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====Backarc Basin====

====Backarc Basin====

Backarc basin forms landward of the volcanic arc. Mechanisms for backarc basin have been proposed by several authors. Karig<ref name=Karig1971a>Karig, D. E., 1971a, Structural history of the Mariana island arc system: Geological Society of America Bulletin, v. 82, p. 323-344.</ref><ref name=Karig1971b>Karig, D. E., 1971, Origin and development of marginal basins in the Western Pacific: Journal of Geophysical Research, v. 76, p. 2542-2561</ref> and 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.

Backarc basin forms landward of the volcanic arc. Mechanisms for backarc basin have been proposed by several authors. Karig<ref name=Karig1971>Karig, D. E., 1971, Origin and development of marginal basins in the Western Pacific: Journal of Geophysical Research, v. 76, p. 2542-2561</ref> and 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<ref>Packham, G. A., and D. A. Falvey, 1971, An hypothesis for the formation of marginal seas in the western Pacific: Tectonophysics, v. 11, p. 79-109</ref> proposed that magma upwelling in backarc basin is passive, generated as a result of extensional regime of the plate. Tamaki<ref>Tamaki, K., 1985, Two modes of back-arc spreading: Geology, v. 13, no. 7, p. 475-478.</ref> 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<ref>Chase, C. G., 1978, Plate kinematics: The Americas, East Africa, and the rest of the world: Earth and Planetary Science Letters, v. 37, p. 355–368.</ref> and Fein & Jurdy<ref>Fein, J. B., and D. M. Jurdy, 1986, Plate motion controls on backarc spreading: Geophysical Research Letters, v. 13, p. 456-459</ref>. 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.

Karig (1971) classifies backarc basin into three classes: active marginal basin, inactive marginal basin with high heat flow, and inactive marginal basin with normal heat flow.

Karig<ref name=Karig1971 /> classifies backarc basin into three classes: active marginal basin, inactive marginal basin with high heat flow, and inactive marginal basin with normal heat flow.

==Petrology of Subduction Zone==  

==Petrology of Subduction Zone==