裂生微地块构造特征及成因模式:来自西太平洋弧后扩张作用的启示

Tectonic and Geodynamic Mechanism of Back-arc-rifting Derived Micro-blocks: Insights from Back-arc Spreading in the West Pacific

微地块是一种在现今空间尺度上相对微小的地质块体,在裂谷系统、被动陆缘、大洋中脊、深海洋盆、俯冲系统和碰撞环境中均有分布。微地块有多种成因类型,其中,由裂谷系统、被动陆缘、弧后扩张作用导致陆块或岛弧发生裂解过程中所生成的新地块或板块,为裂生微地块(rifting-derived micro-block)。本文主要结合西太平洋“沟?弧?盆”体系中弧后扩张形成的马里亚纳微地块、马努斯微地块以及北斐济海盆内部的微地块特征,初步总结了裂生微地块的识别标志、运动学特征以及动力学机制。裂生微地块的地壳属性可以是陆壳或洋壳、陆缘弧或洋内弧地壳,其边界必有一侧为离散型边界,其余边界可复杂多样,如俯冲带、转换断层、深大断裂等,可以是已经死亡的构造,也可以是正在活动的构造。裂生微地块常见于伸展裂解系统、俯冲消减系统中,西太平洋的板缘裂生微地块是在弧后伸展裂解的构造背景下形成的,随着弧后扩张作用的发育而生长演化。基于前人研究,裂生微地块的成因模式可总结为两类:与深部地幔上涌相关的主动裂解型,如板片窗模式、地幔柱模式;与岩石圈拉伸、海沟后撤相关的被动裂解型。裂生微地块的演化往往是多种裂解机制下综合作用的产物,常常伴随着不对称、不稳定的弧后扩张过程以及相对于周缘微地块/板块的旋转,这些都导致了裂生微地块构造演化的独特性和复杂性。对裂生微地块的研究,使得洋?陆俯冲带、造山带的研究将更加精细化,同时也可以丰富地块/板块发展演化的内涵。

Micro-blocks are small geological blocks on the present spatial scale,which are mostly distributed in rift system,passive continental margin,mid-ocean ridge,oceanic basin,subduction system or collision system.There are different types of micro-blocks,and one of them is formed by rifting or splitting of island arcs and breakup of continental blocks which was previously termed as back-arc-rifting derived micro-block.This paper preliminarily summarized its identification markers,kinematic characteristics and dynamic mechanism on the bases of the characteristics of micro-blocks located in the West Pacific“trench-arc-basin”system.Its crust can be continental or oceanic,continental margin arc or intra-oceanic arc with complex and diverse boundaries that include subduction zones,transition faults,and deep faults.However,at least one of them should belong to divergent boundary,such as rifts or spreading centers.The back-arc-rifting derived micro-blocks are generated in extensional system and subduction system.The geodynamic models could be divided into two types based on previous studies.One is called active spreading pattern related to the upwelling of the deep mantle,such as slab window and mantle plum mechanisms;and the other is called passive spreading pattern related to lithosphere extension and trench retreat.Back-arc-rifting derived micro-blocks are commonly related to several rifting mechanisms,and accompanied by asymmetrical unstable spreading,boundary jump and rotation movement.Besides,all of them result in the uniqueness and complexity of micro-blocks.The study of back-arc-rifting derived micro-blocks could refine studies of oceanic-continental subduction and orogeny,and enrich the connotation of the blocks/plates tectonics as well.