, the suture zone consists of various strongly deformed lithologies derived from the
oceanic plate, exotic terrane, or accretionary wedges
of the continents (tectonic mélange). The existence of
ophiolitic remnants between continental blocks is the
most direct evidence for the location of the suture zone.
Geodynamic models suggest that during the initial stage of collision, the suture zone forms a large
shear band where most of the deformation is localized (Toussaint et al., 2004). This stage is followed
by back thrusting around the suture zone and displacement of the initial crustal suture zone towards
the overriding plate (Willet, 1994; Beaumont et al.,
1996; Ellis, 1996). A zone of high topography, erosion, and exhumation grows above the area of active
deformation. After further shortening the suture
zone is no longer the major locus of deformation,
but crustal scale thrusts dominate the surface deformation leading to the formation of another zone of
surface uplift (Toussaint et al., 2004).
(v) A volcanic arc forms in the hanging wall of an oceanic subduction and is the result of melting of the
mantle wedge induced by volatiles released from
the downgoing plate (mainly water), which drastically lower the melting point of the mantle. Therefore, magma ascends above the subducting slab and
forms an arc of volcanoes parallel to the subduction
zone. This arc can form at the rim of a continental
plate (continental arc, e.g., the Andes) or at the rim
of an oceanic plate (island arc, e.g., the Lesser Antilles), depending if the oceanic crust subducts beneath
a continent or another oceanic crust, respectively.
The arcuate shape of volcanic arcs is the result of
the curvature of the Earth’s surface forcing the
subducting plate to deform into a concave shape.
About a quarter of the magmatic rocks on Earth
are produced above subduction zones to form volcanic arcs (Schminke, 2004). Water-bearing fluids from
the subducted slab, which are mainly derived from
destabilization of chlorite at approximately 50 km
depth, rise into the overriding mantle wedge induces
partial melting of and generates low-density, calcalkaline magma (Stern, 2002). The magma buoyantly
rises through the overlying crust because of its lower
density. If the crust above the subduction zone is oceanic, the basaltic to andesitic magma is not significantly altered. Magmas, which intrude into the
continental crust are altered by assimilation of crustal
material and is enriched in SiO 2 and K 2 O.
Recently, the geodynamic processes associated
with subduction and volcanic arc growth have been
studied by various numerical models including the
subduction dynamics influenced by volatile release
(Gerya et al., 2002; Rüpke et al., 2004), melting processes associated with the downgoing slab (Gerya
and Yuen, 2003; Gerya et al., 2004), growth and
mixing dynamics of magmas in mantle wedges
(Gorczyk et al., 2007a; Gorczyk et al., 2007b), and
the crustal growth of volcanic arcs (Nikolaeva
et al., 2008). Some of these models include the physics of various geochemical–petrological–thermomechanical processes investigating slab retreat and
bending, crust dehydration, fluid transport, mantle
wedge melting, and melt extraction. The results of
these numerical experiments show that the rate
of plate retreat significantly influences the rate of
crustal growth of the volcanic arc and the composition of newly formed crust because of the major contribution of a crustal component by hydrated partially
molten upwellings rising from downgoing slabs
(Gerya and Yuen, 2003; Nikolaeva et al., 2008).
Destruction of orogens
Continental collision leads to crustal thickening and building of high topography supported by a thick crustal root.
The transition from the cold oceanic subduction regime
to the warm collision regime weakens the orogenic root,
inducing the formation of a continental plateau. The lower
crust below the plateau can melt and flow out horizontally
in response to crustal thickness variations (Royden, 1996;
Rey et al., 2001). During ongoing convergence, lateral
extrusion of crustal blocks can become the dominant
mechanism accommodating crustal shortening, instead
of crustal thickening (Tapponnier and Molnar, 1976;
Ratschbacher et al., 1991). Depending on free or extensional boundary condition, crustal blocks can accommodate tens to hundreds of kilometers of lateral
displacement along major strike-slip faults (Seyferth and
Henk, 2004). When convergence between the two plates
involved in the orogen stops, gravitational collapse
occurs, inducing thinning of the crust and destruction of
the orogenic belt (Vanderhaeghe, 2009). It may be associated, at depth, with slab rollback, slab detachment, delamination, or convective thinning of the lithospheric mantle
(Houseman et al., 1981; Wortel and Spakman, 1992;
Davis and von Blanckenburg, 1995) and reflected by generalized post-orogenic I- and S-type magmatic activity.
606
OROGENY
mechanical feedback with deformation in major shear
zones (e.g., Schneider et al., 1999; Rosenberg, 2004).
(iv) A suture zone belt marks the original contact between
two plates that have collided or accreted and is therefore of central interest in order to determine the timing
and the nature of collision processes (Haynes and
McQuillan, 1974; Klootwijk et al., 1984; Colchen
et al., 1986; Agard et al., 2005; Anders et al., 2005;
Schmid et al., 2008b; Jolivet and Brun, 2010). In
orogen perpendicular cross sections, suture zones
are generally steeply dipping zones, which are characterized by an asymmetry inherited from subduction
separating block with a marked different geological
history. A collisional suture marks the site of
a subducted oceanic plate, which separated the collided
continents. Therefore, the suture zone consists of various strongly deformed lithologies derived from the
oceanic plate, exotic terrane, or accretionary wedges
of the continents (tectonic mélange). The existence of
ophiolitic remnants between continental blocks is the
most direct evidence for the location of the suture zone.
Geodynamic models suggest that during the initial stage of collision, the suture zone forms a large
shear band where most of the deformation is localized (Toussaint et al., 2004). This stage is followed
by back thrusting around the suture zone and displacement of the initial crustal suture zone towards
the overriding plate (Willet, 1994; Beaumont et al.,
1996; Ellis, 1996). A zone of high topography, erosion, and exhumation grows above the area of active
deformation. After further shortening the suture
zone is no longer the major locus of deformation,
but crustal scale thrusts dominate the surface deformation leading to the formation of another zone of
surface uplift (Toussaint et al., 2004).
(v) A volcanic arc forms in the hanging wall of an oceanic subduction and is the result of melting of the
mantle wedge induced by volatiles released from
the downgoing plate (mainly water), which drastically lower the melting point of the mantle. Therefore, magma ascends above the subducting slab and
forms an arc of volcanoes parallel to the subduction
zone. This arc can form at the rim of a continental
plate (continental arc, e.g., the Andes) or at the rim
of an oceanic plate (island arc, e.g., the Lesser Antilles), depending if the oceanic crust subducts beneath
a continent or another oceanic crust, respectively.
The arcuate shape of volcanic arcs is the result of
the curvature of the Earth’s surface forcing the
subducting plate to deform into a concave shape.
About a quarter of the magmatic rocks on Earth
are produced above subduction zones to form volcanic arcs (Schminke, 2004). Water-bearing fluids from
the subducted slab, which are mainly derived from
destabilization of chlorite at approximately 50 km
depth, rise into the overriding mantle wedge induces
partial melting of and generates low-density, calcalkaline magma (Stern, 2002). The magma buoyantly
rises through the overlying crust because of its lower
density. If the crust above the subduction zone is oceanic, the basaltic to andesitic magma is not significantly altered. Magmas, which intrude into the
continental crust are altered by assimilation of crustal
material and is enriched in SiO 2 and K 2 O.
Recently, the geodynamic processes associated
with subduction and volcanic arc growth have been
studied by various numerical models including the
subduction dynamics influenced by volatile release
(Gerya et al., 2002; Rüpke et al., 2004), melting processes associated with the downgoing slab (Gerya
and Yuen, 2003; Gerya et al., 2004), growth and
mixing dynamics of magmas in mantle wedges
(Gorczyk et al., 2007a; Gorczyk et al., 2007b), and
the crustal growth of volcanic arcs (Nikolaeva
et al., 2008). Some of these models include the physics of various geochemical–petrological–thermomechanical processes investigating slab retreat and
bending, crust dehydration, fluid transport, mantle
wedge melting, and melt extraction. The results of
these numerical experiments show that the rate
of plate retreat significantly influences the rate of
crustal growth of the volcanic arc and the composition of newly formed crust because of the major contribution of a crustal component by hydrated partially
molten upwellings rising from downgoing slabs
(Gerya and Yuen, 2003; Nikolaeva et al., 2008).
Destruction of orogens
Continental collision leads to crustal thickening and building of high topography supported by a thick crustal root.
The transition from the cold oceanic subduction regime
to the warm collision regime weakens the orogenic root,
inducing the formation of a continental plateau. The lower
crust below the plateau can melt and flow out horizontally
in response to crustal thickness variations (Royden, 1996;
Rey et al., 2001). During ongoing convergence, lateral
extrusion of crustal blocks can become the dominant
mechanism accommodating crustal shortening, instead
of crustal thickening (Tapponnier and Molnar, 1976;
Ratschbacher et al., 1991). Depending on free or extensional boundary condition, crustal blocks can accommodate tens to hundreds of kilometers of lateral
displacement along major strike-slip faults (Seyferth and
Henk, 2004). When convergence between the two plates
involved in the orogen stops, gravitational collapse
occurs, inducing thinning of the crust and destruction of
the orogenic belt (Vanderhaeghe, 2009). It may be associated, at depth, with slab rollback, slab detachment, delamination, or convective thinning of the lithospheric mantle
(Houseman et al., 1981; Wortel and Spakman, 1992;
Davis and von Blanckenburg, 1995) and reflected by generalized post-orogenic I- and S-type magmatic activity.
606
OROGENY