俯冲带的后撤与弧后扩张

西太平洋地壳年龄较老,因而岩石层较冷和比重较大,俯冲带的角度也较大,活动和成熟的弧后盆地则较多;条件与之相反的东太平洋弧后盆地则较少.本文探讨这种相关关系的力学成因,计算了俯冲板块诱生的弧后上涌地幔流动.计算表明,俯冲角度大及存在后撤俯冲时,有利于在弧后地区产生明显的上涌地幔流,这种深部热物质的上涌会导致弧后扩张.反之,年龄较轻的海洋地块较热和较轻,俯冲角度一般也较小,不易诱生上涌地幔物质流动和弧后扩张.大陆地壳密度小于地幔物质,大陆碰撞区就更不具备弧后扩张的条件.

ROLL-BACK SUBDUCTION AND BACK-ARC OPENING

It has been found that a correlation exists among ages of oceanic crust, subduction angle, and abundance of back-arc basins in subduction zones. Active and mature back-arc basins are abundant in western Pacific where ages of oceanic crust are relatively old, the lithosphere is cold and heavy, and therefore subduction angles are relatively large; while few back-arc basins are found in eastern Pacific where ages of the crust are young and subduction angles are relatively small. We investigate the mechanics of the correlation by calculating the induced mantle flows by subducted slab in back-arc regions. The results of computation indicates that great subduction angle and existence of roll-back subduction are favorite conditions for inducing upwelling of mantle flows in back-arc region, and upwelling of hot mantle materials may lead to back-arc opening. In contrast, young oceanic crusts are warm and light, thus, subduction angles are usually small, and it is difficult to induce mantle upwelling and back-arc opening. Density of continental crust is smaller than that of the mantle, therefore, back-arc opening is unlikely to occur at continental collision zones.