摘要
阿留申俯冲带位于环太平洋俯冲带最北端,是东太平洋型俯冲和西太平洋俯冲的过渡区域。该俯冲带火山岛弧距离海沟的距离从东向西逐渐增大,而形成地球上独特的岛弧火山链与海沟V字型斜交的现象。这一现象的运动学成因目前并没有统一的认识。本文通过对阿留申俯冲带几何形态数据、运动学数据进行整理分析,尝试运用构造赤道理论探讨该现象形成的运动学背景。阿留申俯冲带的几何学数据表明:从俯冲带东段(175°E)至俯冲带西段(155°W),火山岛弧距俯冲海沟的距离从80 km增加至250 km。与此同时,俯冲板片的倾角由60°减小至30°。板块的运动学分析表明:相对北美板块,太平洋板块的东段的运动矢量为48 mm/a,向北运动;逐渐转变为西段的78mm/a,向西北方向运动。相对于软流圈,太平洋板块的运动方向没有改变,始终向西北方向运动,速率向西逐渐增加。因此,在俯冲带的东段太平洋板块的绝对运动方向和相对运动方向存在30°左右的夹角,而这个夹角在西段几乎不存在。太平洋板块的绝对运动方向和相对运动方向之间的夹角不同,会导致软流圈对俯冲板片的反作用力差异,从而形成不同的俯冲角度和俯冲带宽度。太平洋板块相对北美板块和相对地幔的速度方向夹角的变化被认为是引起阿留申火山弧与海沟"V"字型斜交的运动学成因。
Objectives: The Aleutian subduction zone lies along a 2000-km long in the northern end section of the socalled "Pacific Ring of Fire". This subduction zone is a transition type between Chilean type and Mariana type,with unique geometry. Especially,the distance between Aleutian volcanic arc and subduction trench increases gradually from west to east,and formed the unique V-shaped geometry between volcanic chains and trench. This unique phenomenon has become the central point of an argument among geologists and geophysicists. And there is still no unified understanding on the deep root for this phenomenon.Methods: The kinematic data and geometry data of Aleutian subduction zone were collected and analyzed. The theory of "tectonic equator"is used to explain this unique phenomenon.Results: Geometric data of Aleutian subduction zone show that: from eastern part of this subduction( 175°E)to western part of this subduction zone( 155° W),the distance between volcanic chains and subduction trench is gradually increasing from 80 km to 250 km. Meanwhile,the dip angle of subduction slab is decreasing from 60°to30°. But the relationship between dip angle of subduction slab and longitude is not utterly linear due to block rotation of arc massif. Plate kinematic indicate that: with respect to American plate( relative motion),Pacific plate moving northwards at 48 mm/y in eastern part and transfer to northeastward at 78 mm/y in western part. However,with respect to asthenosphere( absolute motion),the direction of pacific plate motion is invariable. In other words,the angle between relative motion and absolute is about 30° in eastern part of subduction zone,but this angle is close to zero in western part.Conclusions: The decreasing of angle between absolute motion and relative from east to west will increase the influence of asthenosphere to subduction slab. This change will lead the dip angle of subduction slab gentler and the distance between volcanic chains and subduction trench larger in the western part. Herein,the variation of the anglebetween the moving direction of the Pacific plate in different reference systems is considered as the deep root for Vshaped geometry between volcanic chains and trench.
引文
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