Changes

==Principles of Subduction Zone==  

==Principles of Subduction Zone==  

===Forces in Subduction Zone===

===Forces in Subduction Zone===

Oceanic crust generated in MOR will migrate nearer to the trench. Driving mechanism which influences the dynamics of subduction zone is density. Oceanic crust is denser than continental one, therefore the sinking of oceanic crust below continental crust is possible. Forces driving subduction are ridge-push and slab-pull. As oceanic crust moves away from the ridge, conductive cooling turns the crust to be denser. Density of oceanic crust varies laterally, increasing from the ridge axis to the trench. Such force is parallel to the plate since it pushes in topographic slope. Slab-pull force involves cold, dense oceanic lithosphere and hot, lighter mantle. Slab-pull force dominantly accommodates the vertical displacement of plate. This provides the biggest force for oceanic crust to sink.

Oceanic crust generated in MOR will migrate nearer to the trench. Driving mechanism which influences the dynamics of subduction zone is density. Oceanic crust is denser than continental one, therefore the sinking of oceanic crust below continental crust is possible. Forces driving subduction are ridge-push and slab-pull. As oceanic crust moves away from the ridge, conductive cooling turns the crust to be denser. Density of oceanic crust varies laterally, increasing from the ridge axis to the trench. Such force is parallel to the plate since it pushes in topographic slope. Slab-pull force involves cold, dense oceanic lithosphere and hot, lighter [[mantle]]. Slab-pull force dominantly accommodates the vertical displacement of plate. This provides the biggest force for oceanic crust to sink.

Resistive forces are also present in convergent plate boundary. As oceanic crust moves laterally, the base of the plate produces shear resistance, causing deformation of mantle wedge. The contact between oceanic and continental crust along fault plane creates frictional force. On the other hand, when subducted slab enters the depth of 670 km, resistance to penetration occurs because facing discontinuity.

Resistive forces are also present in convergent plate boundary. As oceanic crust moves laterally, the base of the plate produces shear resistance, causing deformation of mantle wedge. The contact between oceanic and continental crust along fault plane creates frictional force. On the other hand, when subducted slab enters the depth of 670 km, resistance to penetration occurs because facing discontinuity.

====Backarc Basin====

====Backarc Basin====

Backarc basin forms landward of the volcanic arc. Mechanisms for backarc basin have been proposed by several authors. Karig (1971) in Keary and Vine<ref name=KV /> stated that the formation of backarc basin is influenced by extensional tectonic regime produced by subduction zone and igneous processes. Basaltic mantle diapirs also contribute to the extension of the plate and increasing heat flow. However, Packham and Falvey (1971) proposed that magma upwelling in backarc basin is passive, generated as a result of extensional regime of the plate. Tamaki (1985) in Keary and Vine<ref name=KV /> stated that the initial rifting of backarc basin takes place at the island arc. The dip of subduction zone controls the nature of rifting. Single rifts form within narrow volcanic zone with steeply dipping Benioff zone. On the other hand, multirift system forms in wider zone with shallow angle of subduction. Another concept of backarc basin formation comes from Chase (1978) and Fein & Jurdy (1986). Regional extension of overriding plate comes from roll-back of the trench. Roll-back occurs when the trench migrates seaward and the oceanic crust retreating. This process produces trench suction force.

Backarc basin forms landward of the volcanic arc. Mechanisms for backarc basin have been proposed by several authors. Karig (1971) in Keary and Vine<ref name=KV /> stated that the formation of backarc basin is influenced by extensional tectonic regime produced by subduction zone and igneous processes. Basaltic [[mantle]] [[diapir]]s also contribute to the extension of the plate and increasing heat flow. However, Packham and Falvey (1971) proposed that magma upwelling in backarc basin is passive, generated as a result of extensional regime of the plate. Tamaki (1985) in Keary and Vine<ref name=KV /> stated that the initial rifting of backarc basin takes place at the island arc. The dip of subduction zone controls the nature of rifting. Single rifts form within narrow volcanic zone with steeply dipping Benioff zone. On the other hand, multirift system forms in wider zone with shallow angle of subduction. Another concept of backarc basin formation comes from Chase (1978) and Fein & Jurdy (1986). Regional extension of overriding plate comes from roll-back of the trench. Roll-back occurs when the trench migrates seaward and the oceanic crust retreating. This process produces trench suction force.

As rifting proceeds in backarc basin, basaltic crust may rise and become the base of the basin. Extensional tectonic regime on overriding plate migrates seaward, generating new backarc basin. Formation of new backarc basin ceases the development of the older one. Time taken for the formation and abandonment of backarc basin requires approximately 20 million years.

As rifting proceeds in backarc basin, basaltic crust may rise and become the base of the basin. Extensional tectonic regime on overriding plate migrates seaward, generating new backarc basin. Formation of new backarc basin ceases the development of the older one. Time taken for the formation and abandonment of backarc basin requires approximately 20 million years.

Volcanic arc above subduction zone is a manifestation of magma generation below the Earth’s surface. Magma generation in subduction zone focuses on the potential sources of partial melting, mechanism of partial melting, and types of magma generated. Magma generated in subduction zone will ascent to the surface as a consequence of buoyancy. Assimilation and fractional crystallization (AFC) will take place, especially in active continental margin.

Volcanic arc above subduction zone is a manifestation of magma generation below the Earth’s surface. Magma generation in subduction zone focuses on the potential sources of partial melting, mechanism of partial melting, and types of magma generated. Magma generated in subduction zone will ascent to the surface as a consequence of buoyancy. Assimilation and fractional crystallization (AFC) will take place, especially in active continental margin.

Potential sources of magma generation are the subducted oceanic crust, mantle wedge, and sea water. Oceanic crust, as previously discussed, consists of terrigenous, carbonate and pelagic sediments, and also sedimentary rock, basalt, and gabbro. Mantle wedge as a part of asthenosphere provides lherzolite and harzburgite. Sea water doesn’t provide silicate materials in magma generation. Otherwise, sea water takes a role in reducing solidus of silicate material. As a result, partial melting can be achieved at lower temperature. Water may reduce the temperature of partial melting about 300oC. Wilson<ref name=Wilson>Wilson, Majorie. 2007. Igneous Petrogenesis: A Global Tectonic Approach. Dordrecht: Springer.</ref> proposed specific potential sources of partial melting:

Potential sources of magma generation are the subducted oceanic crust, [[mantle]] wedge, and sea water. Oceanic crust, as previously discussed, consists of terrigenous, carbonate and pelagic sediments, and also sedimentary rock, basalt, and gabbro. Mantle wedge as a part of asthenosphere provides lherzolite and harzburgite. Sea water doesn’t provide silicate materials in magma generation. Otherwise, sea water takes a role in reducing solidus of silicate material. As a result, partial melting can be achieved at lower temperature. Water may reduce the temperature of partial melting about 300oC. Wilson<ref name=Wilson>Wilson, Majorie. 2007. Igneous Petrogenesis: A Global Tectonic Approach. Dordrecht: Springer.</ref> proposed specific potential sources of partial melting:

* Amphibolite, with or without aqueous fluid

* Amphibolite, with or without aqueous fluid

* Eclogite, with or without aqueous fluid

* Eclogite, with or without aqueous fluid

====Geochemistry of Subduction Zone Igneous Rocks====

====Geochemistry of Subduction Zone Igneous Rocks====

Types of Earth’s crust involved in subduction zone are critical in recognizing the geochemical characteristics of igneous rocks. Igneous petrology classifies subduction-related activity as island-arc and active continental margin. Island-arc only involves oceanic crust in subduction zone. Meanwhile, active continental margin involves both continental and oceanic crust. The two systems generally have similar igneous processes. The difference lies in the more intensive assimilation and fractional crystallization in active continental margin.

Types of Earth’s crust involved in subduction zone are critical in recognizing the geochemical characteristics of [[igneous]] rocks. Igneous petrology classifies subduction-related activity as island-arc and active continental margin. Island-arc only involves oceanic crust in subduction zone. Meanwhile, active continental margin involves both continental and oceanic crust. The two systems generally have similar igneous processes. The difference lies in the more intensive assimilation and fractional crystallization in active continental margin.

====Island-arc====

====Island-arc====

Classification of igneous rocks in geochemistry utilizes the alkaline (K2O and Na2O) and silica content of the rock. Classification based on alkaline and silica content divides igneous rock as alkaline and subalkaline rock. Plotting of K2O and Na2O may be conducted in separate graphs. If the plot in K2O graph states the rock is alkaline while in Na2O says the rock is subalkaline, then the rock is classified as transitional. Graph for alkaline and silica content plotting will be given in Fig. 7 and Fig. 8.

Classification of [[igneous]] rocks in [[geochemistry]] utilizes the alkaline (K2O and Na2O) and silica content of the rock. Classification based on alkaline and silica content divides igneous rock as alkaline and subalkaline rock. Plotting of K2O and Na2O may be conducted in separate graphs. If the plot in K2O graph states the rock is alkaline while in Na2O says the rock is subalkaline, then the rock is classified as transitional. Graph for alkaline and silica content plotting will be given in Fig. 7 and Fig. 8.

Igneous rocks generated in subduction zone generally belong to subalkaline rock. Plotting of K2O and Na2O in Harker diagram produces for classes of subduction magma series: low-K series, calc-alkaline series, high-K series, and shoshonitic series. Potash content in igneous rock is critical because it can represent the degree of contamination of magma. Low-K series is the same as tholeiitic rock. Calc-alkaline magma series have high alumina content. Shoshonitic series represents alkaline rock. Miyashiro (1974) in Wilson<ref name=Wilson /> reveals the difference between calc-alkaline magma series with tholeiitic magma through Fe content. Calc- alkaline magma series show decreasing content of FeO in increasing SiO2 content. Otherwise, tholeiitic magma series show enrichment of Fe in early stage of fractionation.

Igneous rocks generated in subduction zone generally belong to subalkaline rock. Plotting of K2O and Na2O in Harker diagram produces for classes of subduction magma series: low-K series, calc-alkaline series, high-K series, and shoshonitic series. Potash content in igneous rock is critical because it can represent the degree of contamination of magma. Low-K series is the same as tholeiitic rock. Calc-alkaline magma series have high alumina content. Shoshonitic series represents alkaline rock. Miyashiro (1974) in Wilson<ref name=Wilson /> reveals the difference between calc-alkaline magma series with tholeiitic magma through Fe content. Calc- alkaline magma series show decreasing content of FeO in increasing SiO2 content. Otherwise, tholeiitic magma series show enrichment of Fe in early stage of fractionation.

====Spatial and Temporal Distribution of Island-arc Magma Series====

====Spatial and Temporal Distribution of Island-arc Magma Series====

Geochemistry of igneous rocks in subduction zone is not constant in time and space. The evolution of magma series occurs because of subduction geometry and time. Spatial distribution of magma series builds on K – h relationship proposed by Dickinson (1975) in Wilson.<ref name=Wilson /> If silica content is hold constant, the amount of K2O (K) will increase as depth of Benioff zone (h) deepening. Therefore, volcano will produce rock of increasing alkalinity as it migrates away from the trench. Reversal of this characteristic also occurs. Relationship of magma series and time is represented with increasing alkalinity as time progresses. Deeper knowledge is required to build a model for temporal distribution as it is currently poorly understood.

Geochemistry of [[igneous]] rocks in subduction zone is not constant in time and space. The evolution of [[magma]] series occurs because of subduction geometry and time. Spatial distribution of magma series builds on K – h relationship proposed by Dickinson (1975) in Wilson.<ref name=Wilson /> If silica content is hold constant, the amount of K2O (K) will increase as depth of Benioff zone (h) deepening. Therefore, volcano will produce rock of increasing alkalinity as it migrates away from the trench. Reversal of this characteristic also occurs. Relationship of magma series and time is represented with increasing alkalinity as time progresses. Deeper knowledge is required to build a model for temporal distribution as it is currently poorly understood.

====Active Continental Margin====

====Active Continental Margin====

Active continental margin becomes the most complicated site of magma generation of Earth. As discussed in previous section, magma generation begins at the slab and mantle wedge. Partial melting of mantle wedge generates basaltic primitive magma. In island-arc, primitive magma rises to the surface and builds basaltic or andesitic volcano. Igneous processes in island-arc differ with active continental margin in assimilation and fractional crystallization.

Active continental margin becomes the most complicated site of magma generation of Earth. As discussed in previous section, magma generation begins at the slab and [[mantle]] wedge. Partial melting of mantle wedge generates basaltic primitive magma. In island-arc, primitive magma rises to the surface and builds basaltic or andesitic volcano. Igneous processes in island-arc differ with active continental margin in assimilation and fractional crystallization.

Primitive magma generated from the mantle wedge ascent to the boundary of crust and mantle. Due to density contrast, magma from mantle wedge underplates at the base of crust and experiences melting, assimilation, storage, and homogenization (MASH). Assimilation occurs because the crust is molten and enriching the composition of ascending magma. Winter<ref name=Winter>Winter, John D. 2001. An Introduction to Igneous and Metamorphic Petrology. New Jersey: Prentice- Hall Inc.</ref> defines fractionation as mechanical separation of materials with distinct phases. Simplified explanation of fractional crystallization is represented in Bowen reaction series. Magma will ascent from the base of the crust when faults creating fractures for magma migration. This requirement may occur in thinning area.

Primitive magma generated from the mantle wedge ascent to the boundary of crust and mantle. Due to density contrast, magma from mantle wedge underplates at the base of crust and experiences melting, assimilation, storage, and homogenization (MASH). Assimilation occurs because the crust is molten and enriching the composition of ascending magma. Winter<ref name=Winter>Winter, John D. 2001. An Introduction to Igneous and Metamorphic Petrology. New Jersey: Prentice- Hall Inc.</ref> defines fractionation as mechanical separation of materials with distinct phases. Simplified explanation of fractional crystallization is represented in Bowen reaction series. Magma will ascent from the base of the crust when faults creating fractures for magma migration. This requirement may occur in thinning area.