活动海岭俯冲对岛弧地质过程的影响

THE GEOLOGICAL EFFECT OF RIDGE SUBDUCTION IN ARC AREA

本文研究了海岭—岛弧体系的地质演化和海岭俯冲过程，通过有限元法对海岭俯冲的全过程进行了热模拟，分析了海岭俯冲过程中岛弧岩浆活动、变质作用及周围地区地表地形的变化。热模拟的计算结果表明：在海岭俯冲之前和俯冲完成之后，摩擦剪切生热使岛弧下１００ｋｍ深度形成温度反转，俯冲海洋板片内角闪岩相矿物在８５０—１０００℃的相对高温下脱水，释放的水进入其上覆板块内热的地幔楔状体降低地幔岩石的熔点造成部分熔融，形成岛弧火山活动；而当热的活动海岭俯冲期间，近海岭处的年轻海洋地壳在较浅深度达到较高温度而提前脱水，原来的地幔楔状体内部分熔融区因缺水而使熔融停止，岛弧火山活动中断。但此时，活动海岭俯冲产生的热将会使前弧一定区域出现低温变质作用。在整个俯冲过程中，随海岭逼近海沟，温度升高，岛弧将因此逐渐抬升。

Ridge subduction is an important stage of Wilson cycle.If the spreading rate of the active ridge is smaller than the rate of subduction,the subducting oceanic plate will decrease in size,and the ridge itself will be eventually subducted.There are four possible kinematic scenarios of ridge subduction,we investigate one of the possibility:both the leading and the trailing plates are subducted.Subduction of a hot active ridge will significantly affect the thermal state of the subduction zone,and leave tectonic and petrologic signatures.In this study,we use twodimensional finite element method to calculate the thermal consequence of an active ridge subduction.Upwind technique is adopted to secure a stable and accurate solution for advective heat transfer.Frictional/shear heating is included in our modeling.The shear stress is limited by the friction law in the brittle regime and by the flow law in the ductile regime.Sequential changes of temperature created by ridge subduction are calculated,and the resulting geological consequences at arc areas are discussed.The results of computation can well explain cessation of arc volcanism,low temperature metamorphism,uplift and subsidence of arc trench margin.